Multi-mode hollow-core anti-resonance optical fiber

Abstract

The invention provides a multi-mode hollow-core anti-resonance optical fiber, and relates to the technical field of optical fibers, and the multi-mode hollow-core anti-resonance optical fiber comprises a hollow outer layer tube and a plurality of cladding capillary tubes arranged on the inner wall of the outer layer tube; each cladding capillary tube comprises a circular tube, an elliptical tube and an anti-resonance plate, and the elliptical tube is nested in the circular tube through the anti-resonance plate and is coaxially arranged with the circular tube. In the technical scheme provided by the invention, the elliptical tube is nested in the circular tube by the anti-resonance plate, and the elliptical tube and the circular tube are coaxially arranged, so that a nested integrated cladding capillary tube is formed, and the multi-mode hollow-core anti-resonance optical fiber with the structure can reduce bending loss and limiting loss of the fiber core in a high-order mode.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fibers, and in particular to a multi-mode hollow anti-resonant optical fiber. Background Art

[0002] Multi-mode optical fibers have important application values in fields such as high-power laser transmission, short-distance data communication, and sensing. However, in terms of high-power laser transmission, traditional multi-mode solid-core optical fibers with large core diameters and large mode field areas are limited by the inherent nonlinear effects and damage thresholds of quartz materials themselves. Therefore, there are certain limitations on the laser power that can be transmitted. Compared with traditional solid optical fibers, hollow optical fibers guide light in air and have unique advantages such as higher laser damage thresholds, ultra-low nonlinear effects, and lower sensitivity to the environment. According to the light guiding principle, hollow optical fibers are mainly divided into photonic bandgap hollow-core fibers (PBG-HCF) and anti-resonant hollow-core fibers (AR-HCF). Different from photonic bandgap hollow-core fibers, the light guiding principle of anti-resonant hollow-core fibers can be explained by combining the anti-resonant reflecting optical waveguide (ARROW) and the suppressed mode coupling model. Its cladding structure is simpler, it has a wider transmission bandwidth, and has the potential to achieve ultra-low loss.

[0003] Currently, the design of hollow anti-resonant optical fibers mainly supports multi-mode transmission by increasing the core radius. However, too large a core size often brings more sensitive bending losses, and the confinement losses in the high-order modes of the core are also large. Summary of the Invention

[0004] The present invention is completed in order to at least partially solve the technical problems that the current multi-mode hollow anti-resonant optical fiber has large bending losses and large confinement losses in the high-order modes of the core.

[0005] The present invention provides a multi-mode hollow anti-resonant optical fiber, including: a hollow outer tube, and a plurality of cladding capillaries arranged on the inner wall of the outer tube; each cladding capillary includes a circular tube, an elliptical tube, and an anti-resonant plate, and the elliptical tube is nested inside the circular tube through the anti-resonant plate and is coaxially arranged with the circular tube.

[0006] Optionally, the plurality of cladding capillaries are spaced apart from each other and uniformly arranged on the inner wall of the outer tube, and the outer wall of each cladding capillary is in contact with and fixedly connected to the inner wall of the outer tube; the region surrounded by the plurality of cladding capillaries is the core region.

[0007] Optionally, the hollow regions between the inner wall of the outer tube and the outer walls of the respective cladding capillaries, and the hollow regions inside the respective cladding capillaries are all filled with air; and / or, the number of the cladding capillaries is 5 to 8.

[0008] Optionally, the circumference of the core region is tangent to the respective cladding capillaries; the radius corresponding to the circumference of the core region is the core radius, and the core radius R is 18 μm to 22 μm.

[0009] Optionally, in the cross-sectional direction of each cladding capillary, the anti-resonant plate coincides with the radial direction of the circular tube and the major axis of the elliptical tube respectively, and both ends of the anti-resonant plate are connected to the inner wall of the circular tube; the radial direction of the outer tube is perpendicular to the anti-resonant plate in each cladding capillary.

[0010] Optionally, the circular tube radius r of each cladding capillary is obtained by the following formula:

[0011]

[0012] where, r is the circular tube radius, R is the core radius, N is the number of cladding capillaries, and g is the tube gap between two adjacent cladding capillaries;

[0013] The wall thicknesses of the circular tube, the elliptical tube and the anti-resonant plate in each cladding capillary are equal, and the wall thickness t is obtained by the following formula:

[0014]

[0015] where, t is the wall thickness, m is the anti-resonant order, λ is the wavelength of light, and n is the refractive index of the cladding capillary.

[0016] Optionally, the value of the tube gap g between two adjacent cladding capillaries is λ ≤ g ≤ 4λ.

[0017] Optionally, in each cladding capillary, the anti-resonant plate cuts the circular tube into two parts, and the region surrounded by each part of the circular tube and the anti-resonant plate is the first region. The anti-resonant plate cuts the elliptical tube into two parts, and the region surrounded by each part of the elliptical tube and the anti-resonant plate is the second region; by adjusting the major semi-axis and the minor semi-axis of the elliptical tube, the shape and thickness of the first region, and the shape and thickness of the second region are adjusted, so that the cladding mode and the core high-order mode are phase mismatched, and the coupling between the core high-order mode and the cladding mode is suppressed.

[0018] Optionally, the length of the major semi-axis a of the elliptical tube is 12 μm to 14 μm, and the length of the minor semi-axis b of the elliptical tube is 7 μm to 11 μm.

[0019] Optionally, when the anti-resonant order m is 1 to 5, the wall thickness t of the circular tube, elliptical tube, and anti-resonant plate in each cladding capillary is 0.71 μm to 2.14 μm.

[0020] The technical solution provided by the present invention may include the following beneficial effects:

[0021] The multi-mode hollow anti-resonant optical fiber provided by the present invention nests the elliptical tube inside the circular tube through the anti-resonant plate and makes the elliptical tube and the circular tube coaxially arranged, forming a nested and connected cladding capillary. The multi-mode hollow anti-resonant optical fiber with this structure can reduce the bending loss and the confinement loss in the high-order modes of the core.

[0022] Other features and advantages of the present invention will be described in the following specification, and, in part, will be obvious from the specification, or will be understood by implementing the present invention. The objectives and other advantages of the present invention can be realized and obtained by the structures specifically pointed out in the specification, claims, and drawings. Description of the Drawings

[0023] The drawings are used to provide a further understanding of the technical solution of the present invention, and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the technical solution of the present invention, and do not constitute a limitation to the technical solution of the present invention.

[0024] Figure 1 It is a cross-sectional schematic diagram of the multi-mode hollow anti-resonant optical fiber provided by the embodiment of the present invention;

[0025] Figure 2 It is a modal contour diagram of 12 LP modes of the multi-mode hollow anti-resonant optical fiber provided by the embodiment of the present invention;

[0026] Figure 3a It is a schematic diagram showing the change of the confinement loss CL in 12 LP modes when the tube gap g of the multi-mode hollow anti-resonant optical fiber provided by the embodiment of the present invention changes;

[0027] Figure 3b It is a schematic diagram showing the change of the effective mode index n in 12 LP modes when the tube gap g of the multi-mode hollow anti-resonant optical fiber provided by the embodiment of the present invention changes; eff ;

[0028] Figure 4a It is a schematic diagram showing the change of the confinement loss CL in 12 LP modes when the short semi-axis b of the elliptical tube in the cladding capillary of the multi-mode hollow anti-resonant optical fiber provided by the embodiment of the present invention changes;

[0029] Figure 4b It is a schematic diagram showing the change of the effective mode index n in 12 LP modes when the short semi-axis b of the elliptical tube in the cladding capillary of the multi-mode hollow anti-resonant optical fiber provided by the embodiment of the present invention changes; effSchematic diagram of the change situation;

[0030] Figure 5 It is a histogram of the confinement loss CL in 12 LP modes when the cladding capillary of the multimode hollow anti-resonant fiber provided by the embodiment of the present invention adopts different wall thicknesses t.

[0031] In the figure: 10 - outer tube; 20 - cladding capillary; 21 - circular tube; 22 - elliptical tube; 23 - anti-resonant plate; A1 - first region; A2 - second region. Detailed implementation manners

[0032] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the following will describe the detailed implementation manners of the present invention with reference to the accompanying drawings. It should be understood that the detailed implementation manners described herein are only for the purpose of illustration and explanation of the present invention, and are not used to limit the present invention.

[0033] It should be noted that the orientations or positional relationships indicated by various orientation terms are based on the orientations or positional relationships shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus cannot be understood as a limitation to the present invention. In addition, the terms "first", "second", etc. in the description and claims of the present invention are used to distinguish similar objects, and do not necessarily need to describe a specific order or sequence; and, without conflict, the embodiments in the present invention and the features in the embodiments can be arbitrarily combined with each other.

[0034] Those of ordinary skill in the art should understand that the diagrams provided herein are for illustrative purposes only, and the diagrams are not necessarily drawn to scale. When an element is referred to as being "connected" or "coupled" to another element, it can be directly connected or coupled to the other element or there can be intervening elements; the connection can be classified into fixed connection, sliding connection and rotational connection by type. In the case where it is not clearly specified, the connection is generally a fixed connection, and the fixed connection can be a detachable connection or a non-detachable connection.

[0035] To solve the problems that the current multimode hollow anti-resonant fiber has large bending loss and limited low-loss core high-order modes supported, the present invention provides a multimode hollow anti-resonant fiber with nested connected tubes, which can achieve multimode transmission with low confinement loss and low bending loss. The following will be described in detail through specific embodiments.

[0036] As Figure 1As shown in the figure, the multimode hollow anti-resonant fiber includes: a hollow outer tube 10, and a plurality of cladding capillaries 20 disposed on the inner wall of the outer tube 10. Each cladding capillary 20 includes a circular tube 21, an elliptical tube 22, and an anti-resonant plate 23. The elliptical tube 22 is nested inside the circular tube 21 through the anti-resonant plate 23 and is coaxially arranged with the circular tube 21.

[0037] Among them, the outer tube 10 mainly plays a supporting role; and the axis of the outer tube 10 is parallel to the axes of the respective cladding capillaries 20. The elliptical tube 22 can specifically be called a rugby-shaped elliptical tube. The fiber material used for the outer tube 10, the circular tube 21, the elliptical tube 22, and the anti-resonant plate 23 of each cladding capillary 20 is silica glass, which may contain a very small amount of dopants (such as germanium dioxide, phosphorus pentoxide, etc.) to increase the refractive index of the core to light and appropriately reduce the refractive index of the cladding to light.

[0038] In this embodiment, the elliptical tube is nested inside the circular tube through the anti-resonant plate, and the elliptical tube is coaxially arranged with the circular tube, forming a nested and connected cladding capillary. After simulation verification (the content and results of the simulation will be described in detail later), the multimode hollow anti-resonant fiber with this structure can reduce the bending loss and the confinement loss in the high-order modes of the core.

[0039] In a specific embodiment, as Figure 1 shown, a plurality of cladding capillaries 20 are spaced apart from each other and uniformly arranged on the inner wall of the outer tube 10. The outer wall of each cladding capillary 20 is in contact with and fixedly connected to the inner wall of the outer tube 10. The region surrounded by the plurality of cladding capillaries 20 is the core region, which is the main channel for optical signal transmission.

[0040] In this embodiment, the respective cladding capillaries are spaced apart from each other and uniformly arranged. The optical fiber with this structure not only has strong structural stability and is easy to fabricate, but also can achieve low-loss and high-power transmission.

[0041] In a specific embodiment, as Figure 1 shown, the hollow regions between the inner wall of the outer tube 10 and the outer walls of the respective cladding capillaries 20, and the hollow regions inside the respective cladding capillaries 20 are all filled with air.

[0042] In this embodiment, all the hollow regions inside the optical fiber are filled with air. Specifically, the hollow regions between the inner wall of the outer tube 10 and the outer walls of the respective cladding capillaries 20, including the core region, are filled with air; the hollow regions inside each cladding capillary 20 are also filled with air, that is, the hollow region between the inner wall of the circular tube 21 and the outer wall of the elliptical tube 22 is filled with air, and the hollow regions between the inner wall of the elliptical tube 22 and both sides of the anti-resonant plate 23 are filled with air. Filling the hollow regions inside the optical fiber with air has significant advantages such as low time delay, low loss, low non-linear effect, high laser damage threshold, low dispersion, radiation resistance, and ultra-wide transmission bandwidth. These characteristics enable the hollow-core optical fiber to have broad application prospects in the fields of optical communication, high-power laser transmission, fiber sensing, and extreme environment applications.

[0043] In a specific embodiment, the number of the cladding capillaries 20 is 5 to 8. Preferably, the number of the cladding capillaries 20 is 6.

[0044] In this embodiment, the structure with 6 cladding capillaries can better control the light propagation mode, reduce the leakage loss of high-order modes, and improve the purity of the fundamental mode; it also has the advantages of compact structure and strong environmental anti-interference ability, and can work stably in complex environments.

[0045] In a specific embodiment, the circumference of the core region is tangent to the respective cladding capillaries 20. The radius corresponding to the circumference of the core region is the core radius, and the core radius R is 18 μm to 22 μm. Preferably, the core radius R is 20 μm.

[0046] When parameters such as the wavelength λ of light, the tube wall thickness t, and the refractive index n remain unchanged, the lowest limiting loss of the core mode is inversely proportional to the core radius R 4 which means that the limiting loss of the core mode can be reduced by increasing the size of the core to achieve low-loss multimode transmission. However, a larger core size often brings greater bending loss, which is not conducive to the flexible transmission of high-power lasers. To solve this problem, the present invention limits the core radius R within the range of 18 μm to 22 μm, preferably 20 μm. Through simulation and verification, the core radius of this size, combined with the cladding capillaries of the aforementioned structure, can achieve low-loss transmission of the core high-order modes.

[0047] In a specific embodiment, in the cross-sectional direction of each cladding capillary 20, the anti-resonant plate 23 coincides with the radial direction of the circular tube 21 and the major axis of the elliptical tube 22 respectively, and both ends of the anti-resonant plate 23 are connected to the inner wall of the circular tube 21; the radial direction of the outer tube 10 is perpendicular to the anti-resonant plate 23 in each cladding capillary 20.

[0048] In this embodiment, the elliptical tube 22 is nested in the center of the circular tube 21 through the anti-resonant plate 23 passing through the center of the circular tube 21, forming a central nested and connected structure, so that the structure of multiple cladding capillaries in the outer tube is designed as a centrally symmetric structure, forming a multi-mode hollow anti-resonant optical fiber with a centrally symmetric nested and connected tube, which is more conducive to realizing multi-mode transmission with low confinement loss and low bending loss.

[0049] In a specific embodiment, the radius r of the circular tube of each cladding capillary 20 is obtained by the following formula (1):

[0050]

[0051] where r is the radius of the circular tube, R is the core radius, N is the number of cladding capillaries, N can take 5 to 8, and g is the tube gap between two adjacent cladding capillaries. The above formula (1) gives the relationship between the core radius R, the radius r of the circular tube and the tube spacing g.

[0052] The wall thicknesses of the circular tube 21, the elliptical tube 22 and the anti-resonant plate 23 in each cladding capillary 20 are equal, and the wall thickness t is obtained by the following formula (2):

[0053]

[0054] where t is the wall thickness, m is the anti-resonant order, λ is the wavelength of light, and n is the refractive index of the cladding capillary. When the wavelength λ of light is 1 μm and the material of the cladding capillary is silica glass, the refractive index n of the cladding capillary = 1.45, and the refractive index of the air region is assumed to be 1. The above formula (2) is an anti-resonant relational expression for the wall thickness t of the cladding capillary according to the anti-resonant reflection optical waveguide model.

[0055] Currently, the research on hollow anti-resonant optical fibers in the industry mainly focuses on their single-mode transmission performance. Researchers are committed to realizing low-loss transmission of the fundamental mode, which is a pseudo-single-mode transmission. The higher-order modes are not cutoff, but a larger higher-order mode suppression ratio is obtained by adjusting the fiber structure. The higher-order modes with high transmission loss decay rapidly during transmission, and then single-mode transmission is realized, which means that the hollow optical fiber itself supports multi-mode transmission.

[0056] In practical applications, in addition to the more sensitive bending loss caused by increasing the core radius to support multi-mode transmission, the effective mode index of the higher-order modes in the core of the multi-mode hollow anti-resonant optical fiber often decreases as the order of the mode increases. The higher-order modes in the core are more likely to form phase matching with the cladding modes and are more likely to resonate and couple with the cladding modes. Therefore, the differential loss between modes of the multi-mode hollow anti-resonant optical fiber is still large, and the number of low-loss higher-order modes in the core that can be supported is limited.

[0057] To solve the above problems, based on the suppression mode coupling theory and the modified Marcatili-Schmeltzer mode index expression, the present invention designs the structural parameters of the multimode hollow anti-resonant fiber described in the present invention. Specifically, it designs the tube gap g between two adjacent cladding capillaries, the major semi-axis a and minor semi-axis b of the elliptical tube, and the wall thickness t of the circular tube, elliptical tube and anti-resonant plate in each cladding capillary. Through simulation verification, the designed structural parameters can weaken the coupling between the core high-order mode and the cladding mode, effectively reduce the confinement loss under the core high-order mode, and achieve multimode transmission with low confinement loss and low bending loss. The following is a specific analysis respectively:

[0058] The present invention uses the Full-Vector Finite Element Method (FV-FEM) to simulate and analyze the designed multimode hollow anti-resonant fiber. The full-vector finite element method is a numerical analysis method for solving electromagnetic field problems. It discretizes complex electromagnetic field problems into multiple small elements and approximately solves them on each element to obtain an approximate solution to the entire problem. This method is particularly suitable for dealing with electromagnetic field problems with complex geometries and boundary conditions.

[0059] To make the simulation results more accurate, it is necessary to carefully select the mesh accuracy. The mesh accuracy in finite element method simulation refers to the ability to simulate a real physical system through mesh division. Its core lies in the fineness of the mesh and the choice of element size, which directly affects the accuracy and reliability of the simulation results. According to experience, the maximum element size of the mesh in the air region can be set to λ / 4, and the maximum element size of the mesh in the fused silica region can be set to λ / 6.

[0060] In addition, it is also necessary to use the Perfect Matched Layer (PML) in combination with the standard cylindrical coordinates to enclose the simulation area, so that the confinement loss of the core mode can be obtained from the imaginary part of the effective mode index of the core mode obtained by the solution. The perfect matched layer is a technique used in numerical simulation to absorb electromagnetic waves at the open boundary. By setting a special dielectric layer at the boundary of the calculation area, the electromagnetic characteristics of this dielectric layer are carefully designed so that the wave incident on the boundary can propagate without reflection in the layer. In a finite-sized calculation area, the PML can simulate the effect that the wave is absorbed without reflection when it propagates to the boundary.

[0061] Due to the fact that the anti-resonant reflection can greatly reduce the overlap between the core mode field and the core boundary, the electric field gradient at the silica glass-air interface is very small, resulting in a very small difference in the mode indices between degenerate vector modes. Therefore, in the following, the present invention will use the LP (Linear Polarization) mode representation method to represent different core modes. In optical fiber communication, the LP mode is also known as the linear polarization mode, which is a classification method for describing the optical propagation modes in optical fibers. The LP mode is represented by the subscripts l and m, where l is the azimuthal mode number, indicating the number of oscillations of the mode in the circumferential direction, and m is the radial mode number, indicating the number of oscillations of the mode in the radial direction. According to the orders of these modes, they can be divided into low-order modes and high-order modes. The low-order modes can include modes such as LP01, LP11, and LP02, and the high-order modes can include modes such as LP21, LP31, LP12, LP22, LP03, LP32, LP41, and LP51.

[0062] Figure 2 Shown are the modal profiles of 12 spatially different core modes supported by the multimode hollow anti-resonant fiber designed by the present invention, namely LP01, LP11, LP21, LP02, LP31a, LP31b, LP12, LP41, LP22, LP03, LP51, and LP32. Among them, the LP31 mode is divided into the LP31a mode and the LP31b mode because the simulation results show that the difference in the effective mode indices between these two groups of modes is always greater than the critical value (1×10 -4 ) for vector mode decoupling. Therefore, it can be divided into two different spatial modes.

[0063] 1) Influence of the tube gap g:

[0064] The tube gap g between any two adjacent cladding capillaries is an important structural parameter in the design of hollow anti-resonant optical fibers. If the tube gap is too large, light cannot be well confined, resulting in excessive leakage of most of the light energy through the tube gap and excessive confinement loss of the core mode. If the tube gap is too small, it is difficult to ensure the tube spacing during the manufacturing process, resulting in the easy adhesion of two adjacent cladding capillaries to form nodes during the melting process of the capillaries. The Fano resonance caused by the nodes will cause a sharp increase in the confinement loss. Among them, the Fano resonance is a scattering resonance phenomenon that produces an asymmetric line shape. Due to the interaction between discrete states (such as bound states or resonant states) and continuous states (such as free states or background states), an asymmetric line shape will appear in the scattering spectrum or extinction spectrum.

[0065] In order to obtain the influence of tube gaps g of different sizes on the performance of optical fibers, when simulating multimode hollow anti-resonant optical fibers, the anti-resonant order m = 2, the wavelength of light λ = 1 μm, and the refractive index of the cladding capillary n = 1.45 are set. Then, according to the aforementioned formula (2), the wall thickness t = 0.71 μm can be obtained. At the same time, b = r / 2 and b / a = 0.6 are set, and a parametric scan of the tube gap g is performed. When the tube gap g changes, the variation of the confinement loss CL (Confinement Loss) of 12 core modes is calculated (as Figure 3a shown) and the variation of the effective mode index n eff (as Figure 3b shown), where Figure 3b also shows the variation of the effective mode index n eff of a cladding mode. The effective mode index n eff is a quantity that quantitatively describes the phase delay per unit length in a waveguide and is used to describe the equivalent refractive index when light propagates in a waveguide (such as an optical fiber).

[0066] As Figure 3a and Figure 3b shown, as the tube gap g increases, Figure 2 for the 12 LP modes in

[0067]

[0068]

[0069]

[0070] 2) Influence of the short semi-axis b of the elliptical tube:

[0071] Since the mode index in a single-hole thick-walled capillary can be approximately expressed by a modified Marcatili-Schmeltzer expression, the mode index in a single-hole thick-walled capillary is obtained using the following formula (3):

[0071]

[0072] where n lm is the mode index in a single-hole thick-walled capillary, u lm is the m-th root of the Bessel function of the first kind J l , λ is the wavelength of light, f is the fitting parameter of the equation, and d i is the inner diameter of capillary i.

[0073] According to formula (3), the effective mode index of the core mode is related to the air thickness inside the cladding capillary. Therefore, the shapes of the first region A1 and the second region A2 of the cladding (both the first region A1 and the second region A2 are filled with air), as well as the thickness d1 of the first region A1 and the thickness d2 of the second region A2, can be adjusted by adjusting the lengths of the major semi-axis a and the minor semi-axis b of the elliptical tube, so that the cladding mode and the core high-order mode are phase mismatched, the coupling between the core high-order mode and the cladding mode is suppressed, and multi-mode low-loss transmission is achieved. Figure 1 To obtain the influence of the minor semi-axis b of the elliptical tube with different lengths on the fiber performance, when simulating and analyzing the multi-mode hollow anti-resonant fiber, the major semi-axis a of the elliptical tube is set to 13 μm, and other structural parameters are fixed. The minor semi-axis b of the elliptical tube is parametrically scanned, and when the minor semi-axis b changes, the changes in the confinement loss CL of 12 core modes (as shown in

[0074] ) and the changes in the effective mode index n Figure 4a eff (as shown in Figure 4b ) are calculated, where Figure 4b also shows the changes in the effective mode index n eff of the three main cladding modes Cladding1, Cladding2, and Cladding3.

[0075]

[0075] As shown in Figure 4a and Figure 4b , among the 12 LP modes in Figure 2 , the confinement losses of the first 8 LP modes (i.e., the mode groups LP01 - LP41) show low sensitivity to the change of the minor semi-axis b of the elliptical tube, mostly remaining below 0.1 dB / km. However, as the minor semi-axis b of the elliptical tube changes, the confinement losses of the last 4 LP modes (i.e., the mode groups LP22 - LP32) fluctuate violently. In particular, near b = 7 μm and near b = 11 μm, the last 4 LP modes successively exhibit loss peaks.

[0076] Moreover, as the minor semi-axis b of the elliptical tube increases, the thickness of the first region A1 where the cladding mode Cladding1 and the cladding mode Cladding2 are located decreases, while the thickness of the second region A2 where the cladding mode Cladding3 is located increases.

[0077] The cladding mode refers to the mode in which light propagates in the fiber cladding. Its intensity distribution is mainly concentrated in the cladding rather than the core. The existence of the cladding mode is due to the structural characteristics of the fiber, which allows some light to propagate in the cladding.

[0078] The cladding modes can be classified into the following types:

[0079] (1) Cladding1 (cladding propagation mode)

[0080] The cladding propagation mode refers to the mode in which light propagates in the cladding without leaking to the outside. The optical field of this mode is mainly concentrated in the cladding, and the propagation distance is relatively long, but usually has a larger propagation loss than the core mode.

[0081] (2) Cladding2 (cladding leakage mode)

[0082] The cladding leakage mode refers to the mode in which part of the energy leaks out of the cladding when light propagates in the cladding. The optical field of this mode gradually attenuates when propagating in the cladding and may eventually leak completely to the outside.

[0083] (3) Cladding3 (cladding radiation mode)

[0084] The cladding radiation mode refers to the mode in which part of the optical field extends beyond the fiber boundary into the cladding and finally radiates outward when light propagates in the fiber. The optical field distribution of this mode is different from the fundamental mode in the core and usually attenuates relatively quickly due to light leakage.

[0085] From Figure 4b it can be seen that, consistent with the modified Marcatili-Schmeltzer expression, as the thickness of the first region A1 decreases, the effective mode indices n of the cladding modes Cladding1 and Cladding2 existing in the first region A1 eff decrease. At the same time, as the short semi-axis b of the elliptical tube increases, the effective mode index n of the cladding mode Cladding3 in the second region A2 eff increases, and the main mode in the cladding capillary transitions from the cladding modes Cladding1 and Cladding2 to the cladding mode Clading3.

[0086] When b = 7 μm, the effective mode indices n of the latter 4 LP modes (i.e., the mode group LP22 - LP32) eff are close to the effective mode index n of the cladding mode Cladding1. And when b = 11 μm, the effective mode indices n of the latter 4 LP modes (i.e., the mode group LP22 - LP32) eff eff are close to the effective mode index n of the cladding mode Cladding3. eff ​Nearby, there is a high phase matching. According to the suppression coupling theory, the strong coupling between the latter 4 LP modes (i.e., the mode group LP22 - LP32) and the cladding modes Cladding1 and Cladding3 leads to a sharp increase in their confinement losses. Inspired by this, when designing a multimode hollow anti-resonant fiber, the thickness of different regions in the cladding capillary can be reasonably allocated to increase the phase mismatch between the core high-order modes and the cladding modes, thereby suppressing the coupling between the cladding modes and the core high-order modes.

[0087] The simulation results show that when other structural parameters are determined, by adjusting the major semi-axis a and the minor semi-axis b of the elliptical tube, specifically a = 13μm and 7μm ≤ b ≤ 11μm, the shapes and thicknesses d1 and d2 of the first region A1 and the second region A2 of the cladding are adjusted to make the phase mismatch between the cladding modes and the core high-order modes, suppressing the coupling between the core high-order modes and the cladding modes, and enabling the multimode light of 1μm to be well transmitted in the multimode hollow anti-resonant fiber, achieving multimode low-loss transmission.

[0088] In a specific embodiment, as Figure 1 shown, in each cladding capillary 20, the anti-resonant plate 23 cuts the circular tube 21 into two parts, and the region surrounded by each part of the circular tube and the anti-resonant plate 23 is the first region A1. The anti-resonant plate 23 cuts the elliptical tube 22 into two parts, and the region surrounded by each part of the elliptical tube and the anti-resonant plate 23 is the second region A2. By adjusting the major semi-axis and the minor semi-axis of the elliptical tube 22, the shape and thickness of the first region A1, as well as the shape and thickness of the second region A2, are adjusted to make the phase mismatch between the cladding modes and the core high-order modes, and suppress the coupling between the core high-order modes and the cladding modes.

[0089] In this embodiment, as previously mentioned, all the hollow regions inside the fiber are filled with air, so the first region A1 can be called the first air region, and the second region A2 can also be called the second air region.

[0090] In a specific embodiment, the length of the major semi-axis a of the elliptical tube 22 is 12μm - 14μm. Preferably, the length of the major semi-axis a of the elliptical tube 22 is 13μm; the length of the minor semi-axis b of the elliptical tube 22 is 7μm - 11μm.

[0091] 3) Influence of the wall thickness t of the cladding capillary

[0092] To determine the influence of the wall thickness t of the cladding capillary on the confinement loss of the multimode hollow anti-resonant fiber, according to the anti-resonant condition, that is, formula (2), when the wavelength λ of the light is 1μm and the anti-resonant order m takes different values, the corresponding wall thickness t of the cladding capillary is as shown in Table 1 below.

[0093] Table 1

[0094] m 1 2 3 4 5 t / μm 0.24 0.71 1.19 1.67 2.14

[0095] Then, simulations were carried out on the optical fibers with different wall thicknesses t in Table 1, and the specific situations of the confinement losses CL of 12 core modes under different wall thicknesses t were obtained (as Figure 5 shown).

[0096] When the target operating wavelength is in the first-order anti-resonant region, compared with other anti-resonant regions, it has a higher high-order loss ratio, as Figure 5 shown. The loss ratio of the LP32 mode to the LP01 mode is about 108. When the target operating wavelength is in a higher-order anti-resonant region, the confinement losses of different core modes only increase slightly, while the differential losses between different core modes decrease significantly. When the wall thickness of the cladding capillary of the multimode hollow anti-resonant optical fiber designed by the present invention is set to t = 1.19 μm, Figure 2 the confinement losses CL of the 12 LP modes in Figure 2 are all lower than 0.5 dB / km, and the confinement losses CL of 9 LP modes are lower than 0.04 dB / km. When t = 1.67 μm,

[0097] the confinement losses CL of the 12 LP modes in Figure 2 are all lower than 0.3 dB / km, and the confinement losses CL of 10 LP modes are all lower than 0.05 dB / km.

[0097] The simulation results show that when the other structural parameters are determined, when the wall thickness t of the cladding capillary is 0.71 μm, 1.19 μm, 1.67 μm, and 2.14 μm respectively, it can meet the design requirements of the multimode hollow anti-resonant optical fiber, and can well transmit multimode light of 1 μm in the multimode hollow anti-resonant optical fiber to achieve multimode low-loss transmission. When t takes the value of 1.19 μm, the best value is obtained, and multimode transmission at a lower confinement loss level can be achieved.

[0098] In a specific embodiment, when the anti-resonant order m is 1 to 5, the wall thicknesses t of the circular tube 21, the elliptical tube 22, and the anti-resonant plate 23 in each cladding capillary 20 are 0.71 μm to 2.14 μm. Preferably, the wall thickness t is 0.71 μm, 1.19 μm, 1.67 μm, or 2.14 μm.

[0099] The simulation results obtained through the above simulations show that when the structural parameters of the multimode hollow anti-resonant optical fiber change, the confinement loss levels of the first 8 LP modes (i.e., the mode group LP01-LP41) show strong robustness. The design of the multimode hollow anti-resonant optical fiber has strong flexibility, and the structural parameters of the optical fiber can be reasonably selected according to specific application scenarios to support different application requirements.

[0100] The multimode hollow anti-resonant fiber provided by the embodiment of the present invention is provided with a plurality of cladding capillaries that are spaced apart from each other and arranged uniformly on the inner wall of the outer tube. In each cladding capillary, the elliptical tube is nested in the center of the circular tube through an anti-resonant plate passing through the center of the circular tube to form a central nested connected structure. Then, by adjusting the tube gap g between two adjacent cladding capillaries, the major semi-axis a and the minor semi-axis b of the elliptical tube, and the wall thickness t of each cladding capillary, multi-mode transmission with low confinement loss and low bending loss is achieved. Further, by adjusting the magnitudes of the major semi-axis a and the minor semi-axis b of the elliptical tube, the shapes and thicknesses d1 and d2 of the first region A1 and the second region A2 of the cladding can be adjusted, so that the phase mismatch between the cladding mode and the core high-order mode is caused, and the coupling between the core high-order mode and the cladding mode is suppressed, thereby effectively reducing the differential loss between different core modes.

[0101] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features. These modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A multimode hollow core antiresonant optical fiber, characterized in that: include: A hollow outer tube and a plurality of cladding capillaries arranged on the inner wall of the outer tube; each cladding capillary comprises a circular tube, an elliptical tube and an anti-resonance plate, the elliptical tube is nested inside the circular tube through the anti-resonance plate and is coaxially arranged with the circular tube.

2. The multimode hollow core antiresonant optical fiber according to claim 1, characterized in that: A plurality of cladding capillaries are spaced apart from each other and evenly arranged on the inner wall of the outer tube, and the outer wall of each cladding capillary is in contact with and fixedly connected to the inner wall of the outer tube; the area surrounded by the plurality of cladding capillaries is the core area.

3. The multimode hollow core antiresonant optical fiber according to claim 2, characterized in that: The hollow area between the inner wall of the outer tube and the outer wall of each cladding capillary tube, and the hollow area inside each cladding capillary tube, are filled with air; and / or the number of the cladding capillaries is 5 to 8.

4. The multimode hollow core antiresonant optical fiber according to claim 2, characterized in that: The circumference of the core region is tangent to each cladding capillary; the radius corresponding to the circumference of the core region is the core radius, and the core radius R is 18 μm to 22 μm.

5. The multimode hollow core antiresonant optical fiber according to claim 2, characterized in that: In the cross-sectional direction of each cladding capillary, the anti-resonance plate coincides with the radial direction of the circular tube and the major axis of the elliptical tube respectively, and both ends of the anti-resonance plate are connected to the inner wall of the circular tube; the radial direction of the outer tube is perpendicular to the anti-resonance plate in each cladding capillary.

6. The multimode hollow core antiresonant optical fiber according to any one of claims 2 to 5, characterized in that: The radius r of each cladding capillary is obtained using the following formula: Among them, r is the radius of the circular tube, R is the radius of the core, N is the number of cladding capillaries, and g is the tube gap between two adjacent cladding capillaries; The wall thickness of the circular tube, elliptical tube and anti-resonance plate in each cladding capillary is equal, and the wall thickness t is obtained using the following formula: Where t is the wall thickness, m is the antiresonance order, λ is the wavelength of light, and n is the refractive index of the cladding capillary.

7. The multimode hollow core antiresonant optical fiber according to claim 6, characterized in that: The value of the tube gap g between two adjacent cladding capillaries is λ≤g≤4λ.

8. The multimode hollow core antiresonant optical fiber according to claim 6, characterized in that: In each cladding capillary, the anti-resonance plate cuts the circular tube into two parts, and the area enclosed by each part of the circular tube and the anti-resonance plate is the first area; the anti-resonance plate cuts the elliptical tube into two parts, and the area enclosed by each part of the elliptical tube and the anti-resonance plate is the second area; The shape and thickness of the first region and the shape and thickness of the second region are adjusted by adjusting the major and minor axes of the elliptical tube, so that the cladding mode and the core high-order mode are phase-mismatched and the coupling between the core high-order mode and the cladding mode is suppressed.

9. The multimode hollow core antiresonant optical fiber according to claim 8, characterized in that: The length of the major semi-axis a of the elliptical tube is 12 μm to 14 μm, and the length of the minor semi-axis b of the elliptical tube is 7 μm to 11 μm.

10. The multimode hollow core antiresonant optical fiber according to claim 6, characterized in that: When the antiresonance order m is 1 to 5, the wall thickness t of the circular tube, the elliptical tube and the antiresonance plate in each cladding capillary is 0.71 μm to 2.14 μm.

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