Flexible optical waveguides

Abstract

The invention relates to flexible optical waveguides for transmitting electromagnetic waves, in particular for transmitting image information, and to methods of manufacturing flexible optical waveguides.

Description

[0001] Flexible Optical Waveguides

[0002] The invention relates to flexible optical waveguides for transmitting electromagnetic waves, in particular for transmitting image information, and to methods of manufacturing waveguides, in particular image guides.

[0003] Traditional image guides typically comprise a plurality of individual optical fibers, each of which comprises a core and a cladding surrounding the core, the optical fibers being assembled as a bundle and arranged in cross-section in a grid with a one-to-one relationship between the light input surface and the light output surface to form a plurality of pixels. Basically, each pixel serves to transmit a brightness value or color information via the image guide.

[0004] One type of known waveguide is a flexible leached fiber bundle waveguide. A leached fiber bundle waveguide includes a large number of optical fibers which may be arranged in an ordered array. The leached fiber bundle waveguide may be formed by drawing a fiber bundle preform having a number of pre-arranged optic fiber preforms, in the form of glass rods and / or tubes, together with at least some leachable glass spacers located between or encapsulating each of the desired optic fiber preforms. The fiber bundle preform is drawn down to the desired size for the optical fibers, which are fused together with the leachable glass spacers as they are drawn, with the leachable glass spacers maintaining a space between the individual optic fibers. In order to form a flexible waveguide, the ends of the fused optical fiber bundle can be protected with a soft, etch resistant coating, and the leachable glass spacers are leached from the fused optical fiber bundle, typically using an acid etch bath. Once the leaching process is complete, the individual optical fibers in the middle portion of the leached optical fiber bundle are free to move and allow the leached fiber bundle to be flexed, while the ends are still held together. Ferrules can then be installed on the ends to protect the ends from damage. The flexible middle portion may be placed within a flexible outer sheath to prevent the individual optical fibers from being damaged.

[0005] Another type of known waveguide is a flexible wound fiber bundle. In accordance with one industrywide method of fabricating a flexible wound fiber bundle, a continuous light-conducting fiber is helically wound onto a mandrel in order to form adjacent convolutions (i.e. , windings) about the mandrel. Once a helix of predetermined width is formed, a predetermined "ends region" of the successive convolutions is laterally bonded such that the portions of the continuous fiber within that predetermined ends region are fixed in side-by-side relationship with each other prior to the removal of the helix from the mandrel. The bonding is conventionally achieved by application of a suitable glue, cement or epoxy. Once the bonding agent cures, the helix is removed from the mandrel. In order to form two-dimensional input and output ends, a plurality of similarly fabricated helixes is assembled with the ends regions of multiple helixes stacked in aligned superimposed relationship and these ends regions are then secured in fixed relationship by a clamp, housing or a bonding agent such as cement, glue or epoxy. The assembly of ends regions is then cut transversely to the axes of the constituent fibers. A portion of the cut ends region on either side of the cut becomes one of the input end and the output end of the image bundle.

[0006] The leachable cladding may be a third structural element that fuses the ends of the optical fiber bundles together at the proximal end (2) and fuses the ends of the optical fiber bundles together at the distal end (4).

[0007] Flexible waveguides may be manufactured similar to the known processes to manufacture flexible leached fiber bundles and flexible wound fiber bundles, except the leachable cladding and the bonding agent are optional. The input and output ends of the waveguide may be fused or bonded together, such as shown by element 6 in Fig. 1 C .

[0008] In practice, it is often desirable to have the highest possible resolution of the image guide. In principle, a high resolution can be achieved by reducing the diameter of the individual optical fibers. However, due to physical laws, the resolution cannot be increased linearly because, as the diameters of the individual optical fibers become smaller and smaller, an increasing proportion of the field distribution of the transmitted modes exceeds the dimensions of the optical fibers, in particular the cladding, which leads to increased crosstalk between adjacent optical fibers and thus to increasing blurring.

[0009] One approach to provide image guides with higher resolution is based on the wave phenomenon of transverse Anderson localization (TAL). This takes advantage of the fact that a random distribution of refractive indices over the cross-section of the image guide with simultaneous invariance of the crosssection along the length of the image guide leads to a confinement of the coupled light due to destructive interference. The term “random” as used herein includes the arrangements described in WO 2021 / 259926, the entire contents of which are hereby incorporated by reference, and any other aperiodic or non-regular pattern. The random arrangement of cores can be accomplished by making special perturbations to a regular pattern so that the core positions are shifted randomly around the core locations defined by a regular pattern, or by randomly attributing different refractive indices to regular pattern positions. In practice, for example, a large number of individual fibers with different refractive indices can be combined to form a transverse Anderson localization optical waveguide. If a light beam is coupled into such a waveguide, it propagates along the length of the waveguide with a transverse extension limited in cross-section.

[0010] Accordingly, it is an object of the invention to provide flexible waveguides, in particular flexible image guides, that may or may not function based on the principle of transverse Anderson localization, as well as methods for the production thereof, which have improved image quality.

[0011] Additionally, it is an object of the invention to provide flexible waveguides, in particular flexible image guides, that may or may not function based on the principle of transverse Anderson localization, as well as methods for the production thereof, which are flexible in at least a middle distance of the transport direction such that they have a small bending radius.

[0012] The present invention discloses flexible waveguides for transmitting electromagnetic waves, in particular for transmitting image information from a proximal end of the waveguide to a distal end of the waveguide, along a transport direction extending between the proximal and distal ends, wherein the waveguide comprises one or more fiber bundles extending along the transport direction each comprising a plurality of first structural elements and a plurality of second structural elements that differ from the first structural elements, whereby electromagnetic waves introduced into the proximal end (2) are confined within a cross-sectional region transverse to the transport direction (5) due to the difference between the first structural elements (10a) and the second structural elements (10b). When the waveguides transmit image information according to the principle of transverse Anderson localization, each fiber bundle comprises a random distribution of the first structural elements having a first refractive index and second structural elements having a second refractive index. When the waveguides transmit image information according to the principle of total internal reflection, the first structural elements are a plurality of cores and the second structural elements are claddings surrounding each core, and the refractive index of the second structural elements (claddings) is lower than the refractive index of the first structural elements (cores). The first structural elements and the second structural elements may have different refractive indices with or without different cross-sectional areas. The first structural elements may be formed from a single material, as can each of the other structural elements.

[0013] In certain embodiments, for an image of Groups 6 and 7 of a positive USAF51 target described herein, the waveguides have a multi-scale structural similarity index measure (MS-SSIM) greater than 0.25.

[0014] When the waveguide functions utilizing the principle of transverse Anderson localization, at least two different types of structural elements can be used, namely a first type having a first refractive index and a second type having a second refractive index which is different than the first refractive index. Accordingly, the plurality of structural elements may each comprise at least one structural element of the first type as well as at least one structural element of the second type. Of course, also more than two different types, e.g. three different types of structural elements may be used. For example, if the one or more fiber bundles are surrounded by a non-leachable cladding, which may be provided for mechanical purposes, the non-leachable cladding can be considered to be a third structural element. Furthermore, in embodiments where the waveguide is a leached fiber bundle, the leachable cladding that surrounds each optical fiber bundle can be considered to be a fourth structural element, although the leachable cladding is leached away and is not present in the middle along the length of the final waveguide. In all embodiments, the waveguide may be surrounded by a non-leachable mechanical protection buffer. A leachable material is a material that can be dissolved by acids, bases, deionized water or organic solvents at least two times faster than the elements that are not intended to be fully removed in the conventional processes of forming flexible optical fiber bundles, such as the structural elements and the optional non-leachable cladding.

[0015] The structural elements can each extend along the transport direction as well as over the cross-section of the waveguide in such a way that a plurality of cross-sectional regions is defined in the cross-section of the waveguide, each corresponding to the cross-section of a single structural element. Accordingly, the structural elements can extend side by side, in particular parallel to one another, along the transport direction of the waveguide and their cross-sections can each occupy a planar portion of the crosssection of the waveguide and therefore can each define a cross-sectional region of the cross-section of the waveguide. The cross-sectional regions can thus correspond in particular to the surface regions formed by the structural elements when looking at a cross-sectional surface of the waveguide, for example the light entry or light exit surface.

[0016] According to some embodiments of the invention, the first structural element may differ from the second structural element by its refractive index, cross-sectional area and / or composition.

[0017] The cross-sectional regions of the structural elements may have geometries which are non-uniform with respect to one another, for example non-uniform diameters. However, the geometries of the cross- sectional regions can also be of the same type.

[0018] The structural elements can be arranged in such a way that electromagnetic waves transmitted by the waveguide remain localized in a direction extending transversely to the transport direction.

[0019] The structural elements can be arranged mathematically in such a way that the waveguide has a reproducible structure, in particular in such a way that further waveguides with a structure identical or substantially similar to the waveguide can be produced, for example as described in WO 2021 / 259926.

[0020] The ratio of the total area of the cross-sectional regions of the first structural elements to the total area of the cross-sectional regions of the second structural elements can be, for example, in a range between 1 :150 and 150:1 , 1 :100 and 100:1 , 1 :50 and 50:1 ; 1 :10 and 10:1, 3:7 and 7:3, 4:6 and 6:4, or 5:5. This can also be understood as a degree of filling.

[0021] The refractive index of the first structural elements and the refractive index of the second structural elements may differ by at least 10 , for example by at least 103, for example by at least 102, for example by at least 101, for example by at least 1 , for example by at least 2, for example by at least 3, for example by at least 4.

[0022] With respect to the lateral extent of the structural elements, it may be provided that the diameter is 100 nm to 50 pm, 500 nm to 20 pm, or 650 nm to 10 pm.

[0023] With respect to the lateral extent of the fiber bundles, it may be provided that the diameter is 10 to 250 pm, 20 to 150 pm, 20 to 100 pm, or 30 to 60 pm. Furthermore, it can be provided that at least one cross-sectional region of each structural element has a diameter which lies between 0.1 times and 10 times the average wavelength, in particular of a wavelength range of electromagnetic waves to be preferably transmitted, between 0.2 times and 5 times the average wavelength, or between 0.5 times and 2 times the average wavelength.

[0024] The diameter of the waveguide can be 0.4 to 50 mm.

[0025] In the case of leached fiber bundle waveguides, the fiber bundles are distinguishable from each other by the leachable cladding that surrounds the fiber bundles, which is leached away other than at the end of the waveguide, if a leachable cladding is used. In the case of wound fiber bundle waveguides, the fiber bundles are distinguishable from each other due to the interstitial bonding agent at the ends of the waveguide.

[0026] With respect to the geometric shape of the structural elements and the fiber bundles, it may be provided that the structural elements have a non-circular or polygonal geometry, for example pentagonal or hexagonal.

[0027] The first structural elements can be in particular formed as a, for example monolithic, base body with or from a first medium, wherein the first medium has the first refractive index. The second structural elements may be formed as cavities in the base body, wherein the cavities preferably form the second refractive index, for example by the refractive index of liquid, air or a gas which may be present as a medium in the cavities. The structural elements of the second type can be in particular formed as a, for example monolithic, base body with or from a second medium, wherein the second medium has the second refractive index. One or more of the monolithic base bodies of the first medium can be fused to one or more of the monolithic base bodies of the second medium prior to drawing and redrawing the fused base bodies to form a waveguide comprising a plurality of the first base bodies and a plurality of the second base bodies.

[0028] A base body with cavities can be produced or manufactured in various ways. The cavities in the base body can be formed by additive construction of the base body, for example by means of 3D printing processes. Alternatively or additionally, cavities may be subtractively introduced into the base body, in particular as bores which are introduced into the base body in particular by abrasive material processing methods, for example mechanical drilling. Depending on the method used, bores are not exclusively limited to round geometries.

[0029] The waveguide may be manufactured in a multi-train process, in particular such that the waveguide comprises, in addition to the plurality of structural elements, at least a second plurality of structural elements, wherein the waveguide has, in cross-section, at least two surface regions which each comprise the cross-sectional regions of one of the two pluralities of structural elements and these may have an identical structure apart from a rotation and / or a reflection.

[0030] With regard to the length of the waveguide along the transport direction, it may be provided that the waveguide has a length along the transport direction of at least 10 millimeters, of at least 20 millimeters, of at least 50 millimeters, or of at least 100 millimeters.

[0031] In the case that the waveguide is formed as a base body with cavities, the cavities in the base body, in particular the bores, may be filled with a second medium or the second structural element, the second medium or the second structural element having the second refractive index.

[0032] With regard to the materials, it may be provided that at least one structural element, in particular the or a first structural element, in particular the structural element formed as a base body, comprises or consists of one or more of the following materials as a medium: glass (including oxide and non-oxide materials such as chalcogenides), quartz glass, polymer, crystals, monocrystals, polycrystalline materials and / or glass ceramic.

[0033] Furthermore, at least one structural element, in particular the or a first structural element, in particular the structural element formed as a base body, may comprise or consist of a material as a medium which, in the wavelength range to be transmitted, esp. from 2 m to 20 pm, in particular an attenuation of less than 100 dB / m, in particular of less than 50 dB / m, in particular of less than 10 dB / m, in particular of less than 1 dB / m, in particular an infrared-transmissive material, in particular a chalcogenide, in particular comprising at least one element from the group comprising oxygen, sulphur, selenium and tellurium, and at least one element from the group comprising arsenic, germanium, phosphorus, antimony, lead, boron, aluminum, gallium, indium, titanium, sodium. Furthermore, optically active materials may be provided, e.g. as part of a medium or a filling and / or also as a layer or coating or other modification on the surfaces of an assembly of structural elements formed as rods or tubes. Thus, for example, a modification of the guided electromagnetic, e.g. in the sense of an amplification or conversion, can be achieved.

[0034] The first structural elements may be formed as, in particular, rod-shaped or tubular bodies with or made of a first medium, the first medium having the first refractive index.

[0035] The second structural elements can be formed as, in particular, rod-shaped or tubular bodies with or from a second medium, the second medium having the second refractive index, and / or as cavities in the first structural elements, the cavities forming the second refractive index or being filled with a second medium having the second refractive index.

[0036] In particular, in the case where the second structural elements can be present as filled cavities in the first structural elements, the structural elements may be formed as core-shell systems such that the core corresponds to the filled cavity.

[0037] Rod-shaped or tubular bodies are not to be understood exclusively as those with a round cross- sectional geometry.

[0038] The invention further relates to a method for producing a flexible waveguide, in particular a waveguide having one or more of the features described herein, for transmitting electromagnetic waves from a proximal end of the waveguide to a distal end of the waveguide along a transport direction extending between the proximal and distal ends, the method comprising forming a fiber bundle that extends along the transport direction comprising a plurality of first structural elements and a plurality of second structural elements that differ from the first structural elements, whereby electromagnetic waves introduced into the proximal end are confined within a cross-sectional region transverse to the transport direction due to the difference between the first structural elements and the second structural elements.

[0039] In the following embodiments of the invention are described with reference to the Figures described. They show: Fig. 1A: Schematic illustration of a flexible waveguide having a plurality of fiber bundles.

[0040] Fig. 1 B: Schematic illustration of a cross-section of a fiber bundle.

[0041] Fig. 1 C: Schematic illustration of a flexible waveguide having a plurality of fiber bundles.

[0042] Fig. 2: Schematic illustration of cross-sections of various fiber bundles having (a), (b), (c) at least two types of structural elements and (d), (e) at least three types of structural elements, respectively, wherein the cross-sectional areas of the structural elements may differ.

[0043] Fig. 3: Schematic perspective views of two fiber bundles having (a) two types of structural elements that are non-uniformly distributed and (b) a plurality of structural elements of non-uniform refractive indices (plurality of types) and / or non-uniform geometries (diameters).

[0044] Fig. 4: Schematic cross-section of a fiber bundle with two types of structural elements that are non- uniformly distributed on a hexagonal lattice.

[0045] Fig. 5: Schematic perspective views of (a) fiber bundles assembled into a preform, which are drawn into length, (b), (c) a plurality of fiber bundles assembled therefrom again into a preform, which are drawn into length, and (d) assembled again, and (e) a plurality of fiber bundles fused under pressure to form a waveguide.

[0046] Fig. 6: Schematic cross-sections of (a), (c) fiber bundles being unrotated relative to one another, (b), (d) fiber bundles being rotated relative to one another.

[0047] Fig. 7: Schematic illustration of various further possibilities for fiber bundles having structural elements or cross-sectional regions thereof which are different, the waveguides each comprising a plurality of structural elements of a first type and a plurality of structural elements of a second type. Exemplary flexible waveguides of the current disclosure which have a small bending radius may have a bending radius of less than 50 mm. Such waveguides include flexible leached fiber bundle waveguides and flexible wound fiber bundle waveguides.

[0048] Processes for producing flexible leached fiber bundle waveguides are known, for example from WO 02 / 40416 A1 and U.S. Pat. No. 4,389,089, the entire contents of which are hereby incorporated by reference. Leached fiber bundle waveguides can be produced by drawing out correspondingly arranged fiber bundle preforms, for example glass rods or tubes or a plurality of first and second structural elements, which additionally may include at least a few spacers or claddings made from leachable material, i.e. a glass which can be at least partially dissolved by treatment with acids, bases or deionized water. The spacer preforms are generally in the form of tubes or rods. The distance between and arrangement of the structural elements with respect to one another can be defined by a suitable arrangement of the leachable material when a leachable material is used. The fiber bundle preforms obtained in this way are then drawn out under the action of heat using known processes until the desired fiber or fiber bundle cross section is reached. In the process, the spacers when used are fused to the optical fibers and fill the space between the individual optical fibers. To produce a waveguide, the ends of the fused fiber bundles are provided with a protective layer which is resistant to acids and / or lyes, and the spacers and cladding located in the unprotected regions are removed in hot acid or lye baths or in deionized water baths. In this way, the fiber bundles are uncovered or separated from the spacers and cladding if they are present and the waveguide becomes flexible at these locations. The ends when spacers or a leachable cladding are used remain connected to one another and therefore rigid and fixed, or when spacers or a leachable cladding are not used, the ends may be fused or adhered to become rigid and fixed.

[0049] In another embodiment, to provide the flexibility, each of the one or more optical fiber bundles comprising the first and second structural elements (10a, 10b) can be surrounded by a leachable cladding (3) as shown in Fig. 1 B. When the leachable cladding is removed, the waveguide (100) becomes flexible because one or more of the fiber bundles (1) are not bound to each other along at least a middle distance of the transport direction (5) such that the waveguide (100) has a bending radius of less than 50 mm. When the waveguide is flexed, there is a spacing between each of the fiber bundles for example due to the absence of the leachable cladding after the leachable cladding is leached away. In some embodiments, a non-leachable cladding (7) can surround the optical fiber bundles to provide mechanical stability and / or for optical purposes. Fig. 1 C shows an embodiment where item 6 is a portion of the leachable cladding that is not leached away at the ends of the waveguide or a bonding agent introduced at the ends of the waveguide, where both can be used to fuse or bond the fiber bundles together at the proximal end and fuse or bond the fiber bundles together the distal end (4) to provide rigidity. Waveguides can also be flexible when they are made from a single fiber bundle without the need to introduce and leach away a leachable cladding.

[0050] The optically active area of a traditional core-cladding leached fiber bundle consists of the core that transmits the light. The cladding that surrounds the core and the leachable cladding that surrounds each individual core-cladding fiber is not part of the optically active area. The optically active area of a traditional core-cladding leached fiber bundle is about 40% over the cross section at the proximal and distal ends of the waveguide. In contrast, since the optical fiber bundles of the current disclosure are not required to have individual claddings surrounding individual cores, and instead may have individual claddings surrounding individual fiber bundles, the optically active area over the cross section at the proximal and distal ends of the waveguide is much greater, for example at least 75%, at least 80%, at least 85%, at least 90% or at least 95%.

[0051] Along at least a middle distance of the transport direction (5) of such waveguides (100), the waveguide (100) is flexible because one or more of the fiber bundles (1) are not bound to each other. In other words, there is a separation between one or more of the fiber bundles (1) at the middle distance which allows adjacent fiber bundles (1) to freely move with respect to each other. The waveguides (100) can therefore have a bending radius of less than 50 mm.

[0052] The bending radius is a measurement of the flexibility of the waveguide. A waveguide has a bending radius of X when it can be bent 360 degrees around a mandrel having a radius of X. For example, a waveguide has a bending radius of 50 mm when it can be bent 360 degrees around a mandrel having a radius of 50 mm. In some embodiments disclosed herein, the waveguide has a bending radius of less than 50 mm, less than 40 mm, less than 30 mm, less than 20 mm, less than 10 mm, less than 7 mm, less than 4 mm, and / or less than 1 mm.

[0053] Fig. 2 shows various principal examples of fiber bundles (1) which can be used in particular as waveguides. The fiber bundles (1) shown in cross-section each comprise a plurality of structural elements (10), each of which extends along the direction of transport of the waveguide (100), which is perpendicular to the figure here, and each of which extends proportionally over the cross-section thereof. Each of the structural elements (10) thus defines a cross-sectional region (20), i.e. a proportion of the area of the cross-section of the fiber bundle (1). The examples of fiber bundles (1) shown each have at least two different types of structural elements, which differ in their refractive indices and / or diameters. These principle embodiments serve to illustrate some variants of non-uniformity and may deviate in detail from a deterministic positioning of structural elements.

[0054] Such waveguides (100) for transmitting electromagnetic waves from a proximal end (2) of the waveguide (100) to a distal end (4) of the waveguide along a transport direction (5) extending between the proximal and distal end can each comprise one or more fiber bundles (1) extending along the transport direction (5) each comprising a plurality of first structural elements (10a) and a plurality of second structural elements (10b) that have a different refractive index from the first structural elements (10a), wherein the waveguide (100) is flexible such that it has a bending radius of less than 50 mm because the one or more fiber bundles (1) are not bound to each other along at least a middle distance of the transport direction (5).

[0055] When the waveguides transmit image information according to the principle of transverse Anderson localization, there is a random distribution of the plurality of first and second structural elements. When the waveguides transmit image information according to the principle of total internal reflection, the first structural elements are a plurality of cores and the second structural elements are claddings surrounding each core, and the refractive index of the second structural elements is lower than the refractive index of the first structural elements. In some embodiments, the waveguide (1) has a resolution of at least 128.0 Ip / mm in a negative USAF target.

[0056] The fiber bundle (1) shown in cross-section in Fig. 2(a) has a first structural element 10a formed as a base body, which accommodates a plurality of second structural elements 10b. The second structural elements 10b may thereby be formed, for example, as cavities or hollow channels extending along the transport direction in the first structural element 10a. In this case, the first structural element 10a formed as a base body comprises a first material having a first refractive index, and the second structural elements 10b formed, for example, as cavities form the second refractive index, for example, by the air or another gas contained therein. In this case, the cross-sectional region 20 of the first structural element corresponds to the cross-sectional area of the waveguide (100) minus the holes in this area defined by the cavities, while the cross-sectional regions 20 of the second structural elements 10b each correspond to the cross-sectional area of the cavities. However, the cavities in the main body may also be filled with a second material, such that the second structural elements 10b correspond to the filled cavities. As schematically shown in the figure, the cross-sectional regions 20 of the second structural elements 10b are non-uniform in that their positions are non-uniformly distributed over the cross-section, in particular do not lie on a periodic grid.

[0057] The fiber bundle (1) shown in cross-section in Fig. 2(b) has two types 10a, 10b of structural elements, namely one structural element 10a formed as a base body and having a first refractive index and a plurality of structural elements 10b having a second refractive index different therefrom. In the example shown here, the cross-sectional regions 20 of the second structural elements 10b are not only non- uniformly arranged, but also have non-uniform geometries, in this case non-uniform diameters.

[0058] The fiber bundle (1) shown in cross-section in Fig. 2(c) again has two types 10a, 10b of structural elements, wherein the cross-sectional regions of the second structural elements 10b are each arranged within a first structural element 10a, particularly as core-sheath systems. Thus, in this case, a plurality of first structural elements 10a and a plurality of second structural elements 10b are provided. The structural elements or their cross-sectional regions are formed non-uniformly in that the first structural elements 10a (which accommodate the second structural elements 10b) are arranged non-uniformly, in particular aperiodically, over the cross-section of the fiber bundle (1), this arrangement being mathematically determined.

[0059] The fiber bundles (1) shown in cross-section in Figs. 2(d) and (e) correspond in some aspects to the fiber bundles (1) shown in Figs. 2(a) and (b), respectively, but having structural elements of three types 10a, 10b, 10c having different refractive indices. In particular, cavities in the structural element 10a formed as a base body may be filled with different media. Accordingly, the structural elements 10b, 10c have in particular a non-uniformity in that their refractive index differs from each other.

[0060] Fig. 3 shows two further examples of fiber bundles (1) which can be used in particular as waveguides. The fiber bundles (1) comprise a plurality of first and second structural elements 10, each of which extends from a proximal end 2 to a distal end 4 of the fiber bundle (1) along the transport direction 5 and is, for example, rod-shaped. The fiber bundle (1) shown in Fig. 3(a) has a plurality of first structural elements 10a and a plurality of second structural elements 10b. The structural elements have a non- uniform arrangement in that the first structural elements 10a and the second structural elements 10b are non-uniformly arranged and / or distributed. The fiber bundle (1) shown in Fig. 3(b) comprises a plurality of structural elements 10, wherein in this example the cross-sectional regions of the structural elements have non-uniform geometries. In particular, the geometries may differ in that the diameters of the structural elements or their cross-sectional regions differ from each other. Furthermore, the structural elements 10 in each of Figs. 3(a) and 3(b) may exhibit a non-uniformity in that the refractive indices of the structural elements differ from one another. In this respect, a discrete number of different refractive indices, for example two, three, four, etc. may be provided.

[0061] Fig. 4 shows a cross-section of a fiber bundle (1) which corresponds in some aspects to the fiber bundle (1) shown in Fig. 3(a). The fiber bundle (1) shown in Fig. 4 has a plurality of, in particular rodshaped, structural elements 10, namely a plurality of a first structural elements 10a and a plurality of a second structural elements 10b, the structural elements 10 being arranged in cross-section on a hexagonal lattice. At least one of the structural elements 10, or its cross-sectional area 20, is equidistant from, and preferably adjacent to, six immediately adjacent structural elements 10, or their cross-sectional areas 20.

[0062] Fig. 5 shows steps of a method of manufacturing a waveguide according to a multi-draw method. In this process, a plurality of fiber bundles are assembled to form a preform 30 and drawn into length (Fig. 5a). The fiber bundles may be, for example, an arrangement of structural elements 10 or 10a, b, for example, according to Fig. 4, or alternative arrangements, for example, according to those shown in Fig. 2(a) to (e), which may be already drawn out in a known manner.

[0063] The assembled and elongated plurality of fiber bundles ("multi-fiber") are then disassembled into sections and again assembled into a preform 40 (Fig. 5b, "multi-multi-assembly"). The preform 40 can then again be drawn to length (Fig. 5c), and if necessary again broken down into sections and assembled (Fig. 5d). Finally, the assembly thus obtained can be fused at the ends by applying heat and / or pressure, and in particular under vacuum (Fig. 5e). With reference to Fig. 6, the assembled fiber bundles ("Multi-Fiber", here "M1") drawn to length can be assembled unrotated relative to one another (Fig. 6a) or rotated relative to one another (Fig. 6b) during assembly into a further preform. Furthermore, during the assembly, sections from at least two different assembled fiber bundles ("M1", "M2") drawn into length can be assembled unrotated (Fig. 6c) or rotated relative to one another (Fig. 6d). Analogous to the arrangements shown in Figs. 6a and 6b, the fiber bundles can also be or are arranged unrotated or rotated relative to one another when the first preform is assembled.

[0064] Fig. 7 shows some fiber bundles (1 ), each with a plurality of structural elements of a first type and a plurality of structural elements of a second type (and sometimes further types in Fig. 7d). In particular, the fiber bundles (1) shown here do not have any matrix material, rather the structural elements are neighboring each other.

[0065] Fig. 7a shows approximately a fiber bundle (1) having a plurality of structural elements 10a and a plurality of structural elements 10b having different refractive indices.

[0066] Fig. 7b shows a fiber bundle (1 ) having a plurality of structural elements 10d and a plurality of structural elements 10e, which have different refractive indices and a different substructure, the substructure being defined by sub-structural elements 10a and 10b (having refractive indices a and b) and 10a and 10c (having refractive indices a and c), respectively. The substructure here is that the structural elements 10d and 10e are formed as core-shell systems, with the cores being different.

[0067] Fig. 7c similarly shows a fiber bundle (1 ) having a plurality of structural elements 10d and a plurality of structural elements 10e, which have different refractive indices and a different substructure, the substructure being defined by the sub-structural elements 10a and 10b (having refractive indices a and b) and 10c and 10b (having refractive indices c and b), respectively. The substructure here is that the structural elements 10d and 10e are formed as core-cladding systems, with the claddings differing.

[0068] Fig. 7d similarly shows a fiber bundle (1 ) having a plurality of structural elements 10e, a plurality of structural elements 10f, a plurality of structural elements 10g, and a plurality of structural elements 10h, which have different refractive indices and a different substructure, wherein the substructure is represented by the sub-structural elements 10a and 10b (having refractive indices a and b), resp. 10a and 10c (having refractive indices a and c) and 10b and 10d (having refractive indices b and d) and 10c and 10d (having refractive indices c and d), respectively. The substructure here is that the structural elements 10e, 10f, 10g and 10h are formed as core-shell systems, with both the shells and the cores being different.

[0069] Fig. 7e shows a fiber bundle (1 ) having a plurality of structural elements 10c and a plurality of structural elements 10d having different geometries and a different substructure, wherein the substructure of the structural element 10c is defined by the substructural elements 10a and 10b (having refractive indices a and b and a first core diameter), and the substructure of the structural element 10d is defined by the substructural elements 10a and 10b (having refractive indices a and b and a second core diameter).

[0070] Fig. 7f shows a fiber bundle (1) having a plurality of structural elements 10c and a plurality of structural elements 10d, which have different geometries and a different substructure, wherein the substructure of the structural element 10c is defined by the substructural elements 10a and 10b (having refractive indices a and b and a centrally positioned core), and the substructure of the structural element 10d is defined by the substructural elements 10a and 10b (having refractive indices a and b and an eccentrically positioned core).

[0071] The structural elements of the first type 10a and of the second type 10b may be surrounded by a structural element of a third type 10c formed as a cladding tube. The cladding tube may have a refractive index which is lower than both the refractive index of the structural elements of the first type 10a and the refractive index of the second type 10b.

[0072] The fiber bundle (1) can comprise a plurality of first structural elements (10a) and a plurality of second structural elements (10b) and the waveguide (100) can comprise one or more fiber bundles (1). The total number of first structural elements (10a) and second structural elements (10b) in the fiber bundle (1) can be enough to produce a centered polygonal unit, for example a fiber bundle having 20 to 300 structural elements or 50 to 300 structural elements, such as for example sixty one structural elements.

[0073] One or more of the first structural elements (10a) can be fused to one or more of the second structural elements (10b). The second structural element (10b) can be an air channel located within a matrix formed from the first structural element (1 Oa). Each individual first structural element (1 Oa) and each individual second structural element (10b) do not need to be surrounded by a cladding.

[0074] A plurality of the first structural elements (10a) and a plurality of the second structural elements (10b) can be randomly arranged when viewed in a cross-section of the fiber bundle (1). The first structural element (1 Oa) and the second structural element (1 Ob) can have identical compositions but different diameters when viewed in a cross-section of the fiber bundle (1).

[0075] A method of manufacturing a waveguide (100) for transmitting electromagnetic waves from a proximal end (2) of the waveguide (100) to a distal end (4) of the waveguide (100) along a transport direction (5) extending between the proximal and distal ends can comprise the steps of forming one or more fiber bundles (1) that extend along the transport direction (5) each comprising a plurality of first structural elements (10a) and a plurality of second structural elements (10b) that differ from the first structural elements (10a), whereby electromagnetic waves introduced into the proximal end (2) are confined within a cross-sectional region transverse to the transport direction (5) due to the difference between the first structural elements (10a) and the second structural elements (10b).

[0076] The flexible waveguides described herein may have a 128.0 Ip / mm in a negative USAF target. The resolution is determined by having at least ten people skilled in the art of fiber optic image analysis view a negative image of a USAF target through a microscope with a field of view that surrounds Groups 6 and 7 only and minimizes the dead space around the perimeter. The microscope should have a resolution that is good enough to resolve more details when viewing the target directly without the waveguide as compared to viewing the target through the waveguide, in order to determine that the waveguide is the limiting factor not the microscope. The at least ten people should view the target through the microscope plus waveguide at two perspectives that are 45 degrees apart. Each of the at least ten people will identify the smallest Group and Element pairing (and its corresponding resolution value in Ip / mm) where they can resolve three horizontal lines and three vertical lines. The resolution is the average of the at least ten Group and Element selections in terms of line pairs per millimeter. For example, as known in the art, Group 7, Element 1 has a resolution of 128.0 line pairs per millimeter (Ip / mm) and Group 7, Element 2, has a resolution of 143.7 Im / mm, and the resolution which corresponds to 5 selections of 128.0 Ip / mm and 5 selections of 143.7 Ip / mm is 135.9. In some embodiments, the resolution is at least 128.0 Ip / mm (Group 7, Element 1), at least 143.7 Ip / mm (Group 7, Element 2), at least 161.3 Ip / mm (Group 7, Element 3), at least 181.0 Ip / mm (Group 7, Element 4), at least 203.2 Ip / mm (Group 7, Element 5), at least 228.1 Ip / mm (Group 7, Element 2), at least 256.0 Ip / mm (Group 8, Element 1), or at least 287.4 Ip / mm (Group 8, Element 2).

[0077] In certain embodiments, for an image of Groups 6 and 7 of a positive USAF51 target described herein, the waveguide has a multi-scale structural similarity index measure (MS-SSIM) greater than 0.25.

[0078] The measurements conditions for the purposes of calculating the MS-SSIM herein are 300-1 , OOOx magnification.

[0079] When comparing two images, e.g. the blank target image and the sample target image through an imaging system, there are different methods described in literature, to compare the two images and assess the quality of the sample target image. The most widely used are statistical measures like the peak signal-to-noise ratio (PSNR) or the mean squared error (MSE), but they do not correlate well with the perception of the human visual system. The assumption that the human vision is adapted to extract structural information from an image led to the development of the structural similarity index measure (SSIM), a pixel-based comparison, which is used for measuring the similarity between two images [Zhou Wang, A. C. Bovik, H. R. Sheikh and E. P. Simoncelli, "Image quality assessment: from error visibility to structural similarity," in IEEE Transactions on Image Processing, vol. 13, no. 4, pp. 600-612, April 2004, doi: 10.1109 / TIP.2003.819861.].

[0080] For two aligned images with pixel vectors x = {xi 1 1 = 1 , 2, . . ., N } and y = {yi 1 1 = 1 , 2, . . ., N } the SSIM is defined as: with a, p, y being the parameters to define the relative importance of the components l(x,y), c(x,y) and s(x,y), which can be understood as the measures for luminescence, contrast and structure comparison:

[0081] where pi, ai2and oij denote the mean values, variances and covariance of or between the respective image-vectors, while C1 = (K1 L)2; C2 = (K2 L)2and C3 = C2 / 2 are small constants, and L denotes the dynamic range of the pixel values. Since the choice C1 = C2 = 0 leads to unstable measurements, Ki = 0.01 K2 = 0.03 is a common choice. With equal weighting of the components, this yields:

[0082] The maximum value of SSIM(x,y) = 1 is only achieved for identical images.

[0083] As a single scale method, the SSIM lacks the variability of the human image perception to different sampling densities or observation distances. Hence, a multi-scale expansion of the SSIM, considering image details at various resolution levels, the Multi scale - structural similarity index measure (MS- SSIM) is considered [Z. Wang, E. P. Simoncelli and A. C. Bovik, "Multiscale structural similarity for image quality assessment," The Thrity-Seventh Asilomar Conference on Signals, Systems & Computers, 2003, 2003, pp. 1398-1402 Vol.2, doi: 10.1109 / ACSSC.2003.1292216.]:

[0084] Here, for M-1 iterations of images low-pass filtered and downsampled by two, contrast cj (x, y) and structural comparison sj(x,y) is calculated along with the luminance IM(x,y) of the scale M image. The weighting parameters aM, pj, yj were calibrated by Wang et al. for M = 5 to the human image perception, yielding 01 = yi = 0.0448, 02 = y2 = 0.2856, 3 = ys = 0.3001 , 4 = y4 = 0.2363 and, as = 5 = y5= 0.1333. For our comparison, images of the Groups 6 and 7 of positive USAF51 targets groups in different magnifications were taken, where the blank target image serves as the reference image, and the one taken through the image guide is the sample target image. The target was illuminated from below with a Lambertian white light source. Magnification and illumination were not changed between images, and saturation was avoided by checking the histogram of the image and making sure that it is not clipped at the maximum value (e.g. 28 - 1 = 255 for an 8-bit image). Before applying the MS-SSIM calculation, the images where aligned and cropped to the same size, using a template matching algorithm.

[0085] In case the dimensions of the image guide are too small to allow a complete visualization of all group 6 and 7 elements, the image has to be divided into a series of non-overlapping subimages. The size of the subimages is defined by the largest rectangular area that can be transmitted through the waveguide. For every pair of corresponding reference and target subimages (that again can be aligned via a template matching algorithm), the MS-SSIM and its area percentage of the composed image are calculated. The area-weighted sum constitutes the MS-SSIM of the complete image.

[0086] All of the ranges disclosed herein include all subranges and all combinations of the ranges and subranges.

[0087] The current disclosure relates to one or more of the following embodiments.

[0088] Embodiment 1 . A flexible waveguide (100) for transmitting electromagnetic waves from a proximal end (2) of the waveguide (100) to a distal end (4) of the waveguide (100) along a transport direction (5) extending between the proximal and distal ends comprising: one or more fiber bundles (1) extending along the transport direction (5) each comprising a random distribution of a plurality of first structural elements (10a) and a plurality of second structural elements (10b) that have a different refractive index from the first structural elements (10a) such that the waveguides transmit image information along the transport direction (5) according to the principle of transverse Anderson localization, wherein one or more of the fiber bundles (1) are not bound to each other along at least a middle distance of the transport direction (5) such that the waveguide (100) has a bending radius of less than 50 mm. Embodiment 2. Flexible waveguide (100) according to the preceding embodiment, wherein the bending radius is less than 20 mm.

[0089] Embodiment 3. Flexible waveguide (100) according to one or more of the preceding embodiments, wherein the one or more fiber bundles comprise a third structural element.

[0090] Embodiment 4. Flexible waveguide (100) according to one or more of the preceding embodiments, wherein the one or more fiber bundles are fused or bonded together at the proximal end (2) and are fused or bonded together at the distal end (4).

[0091] Embodiment 5. Flexible waveguide (100) according to one or more of the preceding embodiments, wherein the one or more fiber bundles comprises a third structural element which is a leachable cladding, wherein the leachable cladding fuses the ends of the one or more fiber bundles together at the proximal end (2) and fuses the ends of the one or more fiber bundles together at the distal end (4).

[0092] Embodiment 6. Flexible waveguide (100) according to one or more of the preceding embodiments, wherein each of the first and second structural elements (10a, 10b) has a diameter of 100 nm to 50 urn.

[0093] Embodiment 7. Flexible waveguide (100) according to one or more of the preceding embodiments, wherein each of the one or more fiber bundles (1) has a diameter of 10 to 250 pm.

[0094] Embodiment 8. Flexible waveguide (100) according to one or more of the preceding embodiments, wherein the waveguide (100) has an optically active area of at least 75% over the cross section at the proximal distal ends of the waveguide.

[0095] Embodiment 9. Flexible waveguide (100) according to one or more of the preceding embodiments, wherein the waveguide (100) has an optically active area of at least 80% over the cross section at the proximal distal ends of the waveguide.

[0096] Embodiment 10. Flexible waveguide (100) according to one or more of the preceding embodiments, wherein for an image of Groups 6 and 7 of a positive USAF51 target, the waveguide (100) has a MS- SSIM greater than 0.25. Embodiment 11 . A flexible waveguide (100) for transmitting electromagnetic waves from a proximal end (2) of the waveguide (100) to a distal end (4) of the waveguide (100) along a transport direction (5) extending between the proximal and distal ends comprising: one or more fiber bundles (1) extending along the transport direction (5) each comprising a plurality of first structural elements (10a) and a plurality of second structural elements (10b) that have a different refractive index from the first structural elements, wherein one or more of the fiber bundles (1) are not bound to each other along at least a middle distance of the transport direction (5) such that the waveguide (100) has a bending radius of less than 50 mm, and wherein the waveguide (1) has a resolution of at least 128.0 Ip / mm in a negative USAF target.

[0097] Embodiment 12. Flexible waveguide (100) according to the preceding embodiment, wherein the one or more fiber bundles comprise a third structural element.

[0098] Embodiment 13. Flexible waveguide (100) according to one or more of embodiments 11 to 12, wherein the one or more fiber bundles comprises a third structural element which is a leachable cladding, wherein the leachable cladding fuses the ends of the one or more fiber bundles together at the proximal end (2) and fuses the ends of the one or more fiber bundles together at the distal end (4).

[0099] Embodiment 14. Flexible waveguide (100) according to one or more of embodiments 11 to 13, wherein the one or more fiber bundles are fused or bonded together at the proximal end (2) and are fused or bonded together at the distal end (4).

[0100] Embodiment 15. Flexible waveguide (100) according to one or more of embodiments 11 to 14, wherein the resolution is at least 143.7 Ip / mm.

[0101] Embodiment 16. Flexible waveguide (100) according to one or more of embodiments 11 to 15, wherein the resolution is at least 161 .3 Ip / mm.

[0102] Embodiment 17. Flexible waveguide (100) according to one or more of embodiments 11 to 16, wherein each of the first and second structural elements (10a, 10b) has a diameter of 100 nm to 50 urn. Embodiment 18. Flexible waveguide (100) according to one or more of embodiments 11 to 17, wherein each of the one or more fiber bundles (1 ) has a diameter of 10 to 250 pm.

[0103] Embodiment 19. Flexible waveguide (100) according to one or more of embodiments 11 to 18, wherein the waveguide (100) has an optically active area of at least 75% over the cross section at the proximal distal ends of the waveguide.

[0104] Embodiment 20. Flexible waveguide (100) according to one or more of embodiments 11 to 19, wherein the waveguide (100) has an optically active area of at least 80% over the cross section at the proximal distal ends of the waveguide.

[0105] Embodiment 21 . Flexible waveguide (100) according to one or more of embodiments 11 to 20, wherein for an image of Groups 6 and 7 of a positive USAF51 target, the waveguide (100) has a MS-SSIM greater than 0.25.

[0106] Examples:

[0107] Inventive Example 1 (flexible bundle using transverse Anderson localization):

[0108] A preform of a fiber bundle was assembled to have a random distribution of 6,561 total first and second structural elements, arranged in a square cross section. The first structural elements had a refractive index of 1 .6 and the second structural elements had a refractive index of 1 .5 such that the resulting waveguide transmitted image information along the transport direction according to the principle of transverse Anderson localization.

[0109] The preform was inserted in a first non-leachable tube, which acted as a cladding. The outer diameter of the leachable tube was 35.56 mm and the inner diameter was 29.88 mm.

[0110] The resulting assembly was drawn down to produce a fiber bundle. The diameter of the each of the first and second structural elements was 750 nm. The diameter of the fiber bundle was 60 pm. A waveguide with dimensions 5 mm x 5 mm was assembled from the fiber bundle. Each end of the waveguide was glued (or could instead be fused) while the fiber bundles were kept unbound along the length of the waveguide.

[0111] The waveguide had an optically active area of 80.4%, a bending radius of less than 20 mm, and a resolution of at least 128 Ip / mm in a negative USAF target.

[0112] Inventive Example 2 (flexible bundle using transverse Anderson localization):

[0113] A preform of a fiber bundle was assembled to have a random distribution of 913 total first and second structural elements, arranged in a round cross section. The first structural elements had a refractive index of 1 .7 and the second structural elements had a refractive index of 1 .5 such that the resulting waveguide transmitted image information along the transport direction according to the principle of transverse Anderson localization.

[0114] The preform was inserted in a first non-leachable tube, which acted as a cladding. The outer diameter of the leachable tube was 35.56 mm and the inner diameter was 29.88 mm. The assembly was then inserted in a second leachable tube, which acted as an outer leachable cladding and a binding agent for the ends of the fiber bundle.

[0115] The resulting assembly was drawn down to produce a fiber bundle. The diameter of the each of the first and second structural elements was 700 nm. The diameter of the fiber bundle was 24 m.

[0116] A waveguide with dimensions 0.91 mm x 0.91 mm was assembled from the fiber bundle. The waveguide was leached and the leachable claddings were dissolved to unbind the individual fiber bundles. Each end of the waveguide was glued (or could instead be fused) while the fiber bundles were kept unbound along the length of the waveguide.

[0117] The waveguide had an optically active area of 80.4%, a bending radius of less than 20 mm, and a resolution of at least 128 Ip / mm in a negative USAF target.

[0118] Inventive Example 3 (flexible bundle using transverse Anderson localization): A preform of a fiber bundle was assembled to have a random distribution of 953 total first and second structural elements, arranged in a square cross section. The first structural elements had a refractive index of 1 .7 and the second structural elements had a refractive index of 1 .5 such that the resulting waveguide transmitted image information along the transport direction according to the principle of transverse Anderson localization.

[0119] The preform was inserted in a first non-leachable tube, which acted as a cladding. The outer diameter of the leachable tube was 35.56 mm and the inner diameter was 29.88 mm. The assembly was then inserted in a second leachable tube, which acted as an outer leachable cladding and a binding agent for the ends of the fiber bundle.

[0120] The resulting assembly was drawn down to produce a fiber bundle. The diameter of the each of the first and second structural elements was 1 ,200 nm. The diameter of the fiber bundle was 42 m.

[0121] A waveguide with dimensions 1.51 mm x 1 .51 mm was assembled from the fiber bundle. The waveguide was leached and the leachable claddings were dissolved to unbind the individual fiber bundles. Each end of the waveguide was glued (or could instead be fused) while the fiber bundles were kept unbound along the length of the waveguide.

[0122] The waveguide had an optically active area of 80.4%, a bending radius of less than 20 mm, and a resolution of at least 128 Ip / mm in a negative USAF target.

[0123] Comparative Example 1 (rigid bundle using transverse Anderson localization):

[0124] A preform of a fiber bundle was assembled to have a random distribution of 419,504 total first and second structural elements, arranged in a square cross section. The first structural elements had a refractive index of 1 .7 and the second structural elements had a refractive index of 1 .5 such that the resulting waveguide transmitted image information along the transport direction according to the principle of transverse Anderson localization. The resulting assembly was drawn down to produce a fiber bundle. The diameter of each of the first and second structural elements was 2,500 nm. The diameter of the fiber bundle was 1 .78 mm.

[0125] The fiber bundle was used to prepare a second assembly with a fiber bundle count of 81, arranged in a squared cross section. The resulting assembly was drawn down to produce a waveguide with dimensions 5 mm x 5 mm. The diameter of each of the first and second structural elements in the waveguide was 750 nm. The waveguide had an optically active area of 100% and could not achieve any bending radius without breaking.

[0126] Comparative Example 2 (flexible bundle using total internal reflection):

[0127] A preform of a fiber bundle having core fibers each surrounded by a cladding was assembled so that the total core-clad fiber count was 36, arranged in a square cross section.

[0128] The preform was drawn down to produce a fiber bundle. The diameter of the core-clad fibers was 10 urn. The diameter of the fiber bundle was 60 microns.

[0129] A waveguide with dimensions 5 mm x 5 mm was assembled from the fiber bundle. The waveguide was leached and the leachable claddings were dissolved to unbind the individual fiber bundles. Each end of the waveguide was glued while the fiber bundles were kept unbound along the length of the weveguide.

[0130] The waveguide had an optically active area of 80.4%, a bending radius of less than 20 mm, and a resolution of not greater than 50.8 Ip / mm in a negative USAF target.

Claims

We claim:1 . A flexible waveguide (100) for transmitting electromagnetic waves from a proximal end (2) of the waveguide (100) to a distal end (4) of the waveguide (100) along a transport direction (5) extending between the proximal and distal ends comprising: one or more fiber bundles (1) extending along the transport direction (5) each comprising a random distribution of a plurality of first structural elements (10a) and a plurality of second structural elements (10b) that have a different refractive index from the first structural elements (10a) such that the waveguides transmit image information along the transport direction (5) according to the principle of transverse Anderson localization, wherein one or more of the fiber bundles (1) are not bound to each other along at least a middle distance of the transport direction (5) such that the waveguide (100) has a bending radius of less than 50 mm.

2. Flexible waveguide (100) according to the preceding claim, wherein the bending radius is less than 20 mm.

3. Flexible waveguide (100) according to one or more of the preceding claims, wherein the one or more fiber bundles comprise a third structural element.

4. Flexible waveguide (100) according to one or more of the preceding claims, wherein the one or more fiber bundles are fused or bonded together at the proximal end (2) and are fused or bonded together at the distal end (4).

5. Flexible waveguide (100) according to one or more of the preceding claims, wherein the one or more fiber bundles comprises a third structural element which is a leachable cladding, wherein the leachable cladding fuses the ends of the one or more fiber bundles together at the proximal end (2) and fuses the ends of the one or more fiber bundles together at the distal end (4).

6. Flexible waveguide (100) according to one or more of the preceding claims, wherein each of the first and second structural elements (10a, 10b) has a diameter of 100 nm to 50 urn.

7. Flexible waveguide (100) according to one or more of the preceding claims, wherein the waveguide (100) has an optically active area of at least 75% over the cross section at the proximal distal ends of the waveguide.

8. Flexible waveguide (100) according to one or more of the preceding claims, wherein for an image of Groups 6 and 7 of a positive USAF51 target, the waveguide (100) has a MS-SSIM greater than 0.25.

9. A flexible waveguide (100) for transmitting electromagnetic waves from a proximal end (2) of the waveguide (100) to a distal end (4) of the waveguide (100) along a transport direction (5) extending between the proximal and distal ends comprising: one or more fiber bundles (1) extending along the transport direction (5) each comprising a plurality of first structural elements (10a) and a plurality of second structural elements (10b) that have a different refractive index from the first structural elements, wherein one or more of the fiber bundles (1) are not bound to each other along at least a middle distance of the transport direction (5) such that the waveguide (100) has a bending radius of less than 50 mm, and wherein the waveguide (1) has a resolution of at least 128.0 Ip / mm in a negative USAF target.

10. Flexible waveguide (100) according to the preceding claim, wherein the one or more fiber bundles comprise a third structural element.11 . Flexible waveguide (100) according to one or more of claims 9 to 10, wherein the one or more fiber bundles comprises a third structural element which is a leachable cladding, wherein the leachable cladding fuses the ends of the one or more fiber bundles together at the proximal end (2) and fuses the ends of the one or more fiber bundles together at the distal end (4).

12. Flexible waveguide (100) according to one or more of claims 9 to 11 , wherein the one or more fiber bundles are fused or bonded together at the proximal end (2) and are fused or bonded together at the distal end (4).

13. Flexible waveguide (100) according to one or more of claims 9 to 12, wherein the resolution is at least 143.7 Ip / mm.

14. Flexible waveguide (100) according to one or more of claims 9 to 13, wherein the resolution is at least 161.3 Ip / mm.

15. Flexible waveguide (100) according to one or more of claims 9 to 14, wherein each of the first and second structural elements (10a, 10b) has a diameter of 100 nm to 50 urn.

16. Flexible waveguide (100) according to one or more of claims 9 to 15, wherein the waveguide(100) has an optically active area of at least 75% over the cross section at the proximal distal ends of the waveguide.

17. Flexible waveguide (100) according to one or more of claims 9 to 16, wherein for an image of Groups 6 and 7 of a positive USAF51 target, the waveguide (100) has a MS-SSIM greater than 0.25.

Images