Tightly-arranged optical fiber bundle based on photonic crystal fiber

Abstract

The utility model relates to the technical field of optical fibers. The densely-arranged optical fiber bundle based on the photonic crystal optical fibers comprises a plurality of photonic crystal optical fibers with polygonal cross sections, each photonic crystal optical fiber comprises a fiber core and a wrapping layer wrapping the outer side of the fiber core, and a plurality of air holes penetrating through the wrapping layer in the axial direction of the wrapping layer are further formed in the wrapping layer. And on the cross section of the optical fiber bundle, at least one side of the cladding of one photonic crystal fiber is in contact with one side of the cladding of the other photonic crystal fiber, so that the plurality of photonic crystal fibers are densely arranged to form the optical fiber bundle. The photonic crystal fiber has the characteristics of relatively high numerical aperture and relatively high transmission efficiency, and has excellent performance in scenes such as illumination, image transmission and energy transmission. In addition, the photonic crystal fibers are densely arranged to form the fiber bundle, and the cladding of the photonic crystal fibers is polygonal, so that the photonic crystal fibers can be in direct contact through the edges of the cladding, seamless dense arrangement is facilitated, and the structural stability and the space utilization rate of the fiber bundle are improved.

Description

Technical Field

[0001] This application relates to the field of optical fiber technology, and in particular to close-packed fiber bundles based on photonic crystal fibers. Background Technology

[0002] Optical fiber bundles have wide applications in fields such as medical devices and semiconductors, for example, in lighting and spot shaping.

[0003] Since optical fibers are typically cylindrical with a circular cross-section, gaps inevitably form between the fibers in their close-packed structure, leading to a decrease in duty cycle and reduced light transmission efficiency. Furthermore, most commonly used optical fibers are solid fibers. Because the difference between the core and cladding refractive indices of solid fibers is not large enough, their numerical aperture (NA) is difficult to exceed 0.28, resulting in limited energy that can enter the fiber bundle and further reducing the light transmission efficiency of this type of fiber bundle.

[0004] Currently, most methods use densely packed small-core circular solid optical fibers to increase the duty cycle. However, since more small-core fibers are needed for the same area, the probability of fiber breakage increases, leading to a significant decrease in yield. Summary of the Invention

[0005] This application discloses a close-packed fiber bundle based on photonic crystal fiber, used to solve at least one of the technical problems mentioned in the background art.

[0006] In a first aspect, the close-packed fiber bundle based on photonic crystal fiber provided in this application includes a plurality of photonic crystal fibers with a polygonal cross-section. The photonic crystal fiber includes a core and a cladding covering the outside of the core. The cladding also has a plurality of air holes passing through the cladding along the axial direction of the cladding. On the cross-section of the fiber bundle, at least one side of the cladding of one photonic crystal fiber is in contact with one side of the cladding of another photonic crystal fiber, so that the plurality of photonic crystal fibers are close-packed to form the fiber bundle.

[0007] The photonic crystal fiber used in this application has the characteristics of high numerical aperture and high transmission efficiency, and performs excellently in scenarios such as illumination, imaging, and energy transmission. This application also arranges the photonic crystal fibers into a close-packed fiber bundle, wherein the cladding of the photonic crystal fibers is polygonal, allowing direct contact between the photonic crystal fibers through the edges of the cladding. This facilitates seamless close packing and improves the structural stability and space utilization of the fiber bundle.

[0008] In one possible implementation, at least a portion of the photonic crystal fibers have all their edges contacted and covered by the edges of the other photonic crystal fibers, thus eliminating gaps between adjacent photonic crystal fibers. This application further defines a close-packed configuration of photonic crystal fibers, where the photonic crystal fibers in a fiber bundle are stacked approximately around the axis of the fiber bundle, resulting in a more concentrated fiber arrangement where there may be no gaps between the fibers.

[0009] In one possible implementation, the plurality of photonic crystal fibers have identical cross-sections and are all regular polygons. Having multiple identical photonic crystal fibers means that only one type of photonic crystal fiber needs to be prepared, simplifying the close-packing process.

[0010] In one possible implementation, the photonic crystal fiber has a regular hexagonal cross-section. Fibers with a regular hexagonal cross-section are less prone to slippage and misalignment, have structural stability, and are highly practical.

[0011] In one possible implementation, the photonic crystal fiber has a square or equilateral triangular cross-section. Fibers with square or equilateral triangular cross-sections can also achieve seamless close packing, improving the space utilization of the fiber bundle.

[0012] In one possible implementation, the cross-sectional shape of the core is the same as that of the photonic crystal fiber. Having identical core and cladding shapes is advantageous for achieving a higher core duty cycle.

[0013] In one possible implementation, the fiber core has a circular cross-section. Circular cross-section fiber cores are simple to manufacture and have lower costs.

[0014] In one possible implementation, the number of photonic crystal fibers in the fiber bundle is not less than 10. The more photonic crystal fibers there are, the more significant the effect of the fiber bundle on improving NA and transmission efficiency. Furthermore, once the number of photonic crystal fibers in a fiber bundle exceeds a certain limit, the effect of the fiber quantity on improving NA and transmission efficiency becomes even more pronounced.

[0015] In one possible implementation, the plurality of air holes are formed as two sets of air holes located on both sides of the fiber core, with the center line connecting the two sets of air holes passing through the center of the fiber core. The fiber core and the portion outside the air holes are formed as a solid cladding. Polarized light in the direction of the line connecting the two sets of air holes will be attenuated, leaving only polarized light perpendicular to it, thus achieving the effect of polarized light transmission and facilitating the resolution of different polarization information in the object space.

[0016] In one possible implementation, the edges of the claddings of the two photonic crystal fibers are brought into contact by bonding. Compared to methods such as adhesive bonding, bonding allows the claddings of the two fibers to directly contact each other, reducing impurities between the fibers and improving stability. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a schematic cross-sectional view of a close-packed fiber bundle based on photonic crystal fiber according to an embodiment of this application.

[0019] Figure 2 This is a schematic cross-sectional view of a single photonic crystal fiber according to the first embodiment of this application.

[0020] Figure 3 This is a schematic cross-sectional view of a single photonic crystal fiber according to the second embodiment of this application.

[0021] Figure 4 This is a three-dimensional structural diagram of a close-packed fiber bundle based on photonic crystal fiber according to an embodiment of this application.

[0022] Figure 5 This is a schematic diagram of the energy transmission system of a divergent array light source, which uses a common optical fiber bundle.

[0023] Figure 6 This is a schematic diagram of an energy transmission system for a divergent array light source, which utilizes a close-packed fiber bundle based on photonic crystal fiber according to an embodiment of this application.

[0024] Figure 7 This is a schematic diagram of a light transmission system based on a close-packed fiber bundle of photonic crystal fiber as an image transmission bundle according to an embodiment of this application.

[0025] Explanation of reference numerals in the attached figures:

[0026] 10-photonic crystal fiber

[0027] 101 Core; 102 Cladding; 103 Air pores;

[0028] 110 non-edge photonic crystal fiber; 120 edge photonic crystal fiber;

[0029] 20 fiber bundles;

[0030] 201 Protective Case

[0031] 202 Connector

[0032] 31. Surface light source; 32. Ordinary optical fiber bundle; 33. Collimating lens group; 34. Area array detector;

[0033] 41 Telescopic lens; 42 Imaging lens; 43 Image sensor; 44 Object to be imaged. Detailed Implementation

[0034] The embodiments of this application are described below with reference to the accompanying drawings.

[0035] For ease of understanding, the relevant technical terms involved in the embodiments of this application will be explained and described below.

[0036] It should be understood that the described embodiments are merely some, not all, of the embodiments in this application. All other embodiments obtained by those skilled in the art based on the embodiments in this application without inventive effort are within the scope of protection of this application.

[0037] The terminology used in the embodiments of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application. The singular forms “a,” “the,” and “the” used in the embodiments of this application and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise.

[0038] It should be understood that the term "and / or" used in this document is merely a description of the same field in the related objects, indicating that three relationships can exist. For example, A and / or B can represent: A alone, A and B simultaneously, and B alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0039] Depending on the context, the word "if" as used here can be interpreted as "when," "when," "in response to determination," or "in response to detection." Similarly, depending on the context, the phrase "if determination" or "if detection (of the stated condition or event)" can be interpreted as "when determination," "in response to determination," "when detection (of the stated condition or event)," or "in response to detection (of the stated condition or event)."

[0040] It should be understood that the terms "first," "second," etc., used in this application are for distinguishing purposes only and should not be construed as indicating or implying relative importance or order.

[0041] In the description of this application, the terms “center,” “upper,” “lower,” “front,” “rear,” “left,” “right,” “vertical,” “horizontal,” “top,” “bottom,” “inner,” and “outer,” etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0042] The phrase "within the range" used in this application, unless otherwise specified, includes both endpoints of the range by default. For example, in the range of 1 to 5, it includes the values ​​1 and 5.

[0043] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "installation", "connection" and "joining" should be interpreted broadly, for example, they can be fixed connections, detachable connections, mating connections or integral connections; those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances.

[0044] Photonic Crystal Fiber (PCF): It is usually composed of pores arranged in different ways. The size of these pores is roughly on the same order of magnitude as the wavelength of light and runs through the entire length of the fiber. This allows for a complex refractive index distribution on the cross-section of the fiber to achieve light guiding effects for different purposes.

[0045] Polarization-maintaining fiber: Light exists in different polarization states. In order to keep the polarization state of light from changing during transmission in the optical fiber, polarization-maintaining fiber uses a special stress structure to ensure that the light has only one polarization state in the fiber, thereby achieving polarization-maintaining transmission of light.

[0046] Image bundle: An image bundle is a common application of fiber optic bundles, often used for image transmission. It acquires the optical signal of the object under test (DUT) through active emission or passive emission and transmits the acquired signal to a display screen at the other end, thus obtaining the image information of the DUT.

[0047] This application provides a close-packed fiber bundle (hereinafter, sometimes simply referred to as "fiber bundle") based on photonic crystal fiber, which, as an example, can be used in the energy transmission system of a divergent array light source, as an image bundle in an image transmission system, or in the field of lighting (detailed below).

[0048] See Figure 1 , Figure 2The fiber bundle 20 may include a plurality of photonic crystal fibers 10 with a polygonal cross-section. The photonic crystal fiber 10 may include a core 101 and a cladding 102 covering the outside of the core 101. A plurality of air holes 103 may also be provided in the cladding 102, passing through the cladding 102 along the axial direction of the cladding 102.

[0049] The air holes 103 in the photonic crystal fiber 10 can significantly reduce the equivalent refractive index of the cladding 102, thereby greatly improving the NA (reflection angle) of the fiber. For example, the NA of traditional silica fiber is typically limited to a maximum of 0.28, and if the cladding is made of plastic, it can be reduced to around 0.40. The NA of the photonic crystal fiber 10 of this application can reach 0.80 or even higher. The larger the NA of the fiber, the larger its reception angle, thus enabling it to accept light rays incident at larger angles. Therefore, for divergent surface light sources, a larger NA means a larger amount of light received and higher transmission efficiency. Therefore, the photonic crystal fiber 10 provided in this application can significantly improve transmission efficiency.

[0050] On the cross-section of the fiber bundle 20, at least one side of the cladding 102 of one photonic crystal fiber 10 is in contact with one side of the cladding 102 of another photonic crystal fiber 10, so that multiple photonic crystal fibers 10 are closely packed to form a fiber bundle 20.

[0051] Traditional circular cross-section optical fibers inevitably have gaps when densely packed, and these gaps require adhesive treatment to prevent contamination. This application designs the cladding of photonic crystal fibers as polygons, allowing direct contact between the fibers through the edges of the cladding. This facilitates seamless laying and dense packing, increases the duty cycle, and thus improves transmission efficiency. For example, with fiber bundles of the same or similar cross-sectional area, a fiber bundle with a higher duty cycle contains more fibers, and correspondingly, more fiber cores are used to transmit optical signals, thus improving transmission efficiency. Furthermore, reducing gaps in the fiber bundle improves its structural stability.

[0052] Specific examples of close-packing are disclosed later in this application. However, it should be understood that setting the cladding as a polygon and having the claddings of the two fibers in direct contact already achieves the technical effect of improving structural stability and space utilization, thus realizing close-packing. This application does not limit the specific close-packing method.

[0053] In one implementation, see Figure 1In this arrangement, at least a portion of the photonic crystal fibers 10 have all their edges contacted and covered by the edges of other photonic crystal fibers 10, resulting in no gaps between adjacent photonic crystal fibers 10. Exemplarily, the photonic crystal fibers 10 may include non-edge photonic crystal fibers 110 and edge photonic crystal fibers 120. In the cross-section of the fiber bundle 20, each edge of the cladding 102 of the non-edge photonic crystal fiber 110 contacts and covers one edge of the cladding 102 of another photonic crystal fiber 10. The cladding 102 of the edge photonic crystal fiber 120 includes edges that do not contact the cladding 102 of other photonic crystal fibers 10. Coverage can be understood as the edges of the two fibers coinciding.

[0054] That is, the non-edge photonic crystal fiber 110 can be distributed in the center of the fiber bundle 20, and the edge photonic crystal fiber 120 can be distributed in the periphery of the fiber bundle 20, so that the photonic crystal fibers 10 in the fiber bundle 20 are roughly stacked together around the axis of the fiber bundle 20, the fibers are more concentrated, and there can be no gaps between the fibers.

[0055] In one embodiment, the multiple photonic crystal fibers 10 have the same cross-section and are all regular polygons. For example, they can be regular hexagons, regular squares, equilateral triangles, etc. When multiple photonic crystal fibers 10 are the same, only one type of photonic crystal fiber 10 needs to be prepared, and the close-packing process is simpler.

[0056] Of course, the fiber bundle 20 can also include various optical fibers with inconsistent cross-sections. For example, it can include two types of photonic crystal fibers 10 with cross-sections of regular octagons and regular quadrilaterals (not shown in the figure), which are interleaved and arranged in a gapless close packing. The fiber bundle 20 can also include optical fibers with cross-sections that are not regular polygons. For example, the fiber bundle 20 includes photonic crystal fibers 10 with cross-sections of isosceles right triangles, which can also be arranged in a gapless close packing.

[0057] In one implementation, see Figure 1 , Figure 2 The cross-section of the photonic crystal fiber 10 can be a regular hexagon. The regular hexagonal structure is not prone to slippage or misalignment, has structural stability, and is highly practical.

[0058] Accordingly, the cross-section of the photonic crystal fiber 10 can also be a square or an equilateral triangle (not shown in the figure). It should be understood that squares and equilateral triangles can also be seamlessly packed together, improving the space utilization of the fiber bundle.

[0059] In one embodiment, when the cross-section of the photonic crystal fiber 10 is a regular hexagon, the cross-section of the core 101 is also formed as a regular hexagon (not shown in the figure). Having both the core 101 and cladding 102 in regular hexagonal shapes is advantageous for achieving a higher core duty cycle (cross-sectional area of ​​the core / cross-sectional area of ​​the fiber). It should be understood that the minimum producible thickness of the cladding 102 is fixed, and gaps will occur between fibers with circular cross-sections. Therefore, given the same cross-sectional area of ​​the fiber bundle and an infinitely close packing of fibers, the core duty cycle when both the core 101 and cladding 102 are regular hexagonal is necessarily higher than the core duty cycle when both the core 101 and cladding 102 are circular.

[0060] Of course, when the cross-section of the photonic crystal fiber 10 is a square or an equilateral triangle, the cross-section of the fiber core 101 can be the same as the cross-section of the photonic crystal fiber 10 (also a square or an equilateral triangle, not shown in the figure) to improve the fiber core duty cycle.

[0061] Of course, the cross-section of the fiber core 101 can also be a traditional circle. The process of making the fiber core 101 with a circular cross-section is mature and the cost is low.

[0062] In one implementation, see Figure 2 Multiple air holes 103 are arranged in an axial array around the fiber core 101. As mentioned earlier, the air holes 103 can significantly reduce the equivalent refractive index of the cladding 102, thereby greatly improving the optical fiber's NA (Negative Refractive Index). The air holes 103 can be arranged in multiple turns, regularly on the outer side of the fiber core 101. This arrangement method is simple in process and low in cost. Of course, this application does not limit its specific spacing parameters. The size (diameter) of the air holes 103 can be on the same order of magnitude or close to the wavelength of the light transmitted by the fiber bundle 120.

[0063] In one implementation, see Figure 3 Multiple air holes 103 are formed into two sets of air holes 103 located on both sides of the fiber core 101, and the center line connecting the two sets of air holes 103 passes through the center of the fiber core 101. The remaining part outside the fiber core 101 and the air holes 103 is a solid cladding 102. At the same time, the cross-section of the photonic crystal fiber 10 can be a regular hexagon or a square.

[0064] Similar to polarization-maintaining fiber, in this embodiment, a set of air holes 103 are concentrated in one region, causing a significant decrease in the equivalent refractive index of that region, forming a low-refractive-index region, thereby confining light to propagate in the high-refractive-index region. At this time, the polarized light in the direction of the line connecting the two sets of air holes 103 will be attenuated, leaving only the polarized light in the direction perpendicular to it, achieving the effect of polarized light transmission, thus facilitating the resolution of different polarization information in the object space.

[0065] In one embodiment, the edges of the cladding 102 of two photonic crystal fibers 10 are brought into contact by bonding. Exemplarily, the cladding 102 can be molten at a high temperature, where the molecular bonds of the cladding 102 of adjacent photonic crystal fibers 10 will attract each other and bond together. Compared to methods such as adhesive bonding, high-temperature bonding allows the cladding 102 of the two fibers to directly contact each other, reducing impurities between the fibers and improving stability.

[0066] In one implementation, see Figure 4 A protective sleeve 201 can be installed on the outside of the fiber optic bundle 20. The protective sleeve 201 can be a plastic protective sleeve or a metal protective sleeve. Connectors 202 can be installed at both ends of the fiber optic bundle 20 to match different usage scenarios. The connector 202 may include structural components, optical elements, flanges, etc.

[0067] In one embodiment, both the core 101 and the cladding 102 can be made of quartz material. Quartz material has suitable stiffness, which can maintain the structural stability of the air holes 103 outside the core 101. Fluorine can be doped into the cladding 102 to reduce its refractive index. For example, the doping concentration of the cladding 102 can be greater than that of the core 102, forming the core 101 and the cladding 102. The doping concentration of the core 101 can be zero, i.e., doped only in the cladding 102. This is further enhanced by the air holes 103 (e.g., with additional fluorine). Figure 2 Air pores distributed in a dispersed manner, or attached Figure 3 Multiple sets of air holes are concentrated in the middle, so that the cladding 102 and the fiber core 101 form a high refractive index difference, thereby increasing the system's receiving angle.

[0068] In one embodiment, the fiber bundle 20 may include 10 to 100,000 photonic crystal fibers 10. Of course, this application does not limit the specific number of photonic crystal fibers 10 in the fiber bundle 20. However, it should be understood that the more photonic crystal fibers 10 there are, the more significant the improvement in NA and transmission efficiency. Furthermore, once the number of photonic crystal fibers 10 in a fiber bundle 20 exceeds 10, the improvement in NA and transmission efficiency becomes even more pronounced with the increase in the number of fibers.

[0069] In one implementation, the diameter of the fiber core can be approximately 5 micrometers to 200 micrometers, and the appropriate value can be determined according to the application scenario.

[0070] The close-packed fiber bundle based on photonic crystal fiber provided in this application can be applied to, for example, energy transmission systems of divergent array light sources.

[0071] See Figure 5Since the divergent surface light source 31 has a large divergence angle, the transmission efficiency of the ordinary optical fiber bundle 32 is low. Therefore, a collimating optical system (such as a collimating lens group 33) is usually set before the ordinary optical fiber bundle 32. The divergent surface array light is collimated by it before entering the ordinary optical fiber bundle 32 for energy transmission, and finally received by the surface array detector 34. The structure is relatively complex.

[0072] See Figure 6 This application replaces the ordinary optical fiber bundle 32 with a close-packed optical fiber bundle based on photonic crystal fiber, which can achieve a large beam receiving angle, thereby enabling direct energy transmission to the divergent surface light source 31 without the need for a specially designed collimating optical system. The structure is simpler, the NA is higher, and the transmission efficiency is higher.

[0073] It should be understood that Figure 5 The diagram below shows the cross-section of a conventional optical fiber bundle 32, with gaps between the fibers. Figure 6 In the present application, the photonic crystal fiber bundle 20 also has a schematic diagram of its cross-section below. The photonic crystal fiber bundle 20 is formed by multiple photonic crystal fibers 10 closely packed together, with no gaps between each photonic crystal fiber 10.

[0074] The close-packed fiber bundle based on photonic crystal fiber provided in this application can also be used as, for example, image transmission bundles for applications such as medical endoscopes, high-precision industrial visual monitoring, and high-precision industrial position measurement.

[0075] See Figure 7 One end of the fiber optic bundle 20 can be equipped with a first optical device (e.g., a telephoto lens 41), and the other end can be equipped with a second optical device (e.g., an imaging lens 42 and an image sensor 43). Light emitted or reflected by the object 44 to be imaged passes sequentially through the telephoto lens 41, the fiber optic bundle 20, and the imaging lens 42 before entering the image sensor 43. The image sensor 43 converts the light signal into an electrical signal, which is then transmitted to a display screen or other devices, enabling long-distance image transmission. For example, the image sensor 43 can be a CCD (charge-coupled device) sensor.

[0076] like Figure 7 As shown by the dashed line, the telescope lens 41 and one end of the fiber bundle 20 can be formed as an integral structure, and the imaging lens 42, the image sensor 43 and the other end of the fiber bundle 20 can be formed as an integral structure.

[0077] Of course, the close-packed fiber bundle based on photonic crystal fiber provided in this application can also be used in the lighting field. This application does not limit the specific application areas of the close-packed fiber bundle based on photonic crystal fiber.

[0078] The above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application, and should all be included within the protection scope of this application.

Claims

1. A close-packed fiber bundle based on photonic crystal fiber, characterized in that, The fiber bundle includes multiple photonic crystal fibers with polygonal cross-sections. Each photonic crystal fiber includes a core and a cladding covering the outside of the core. The cladding also has multiple air holes that pass through the cladding along its axial direction. In the cross-section of the fiber bundle, at least one side of the cladding of one photonic crystal fiber is in contact with one side of the cladding of another photonic crystal fiber, such that the multiple photonic crystal fibers are closely packed together to form the fiber bundle.

2. The close-packed fiber bundle based on photonic crystal fiber according to claim 1, characterized in that, At least a portion of the photonic crystal fibers have all their edges contacted and covered by the edges of the other photonic crystal fibers, so that there are no gaps between adjacent photonic crystal fibers.

3. The close-packed fiber bundle based on photonic crystal fiber according to claim 1 or 2, characterized in that, The multiple photonic crystal fibers have the same cross-section, and all of them are regular polygons.

4. The close-packed fiber bundle based on photonic crystal fiber according to any one of claims 1 to 3, characterized in that, The cross-section of the photonic crystal fiber is a regular hexagon.

5. The close-packed fiber bundle based on photonic crystal fiber according to any one of claims 1 to 3, characterized in that, The cross-section of the photonic crystal fiber is square or equilateral triangle.

6. The close-packed fiber bundle based on photonic crystal fiber according to any one of claims 1 to 5, characterized in that, The cross-sectional shape of the fiber core is the same as that of the photonic crystal fiber.

7. The close-packed fiber bundle based on photonic crystal fiber according to any one of claims 1 to 5, characterized in that, The cross-section of the fiber core is circular.

8. The close-packed fiber bundle based on photonic crystal fiber according to any one of claims 1 to 7, characterized in that, The number of photonic crystal fibers in the fiber bundle is not less than 10.

9. The close-packed fiber bundle based on photonic crystal fiber according to any one of claims 1 to 8, characterized in that, The plurality of air holes are formed as two sets of air holes located on both sides of the fiber core, the center line connecting the two sets of air holes passes through the center of the fiber core, and the portion of the fiber core and the air holes is formed as a solid cladding.

10. The close-packed fiber bundle based on photonic crystal fiber according to any one of claims 1 to 9, characterized in that, The edges of the claddings of the two photonic crystal fibers are in contact by bonding.

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