Optical fiber connection structure and parameter design method thereof

Abstract

The invention relates to the technical field of optical fibers, and provides an optical fiber connection structure and a parameter design method thereof, the optical fiber connection structure comprises a first optical fiber (1), a graded index optical fiber (2), a connection tube (3) and a second optical fiber (4); the graded-index optical fiber (2) and the connecting pipe (3) are respectively provided with a first end and a second end which are oppositely arranged; the first end of the first optical fiber (1) and the first end of the graded-index optical fiber (2) are fixedly connected and then connected with the first end of the connecting pipe (3) along the second end of the graded-index optical fiber (2), and the connecting pipe (3) is fixedly connected with the second optical fiber (4) along the second end. The technical problem that the diameters of different optical fiber mode fields are not matched is solved, the back loss can be effectively inhibited, an optical fiber connector is not needed, and extra insertion loss is not introduced.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fibers, and more specifically, to an optical fiber connection structure and a parameter design method thereof. Background Art

[0002] Optical fiber consists of a core and a cladding. It is a thin, transparent glass or plastic fiber used to transmit optical signals. Since the transmission loss of light in optical fiber is much lower than the transmission loss of electricity in wires, optical fiber is used as a carrier for long-distance information transmission. Optical fibers with different transmission media have different transmission rates and are suitable for different types of network equipment.

[0003] In practical applications, according to specific application scenarios and requirements, combined with the compatibility, cost budget and security requirements of network equipment, it is often necessary to connect different types of optical fibers together; for example, connecting hollow-core optical fibers with solid-core optical fibers; hollow-core optical fibers bind light in the air core for transmission through antiresonance effect or photonic bandgap effect. Compared with ordinary optical fibers, they have the advantages of low loss, low delay, low nonlinear effect and low laser damage threshold, and have great application prospects in the fields of optical fiber communication, optical fiber sensing and laser devices; after decades of rapid development, traditional solid-core optical fibers have great advantages in optical communication and optical sensing; the optical fiber connection structure can combine the advantages of hollow-core optical fibers and solid-core optical fibers, and has broad application prospects. However, different types of optical fibers often have different mode field diameters. Directly connecting different types of optical fibers together will produce huge optical fiber losses due to the mismatch of mode field diameters, and will produce large back reflections due to the large refractive index difference at the connection interface. The traditional connection method reduces back reflections by using connectors at the connection interface of the two optical fibers, but it will cause further optical fiber losses.

[0004] In order to overcome the technical problem of mismatching mode field diameters of different types of optical fibers, the prior art uses optical fiber connectors to match the mode field diameters of different types of optical fibers. However, the prior art not only still has the problem of large back reflection caused by large refractive index differences at the connection interface, but also introduces additional insertion loss because of the optical fiber connector. Summary of the invention

[0005] In order to overcome the defect of the prior art that there is a large back reflection, the present invention provides an optical fiber connection structure that can suppress back reflection and has no insertion loss.

[0006] In order to solve the above technical problems, the technical solution of the present invention is as follows:

[0007] An optical fiber connection structure comprises a first optical fiber, a graded-refractive-index optical fiber, a connecting tube, and a second optical fiber; the graded-refractive-index optical fiber and the connecting tube respectively have a first end and a second end arranged opposite to each other; the first optical fiber is fixedly connected to the first end of the graded-refractive-index optical fiber and then connected to the first end of the connecting tube along the second end of the graded-refractive-index optical fiber, and the connecting tube is fixedly connected to the second optical fiber along the second end.

[0008] The present invention also proposes a parameter design method for an optical fiber connection structure, which uses the above optical fiber connection structure. The method comprises the following steps:

[0009] A simulation model of the connection structure is constructed in the simulation platform, and the value interval of the length L1 of the graded refractive index optical fiber is set to Wherein, k represents a positive integer, and the value of k is selected according to actual needs;

[0010] Select a value in the value range of the length L1 as the initial value of the length L1, set the length L2 of the connecting tube, the inclination angle θ of the connecting surface of the graded-refractive-index optical fiber and the connecting tube, and the initial value of the radial deviation d between the axis of the first optical fiber and the axis of the graded-refractive-index optical fiber;

[0011] The initial mode field diameter A1 of the light beam is set to a value matching the mode field diameter B1 of the first optical fiber, and the mode field diameter A2 of the light beam when the light beam is incident on the second optical fiber after being transmitted through the first optical fiber, the graded refractive index optical fiber and the connecting tube in sequence is calculated using the simulation model of the connection structure;

[0012] Taking minimizing the difference between the mode field diameter A2 and the mode field diameter B2 of the second optical fiber as the first objective function;

[0013] The length L1, length L2, inclination angle θ and radial deviation d are set as variables of the target optimization algorithm, and the initial value and the first objective function are input into the target optimization algorithm. The first objective function is iteratively solved using the target optimization algorithm. When the number of iterations reaches a preset value, or the objective function reaches a minimum value, the calculation is terminated to obtain the optimal values ​​of the length L1, length L2, inclination angle θ and radial deviation d.

[0014] The present invention also provides a method for connecting an optical fiber connection structure, comprising the following steps:

[0015] Fixedly connecting one end of the first optical fiber to one end of the graded-index optical fiber;

[0016] Grinding or cutting the other end of the graded-refractive-index optical fiber so that the other end of the graded-refractive-index optical fiber forms an inclined surface with an inclined angle of a preset value and the length of the graded-refractive-index optical fiber is a first preset length;

[0017] Grinding or cutting one end of the connecting tube to a shape matching the other end of the graded-refractive-index optical fiber, and fixing the connecting tube and the graded-refractive-index optical fiber in a fixed connection;

[0018] The other end of the connecting tube is ground or cut to make the length of the connecting tube a second preset length, and the connecting tube is fixedly connected to the second optical fiber.

[0019] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0020] When the light beam is injected into the first optical fiber, the mode field diameter of the light beam matches the mode field diameter of the first optical fiber; the first optical fiber transmits the light beam to the graded-index optical fiber, the graded-index optical fiber increases the mode field diameter of the light beam, and then injects the light beam into the connecting tube, the connecting tube reduces the mode field diameter of the light beam, and then injects the light beam into the second optical fiber, at this time, the mode field diameter of the light beam matches the mode field diameter of the second optical fiber; the technical problem of mismatch of mode field diameters of different optical fibers is overcome, the back loss can be effectively suppressed, and no optical fiber connector is required, and no additional insertion loss is introduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 A schematic diagram of the optical fiber connection structure proposed in Example 1;

[0022] Figure 2 A schematic diagram of the radial deviation proposed in Example 1;

[0023] Figure 3 This is a schematic diagram of optical fiber loss proposed in Example 1;

[0024] Figure 4 This is a flow chart of the parameter design method proposed in Example 2;

[0025] Figure 5 This is a schematic diagram of the connection method proposed in Example 3;

[0026] Among them, 1-first optical fiber, 11-first optical fiber cladding, 12-first optical fiber core, 2-graded refractive index optical fiber, 21-graded refractive index optical fiber cladding, 22-graded refractive index optical fiber core, 3-connecting tube, 31-connecting tube surrounding body, 32-connecting tube air core, 4-second optical fiber, 41-second optical fiber cladding, 42-second optical fiber core. DETAILED DESCRIPTION

[0027] The drawings are for illustrative purposes only and should not be construed as limiting the present patent;

[0028] In order to better illustrate the present embodiment, some parts in the drawings may be omitted, enlarged or reduced, and do not represent the size of the actual product;

[0029] It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0030] The technical solution of the present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0031] Example 1

[0032] This embodiment provides a connection structure of optical fibers. Figure 1 A schematic diagram of the optical fiber connection structure proposed in this embodiment;

[0033] like Figure 1 As shown, the optical fiber connection structure of this embodiment includes a first optical fiber 1, a graded refractive index optical fiber 2, a connecting tube 3 and a second optical fiber 4; the graded refractive index optical fiber 2 and the connecting tube 3 respectively have a first end and a second end arranged opposite to each other; after the first ends of the first optical fiber 1 and the graded refractive index optical fiber 2 are fixedly connected, they are connected to the first end of the connecting tube 3 along the second end of the graded refractive index optical fiber 2, and the connecting tube 3 is fixedly connected to the second optical fiber 4 along the second end.

[0034] In the specific implementation process, when the light beam is injected into the first optical fiber 1, the mode field diameter of the light beam matches the mode field diameter of the first optical fiber 1; the first optical fiber 1 transmits the light beam to the graded refractive index optical fiber 2, and the graded refractive index optical fiber 2 increases the mode field diameter of the light beam and then injects the light beam into the connecting tube 3, and the connecting tube 3 reduces the mode field diameter of the light beam and then injects the light beam into the second optical fiber 4. At this time, the mode field diameter of the light beam matches the mode field diameter of the second optical fiber 4; the technical problem of mismatch of mode field diameters of different optical fibers is overcome, and the back loss can be effectively suppressed, and there is no need to use an optical fiber connector, and no additional insertion loss will be introduced.

[0035] As an exemplary explanation, the graded-index optical fiber is a graded-index optical fiber or a combination of multiple graded-index optical fibers, wherein the combination of multiple graded-index optical fibers refers to a combination of multiple graded-index optical fibers connected in a front-to-back manner.

[0036] In an optional embodiment, the first optical fiber 1 includes a first optical fiber cladding 11 and a first optical fiber core 12; wherein the first optical fiber core 12 is a solid core;

[0037] The second optical fiber 4 includes a second optical fiber cladding 41 and a second optical fiber core 42; wherein the second optical fiber core 42 is a hollow core;

[0038] The connecting tube 3 comprises a hollow tube filled with air. The diameter of the air region of the connecting tube 3 is larger than the core diameter of the graded-refractive-index optical fiber 2 and larger than the core diameter of the second optical fiber 4 .

[0039] As an exemplary illustration, the first optical fiber comprises a single-mode solid core optical fiber; and a mode field diameter of the first optical fiber is smaller than a mode field diameter of the second optical fiber.

[0040] As an example, Figure 1 As shown, the first optical fiber 1 includes a first optical fiber cladding 11 and a first optical fiber core 12. The diameter of the first optical fiber cladding 11 ranges from [60, 300], in microns; the diameter of the first optical fiber core 12 ranges from [5, 40], in microns.

[0041] As an example, Figure 1 As shown, the second optical fiber 4 includes a second optical fiber cladding 41 and a second optical fiber core 42. The diameter of the second optical fiber cladding 41 ranges from [125, 300], in microns; the diameter of the second optical fiber core 42 ranges from [20, 90], in microns.

[0042] As an example, Figure 1 As shown, the graded-refractive-index optical fiber 2 includes a graded-refractive-index optical fiber cladding 21 and a graded-refractive-index optical fiber core 22. The diameter of the graded-refractive-index optical fiber cladding 21 ranges from [125, 400], in micrometers; the diameter of the graded-refractive-index optical fiber core 22 ranges from [50, 350], in micrometers.

[0043] As an example, Figure 1 As shown, the body portion of the hollow tube of the connecting tube 3 is called a connecting tube enclosure 31, and the portion of the connecting tube 3 filled with air is called a connecting tube air core 32, wherein the connecting tube enclosure 31 is made of a glass tube, a ceramic tube or a metal tube, and the connecting tube enclosure 31 does not need to be completely closed; in this optional embodiment, the light beam emitted by the gradient refractive index optical fiber uses the connecting tube air core 32 to reduce the mode field diameter of the light beam, and by adjusting the length of the connecting tube air core 32, the size of the mode field diameter of the light beam emitted from the connecting tube can be adjusted. In the specific implementation process, only the length of the connecting tube needs to be adjusted to achieve a low-loss connection between the first optical fiber and the second optical fiber of different specifications, and the operation is simple and the cost is low.

[0044] As an exemplary illustration, the length of the fiber optic connector made using a gradient refractive index fiber in the prior art must be 0.5n times the length of the self-imaging period of the gradient refractive index fiber, where n is a positive integer and its length cannot be freely adjusted. Given a specific value of n and the specification of the first optical fiber, the mode field diameter of the light beam after transmission through the gradient refractive index fiber is fixed and can only match a second optical fiber of a specific specification. The connection structure of the present application, after promoting the matching of the mode field diameters of the first optical fiber and the second optical fiber through the gradient refractive index fiber, also adds a connecting tube with an air core, and uses the connecting tube to reduce the mode field diameter of the light beam emitted by the gradient refractive index fiber. By adjusting the length of the connecting tube, a specific first optical fiber can be matched with a second optical fiber of different specifications.

[0045] In an optional embodiment, the expression of the length L1 of the graded-index optical fiber 2 includes:

[0046]

[0047] Wherein, k represents a positive integer, and L represents the self-focusing length.

[0048] As an example, when the light beam is transmitted into the graded-index fiber 2, the mode field diameter of the light beam changes periodically along with the length of the graded-index fiber 2, and the length L1 is constrained to Within the value range of, the convergence function of the graded refractive index optical fiber 2 can be effectively utilized, thereby promoting the matching of the mode field diameters of the first optical fiber 1 and the second optical fiber 4; wherein, given the specific value of k and the specification of the first optical fiber 1, the first optical fiber 1 can be matched with the second optical fiber 4 of different specifications by adjusting the value of the length L1 within a preset range; combined with adjusting the length of the connecting tube 3, it is possible to have a larger selection space for the specification of the second optical fiber 4 when the specification of the first optical fiber 1 is determined.

[0049] In an optional embodiment, the connection surface between the graded-refractive-index optical fiber 2 and the connection tube 3 includes an inclined surface with an inclination angle of a preset value.

[0050] As an exemplary explanation, the present application makes the mode field diameter of the light beam that originally matches the mode field diameter of the first optical fiber 1 match the mode field diameter of the second optical fiber 4 after passing through the graded refractive index optical fiber 2 and the connecting tube 3; wherein the connecting surface between the graded refractive index optical fiber 2 and the connecting tube 3 is set to an inclined plane with an inclination angle of a preset value; the inclined plane is used to convert the direction of the reflection occurring when the light beam enters the connecting tube 3 from the graded refractive index optical fiber 2 into a non-backward direction, so as to suppress back reflection.

[0051] In an optional embodiment, there is a radial deviation between the axis of the first optical fiber 1 and the axis of the graded-refractive-index optical fiber 2 .

[0052] As an exemplary explanation, the radial deviation is used to correct the direction of the light beam entering the connecting tube 3 through the inclined surface to a direction parallel to the axial direction, so as to reduce the connection loss.

[0053] As an exemplary illustration, the radial deviation has a value range of [0,50], in micrometers.

[0054] As an example, Figure 2 Schematic diagram of radial deviation proposed in this embodiment, Figure 2 The d in is the radial deviation.

[0055] As an example, Figure 3 Schematic diagram of optical fiber loss proposed in this embodiment; Figure 3 As shown, Figure 3 Part (a) directly connects the first optical fiber, the graded refractive index optical fiber and the connecting tube in sequence. The connecting surface of the graded refractive index optical fiber and the connecting tube is a vertical surface, and there is no tilt angle θ. At this time, the output light is parallel to the main axis of the optical fiber 2, and no additional connection loss is caused by the non-parallel output light angle. However, the back reflection is relatively high at this time. In order to suppress the back reflection, the present application sets the connecting surface of the graded refractive index optical fiber and the connecting tube to an inclined surface, such as Figure 3 As shown in part (b), the direction of reflection becomes non-backward, which can effectively suppress back reflection. However, the inclined surface makes the direction of the light beam entering the connecting tube non-parallel, which brings additional connection loss. In order to reduce the additional connection loss while causing back reflection, as shown in Figure 3 As shown in part (c), the radial deviation d is introduced to reduce the angle between the outgoing light and the main axis of the connecting tube, so that the light beam is incident into the connecting tube at an angle parallel to the main axis of the connecting tube as much as possible, thereby reducing or even eliminating this additional connection loss.

[0056] Example 2

[0057] This embodiment proposes a parameter design method for an optical fiber connection structure, using the optical fiber connection structure proposed in Embodiment 1. Figure 4 A flow chart of the parameter design method proposed in this embodiment;

[0058] The parameter design method of the optical fiber connection structure comprises the following steps:

[0059] S1: Construct a simulation model of the connection structure in the simulation platform, and set the value interval of the length L1 of the graded refractive index optical fiber 2 to Wherein, k represents a positive integer, and the value of k is selected according to actual needs;

[0060] S2: Select a value in the value range of the length L1 as the initial value of the length L1, set the length L2 of the connecting tube 3, the inclination angle θθ of the connecting surface between the graded-refractive-index optical fiber 2 and the connecting tube 3, and the initial value of the radial deviation d between the axis of the first optical fiber 1 and the axis of the graded-refractive-index optical fiber 2;

[0061] S3: setting the initial mode field diameter A1 of the light beam to a value matching the mode field diameter B1 of the first optical fiber 1, and using the simulation model of the connection structure, calculating the mode field diameter A2 of the light beam when the light beam is incident on the second optical fiber 4 after being transmitted through the first optical fiber 1, the graded refractive index optical fiber 2 and the connecting tube 3 in sequence;

[0062] S4: minimizing the difference between the mode field diameter A2 and the mode field diameter B2 of the second optical fiber 4 as the first objective function;

[0063] S5: Set the length L1, length L2, inclination angle θ and radial deviation d as variables of the target optimization algorithm, and input the initial value and the first objective function into the target optimization algorithm, and use the target optimization algorithm to iteratively solve the first objective function. When the number of iterations reaches a preset value, or the objective function reaches a minimum value, end the calculation and obtain the optimal values ​​of the length L1, length L2, inclination angle θ and radial deviation d.

[0064] As an exemplary illustration, as an exemplary illustration, the inclination angle θ refers to the angle between the inclined plane and the vertical direction, and the value range of the inclination angle θ includes [0, 10], and the unit of the value range is degree.

[0065] In an optional embodiment, the method further comprises a second objective function with the goal of minimizing the optical fiber loss Ls1;

[0066] When the objective optimization algorithm is a multi-objective optimization algorithm, after setting the length L1, length L2, inclination angle θ and radial deviation d as variables of the multi-objective optimization algorithm, the initial value, the first objective function and the second objective function are input into the multi-objective optimization algorithm, and the multi-objective optimization algorithm is used to iteratively solve the first objective function and the second objective function. When the number of iterations reaches a preset value, or the first objective function reaches the minimum value and the second objective function does not exceed a preset threshold, the calculation is terminated to obtain the optimal values ​​of the length L1, length L2, inclination angle θ and radial deviation d.

[0067] In an optional embodiment, the expression of the optical fiber loss Ls1 includes:

[0068]

[0069] Wherein, P1 represents the optical power of the light beam emitted from the first optical fiber, and P2 represents the optical power of the light beam emitted into the second optical fiber.

[0070] It can be understood that the parameter design method of this embodiment applies the connection structure of the above-mentioned embodiment 1, and the options of the above-mentioned embodiment 1 are also applicable to this embodiment, so they will not be described repeatedly here.

[0071] Example 3

[0072] This embodiment provides a connection method for an optical fiber connection structure, using the optical fiber connection structure provided in Embodiment 1; Figure 5 This is a schematic diagram of the flow of the connection method proposed in this embodiment.

[0073] The connecting method of the optical fiber connecting structure comprises the following steps:

[0074] S1: fixedly connecting one end of the first optical fiber 1 to one end of the graded-refractive-index optical fiber 2;

[0075] S2: grinding or cutting the other end of the graded-refractive-index optical fiber 2 so that the other end of the graded-refractive-index optical fiber 2 forms an inclined surface with an inclined angle of a preset value and the length of the graded-refractive-index optical fiber 2 is a first preset length;

[0076] S3: grinding or cutting one end of the connecting tube 3 to a shape matching the other end of the graded-refractive-index optical fiber 2, and fixing the connecting tube 3 and the graded-refractive-index optical fiber 2;

[0077] S4: grinding or cutting the other end of the connecting tube 3 to make the length of the connecting tube 3 a second preset length, and to fix the connecting tube 3 to the second optical fiber 4 in connection.

[0078] In an optional embodiment, the fixed connection method includes welding or bonding with glue;

[0079] When the welding method is used, the welding is achieved by a welding machine, and the heat source of the welding machine is arc discharge, carbon dioxide laser or graphite.

[0080] As an example, the tools needed to make the optical fiber connection structure include an optical fiber fusion splicer, an optical fiber cleaver, and an optical fiber grinder; the optical fiber fusion splicer is used to perform optical fiber fusion splicing; the optical fiber cleaver is used to cut the graded refractive index optical fiber; and the optical fiber grinder is used to grind the fixed graded refractive index optical fiber or the connecting tube. The wavelength of the laser transmitted in the second optical fiber is ultraviolet light, visible light, or infrared band.

[0081] This embodiment proposes a specific implementation example of connecting a hollow-core optical fiber with a solid-core optical fiber, wherein the diameter of the core 12 of the first optical fiber is set to 9 microns, the diameter of the first optical fiber cladding 11 is set to 125 microns, the mode field diameter is set to 10 microns, and the length is set to 1 meter; the diameter of the second optical fiber core 42 is set to 30 microns, the diameter of the second optical fiber cladding 41 is set to 250 microns, the mode field diameter is set to 23 microns, and the length is set to 1 meter; the diameter of the graded-index optical fiber core 22 is set to 300 microns, the diameter of the graded-index optical fiber cladding 21 is set to 350 microns, the core center refractive index of the graded-index optical fiber core 22 is set to 1.45 at 1550nm, the refractive index of the cladding 21 is set to 1.444, and the original coating layer has been stripped. The diameter of the air core 32 of the connecting tube is 80 microns, and the diameter of the surrounding body 31 is 250 microns.

[0082] After the first optical fiber 1 and the graded-refractive-index optical fiber 2 are cut with a cleaver, they are fused together by a CO2 laser fusion splicer. By controlling the fusion splicer, the center deviation distance between the first optical fiber 1 and the graded-refractive-index optical fiber 2 is set to 4.5 microns; then the cleaver is used for cutting and the grinder is used for grinding. After grinding, the length L1 of the graded-refractive-index optical fiber 2 is 400 microns and the angle is 8 degrees.

[0083] After one end of the connecting tube 3 is ground to an angle of 8 degrees by a grinder, the connecting tube 3 and the graded-refractive-index optical fiber 2 are fused together by a CO2 laser fusion splicer. Subsequently, the length of the connecting tube is cut to 1.8 mm by a cutting knife.

[0084] The connecting tube 3 and the second optical fiber 4 are welded by a CO2 laser welding machine. After the light with a mode field diameter of 10 microns in the first optical fiber 1 is transmitted through a 400-micron-long gradient refractive index optical fiber and 1.8 mm of air, its mode field diameter is 23 microns, which can achieve mode field matching coupling between the first optical fiber 1 and the second optical fiber 4.

[0085] The same or similar reference numerals correspond to the same or similar components;

[0086] The terms used in the drawings to describe positional relationships are only used for illustrative purposes and should not be construed as limiting this patent;

[0087] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the embodiments here. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. An optical fiber connection structure, characterized in that: The invention comprises a first optical fiber (1), a graded-refractive-index optical fiber (2), a connecting tube (3) and a second optical fiber (4); the graded-refractive-index optical fiber (2) and the connecting tube (3) respectively have a first end and a second end arranged opposite to each other; the first optical fiber (1) and the graded-refractive-index optical fiber (2) are fixedly connected at their first ends and then connected to the first end of the connecting tube (3) along the second end of the graded-refractive-index optical fiber (2); and the connecting tube (3) is fixedly connected to the second optical fiber (4) along its second end.

2. The optical fiber connection structure according to claim 1, characterized in that: The first optical fiber (1) comprises a first optical fiber cladding (11) and a first optical fiber core (12); wherein the first optical fiber core (12) is a solid core; The second optical fiber (4) comprises a second optical fiber cladding (41) and a second optical fiber core (42); wherein the second optical fiber core (42) is a hollow core; The connecting tube (3) comprises a hollow tube filled with air, and the diameter of the air region of the connecting tube (3) is larger than the core diameter of the graded refractive index optical fiber (2) and larger than the core diameter of the second optical fiber (4).

3. The optical fiber connection structure according to claim 1, characterized in that: The expression for the length L1 of the graded-refractive-index optical fiber (2) includes: Wherein, k represents a positive integer, and L represents the self-focusing length.

4. The optical fiber connection structure according to any one of claims 1 to 3, characterized in that: The connection surface between the graded refractive index optical fiber (2) and the connection tube (3) comprises an inclined surface with an inclination angle of a preset value.

5. The optical fiber connection structure according to claim 4, characterized in that: There is a radial deviation between the axis of the first optical fiber (1) and the axis of the graded-refractive-index optical fiber (2).

6. A method for designing parameters of an optical fiber connection structure, using the optical fiber connection structure of claim 5, characterized in that: The following steps are involved: A simulation model of the connection structure is constructed in a simulation platform, and the value interval of the length L1 of the graded refractive index optical fiber (2) is set to Wherein, k represents a positive integer, and the value of k is selected according to actual needs; A value is selected from the value range of the length L1 as the initial value of the length L1, and the length L2 of the connecting tube (3), the inclination angle θ of the connecting surface between the graded-refractive-index optical fiber (2) and the connecting tube (3), and the initial value of the radial deviation d between the axis of the first optical fiber (1) and the axis of the graded-refractive-index optical fiber (2) are set; The initial mode field diameter A1 of the light beam is set to a value matching the mode field diameter B1 of the first optical fiber (1), and the mode field diameter A2 of the light beam when the light beam is incident on the second optical fiber (4) after being sequentially transmitted through the first optical fiber (1), the graded refractive index optical fiber (2) and the connecting tube (3) is calculated using the simulation model of the connection structure; Minimizing the difference between the mode field diameter A2 and the mode field diameter B2 of the second optical fiber (4) as a first objective function; The length L1, length L2, inclination angle θ and radial deviation d are set as variables of the target optimization algorithm, and the initial value and the first objective function are input into the target optimization algorithm. The first objective function is iteratively solved using the target optimization algorithm. When the number of iterations reaches a preset value, or the objective function reaches a minimum value, the calculation is terminated to obtain the optimal values ​​of the length L1, length L2, inclination angle θ and radial deviation d.

7. The parameter design method of the optical fiber connection structure according to claim 6, characterized in that: The method further includes a second objective function aimed at minimizing the optical fiber loss Ls1; When the objective optimization algorithm is a multi-objective optimization algorithm, after setting the length L1, length L2, inclination angle θ and radial deviation d as variables of the multi-objective optimization algorithm, the initial value, the first objective function and the second objective function are input into the multi-objective optimization algorithm, and the multi-objective optimization algorithm is used to iteratively solve the first objective function and the second objective function. When the number of iterations reaches a preset value, or the first objective function reaches the minimum value and the second objective function does not exceed a preset threshold, the calculation is terminated to obtain the optimal values ​​of the length L1, length L2, inclination angle θθ and radial deviation d.

8. The method for designing parameters of an optical fiber connection structure according to claim 7, characterized in that: The expression of the optical fiber loss Ls1 includes: In the formula, P1 represents the optical power of the light beam emitted from the first optical fiber (1), and P2 represents the optical power of the light beam emitted into the second optical fiber (4).

9. A method for connecting an optical fiber connection structure, characterized in that: The following steps are involved: Fixedly connecting one end of the first optical fiber (1) to one end of the graded-refractive-index optical fiber (2); Grinding or cutting the other end of the graded-refractive-index optical fiber (2) so that the other end of the graded-refractive-index optical fiber (2) forms an inclined surface with an inclined angle of a preset value and the length of the graded-refractive-index optical fiber (2) is a first preset length; Grinding or cutting one end of the connecting tube (3) into a shape matching the other end of the graded-refractive-index optical fiber (2), and fixedly connecting the connecting tube (3) and the graded-refractive-index optical fiber (2); The other end of the connecting tube (3) is ground or cut so that the length of the connecting tube (3) is a second preset length, and the connecting tube (3) is fixedly connected to the second optical fiber (4).

10. The optical fiber connection structure according to claim 9, characterized in that: The fixed connection method includes welding or bonding with glue; When the welding method is used, the welding is achieved by a welding machine, and the heat source of the welding machine is arc discharge, carbon dioxide laser or graphite.

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