OAM mode modulator based on torsional anti-resonance hollow-core fiber and preparation method thereof
By providing a uniform and periodic spiral torsion structure on the anti-resonant hollow core optical fiber, combined with the improved arc discharge method, the polarization sensitivity and compatibility problems of OAM mode conversion in the prior art are solved, and low-cost and high-efficiency are achieved OAM mode modulator.
Patent Information
- Application Number
- CN202411390129.8
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-10-08
- Publication Date
- 2025-02-07
AI Technical Summary
The prior art has polarization sensitivity and compatibility problems when generating and converting orbital angular momentum modes (OAMs), and the equipment is complex and costly.
Using an OAM mode modulator based on torsional anti-resonant hollow core optical fiber, a uniform and periodic spiral refractive index modulation of incident light is achieved by providing a uniform and periodic spiral torsion structure in the axial direction of the optical fiber, thereby stimulating high and fiber samples were prepared by improved arc discharge method.
The polarization-independent and polarization-controllable OAM mode conversion is realized, with low insertion loss and compatibility with existing fiber optic systems, which is low cost and easy to prepare.
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Figure CN119395811A_ABST
Abstract
Description
Technical Field
[0001] The present invention belongs to the technical field of orbital angular momentum application, and in particular relates to an OAM mode modulator based on torsional antiresonant hollow-core optical fiber and a preparation method thereof. Background Art
[0002] OAM has attracted wide attention due to its unique spiral phase and annular intensity distribution. Nowadays, methods for generating OAM have been widely studied and applied. These methods can be divided into two categories. One is to generate OAM based on spatial beam transformation. For example, computer-controlled spatial light modulators (SLMs) can achieve the excitation of different topological charge OAM modes by controlling the arrangement of liquid crystal molecules. However, SLMs are easily affected by polarization during actual use and are not directly compatible with existing networks. The other is to generate OAM modes in optical fiber systems. Due to its easy integration, compatibility with existing optical fiber systems, and low insertion loss, it is an ideal OAM mode converter in optical fiber systems. Among them, OAM mode converters based on twisted fibers have been widely studied in recent years due to their unique polarization characteristics. By twisting solid core fibers, OAM modes with different polarization states can be generated. Antiresonant hollow core fiber is a new type of microstructured fiber. It uses microstructures to achieve antiresonance effects and suppress mode coupling so that light is confined in the air core. This light-guiding mechanism makes antiresonant hollow-core fiber have the characteristics of low dispersion, low nonlinearity, and low loss material loss, which means that it has extremely broad application scenarios in fiber-optic communication, quantum communication, fiber-optic sensing and other fields. Therefore, it is necessary to study OAM mode converters based on torsional antiresonant hollow-core fiber. Summary of the invention
[0003] The main purpose of the present invention is to overcome the shortcomings and deficiencies of the prior art and to propose an OAM mode modulator based on torsional antiresonant hollow-core fiber and a preparation method thereof.
[0004] In order to achieve the above object, the present invention adopts the following technical solutions: An OAM mode modulator based on a torsional antiresonant hollow core fiber, including a torsional antiresonant hollow core fiber; The twisted antiresonant hollow core optical fiber is provided with a uniform and periodic spiral twist structure in the axial direction, which is used to make the incident light undergo a uniform and periodic spiral refractive index modulation, thereby coupling the incident light to a higher-order mode and carrying OAM, thereby generating an OAM beam.
[0005] Furthermore, the twisted antiresonant hollow-core optical fiber is a single-ring seven-cladding tube antiresonant hollow-core optical fiber.
[0006] Further, the torsional antiresonant hollow core optical fiber includes a cladding, a geometric microstructure, and an air core; The cladding provides support for the geometric microstructure, which realizes the anti-resonance effect and suppresses mode coupling, and transmits light in the air core.
[0007] The present invention also includes a method for preparing an OAM mode modulator based on a torsional antiresonant hollow core fiber, the method comprising: Finite element simulation software is used to calculate the optical properties of antiresonant hollow core fibers and collect electric field data of different modes of antiresonant hollow core fibers; The order of the excited OAM mode is determined based on the topological charge matching theory, providing the target mode for calculating the mode coupling coefficient; The mode coupling coefficients of the fundamental mode coupling to the higher-order OAM modes are calculated based on the mode coupling theory and the torsion model. The parameters of the torsional antiresonant fiber sample are set during the preparation process according to the mode coupling theory; The antiresonant hollow core optical fiber was twisted using an improved arc discharge method.
[0008] Furthermore, the order of the excited OAM mode is determined based on the topological charge matching theory, specifically: The order of the excited OAM mode is determined according to the topological charge matching formula: l_p+l_q±m=0; Wherein, l_p and l_q represent the topological charges carried by modes p and q, respectively, m represents the geometric symmetry of the antiresonant hollow-core fiber, and + and - represent the left and right handedness of the torsional antiresonant hollow-core fiber, respectively.
[0009] Furthermore, based on the mode coupling theory and the torsion model, the mode coupling coefficient of the fundamental mode coupling to the higher-order OAM mode is calculated. The specific formula is: ; in, and represents the angular frequency and the dielectric constant of vacuum; N and N q Indicates the change in the refractive index of the material before and after twisting; E p and E q denote the electric field distribution of mode p and mode q respectively; the superscript * denotes conjugation; H p represents the magnetic field distribution of mode p; u z Represents the unit vector in the z direction; N is obtained by twisting the matrix, the twist matrix T -1 It is expressed as: ; Where α represents the torsion rate applied to the optical fiber, based on which the Cartesian coordinates Convert to spiral coordinates On this basis, the mode coupling coefficient is calculated through the data collected by the finite element software, and the efficiency of mode conversion is evaluated through the mode coupling coefficient.
[0010] Furthermore, the parameters of the torsional antiresonant optical fiber sample during the preparation process are set according to the mode coupling theory, specifically: According to mode coupling theory: ; ; Among them, P a (z) and P b (z) represents the normalized power variation of mode a and mode b along the z-axis direction, represents the maximum coupling efficiency; represents the difference in propagation constants between mode a and mode b under the influence of torsion rate α; , is the mode coupling coefficient; z represents the length of the prepared torsional antiresonant fiber sample; according to the mode coupling theory, the torsion rate α is set so that the maximum coupling efficiency F=1 and the length z is set to ensure that the mode conversion efficiency reaches the maximum coupling efficiency.
[0011] Furthermore, an improved arc discharge method is used to twist the anti-resonant hollow core optical fiber, including: A pair of electrodes, two rotation motors and two horizontal displacement motors are provided; The electrodes are used to discharge to form a heating field to keep the antiresonant hollow core optical fiber in a molten state; the two horizontal displacement motors are used to make the antiresonant hollow core optical fiber pass through the arc discharge heating field at a uniform speed; the rotary motor is used to twist the antiresonant hollow core optical fiber in a molten state in the heating field; When the optical fiber is in a molten state, a differential speed is set between the left and right horizontal displacement motors to eliminate the bending phenomenon of the anti-resonant hollow-core optical fiber caused by gravity and torsional stress in the molten state; the speed of one horizontal displacement motor is V2, and the speed of the other horizontal displacement motor is V1, and the speed of the horizontal displacement motor is set so that V2 is greater than V1; The antiresonant hollow-core fiber is controlled by V-grooves to keep both sides in a horizontal position to prevent eccentric twisting from causing uneven samples. Control the distance between the fixture and the V-groove to improve the eccentric torsion caused by the mismatch between the large core diameter fiber and the fixture, which leads to the fiber tilting.
[0012] Furthermore, when twisting the antiresonant hollow core fiber, the electrode spacing is controlled to form a larger heating field, ensuring that the large diameter fiber is in a uniform molten state, ensuring the uniformity of the sample and maintaining the geometric microstructure of the antiresonant hollow core fiber.
[0013] Furthermore, when the optical fiber is in a molten state, the rotary motor is set to twist the antiresonant hollow core optical fiber at a rotation speed of Ω, and the twist rate α applied to the optical fiber is calculated by the following formula: α=(V2*π / 180) / Ω; The working time T of the rotating motor is adjusted to control the length L of the prepared torsional antiresonant hollow-core optical fiber, L=V2*T, and finally the torsional antiresonant hollow-core optical fiber is prepared.
[0014] Compared with the prior art, the present invention has the following advantages and beneficial effects: 1. The present invention realizes a polarization-independent and polarization-controllable OAM mode converter by using an antiresonant hollow-core optical fiber of a twisted single-ring seven-cladding tube. The twisted antiresonant hollow-core optical fiber has a spiral twist structure, and its axial direction has a uniform and periodic spiral structure change. This structure makes the transmitted light have a uniform and periodic refractive index modulation when propagating axially, and can modulate the incident Gaussian light into a high-order mode. The periodic spiral structure makes the mode carry OAM, generating a high-order OAM mode. Moreover, since the OAM light beam is generated by a uniform and periodic spiral structure, it is little affected by the polarization direction of the electric field. Therefore, different polarization states can be input to excite high-order OAM modes, and it has a polarization-insensitive characteristic. The polarization state of the OAM mode excited by the OAM mode modulator is the same as the polarization state of the input Gaussian light, and it has a polarization-controllable characteristic.
[0015] 2. The OAM mode modulator of the present invention is an all-fiber device, which can directly excite the OAM mode through a uniform and periodic spiral structure, does not require additional control devices, is directly compatible with the optical fiber communication system, is easy to prepare, and has a low cost.
[0016] 3. The present invention adopts an improved arc discharge method to twist the antiresonant hollow core optical fiber, and can prepare the torsional antiresonant hollow core optical fiber under the condition of ensuring the uniformity and structure of the optical fiber sample is not destroyed. BRIEF DESCRIPTION OF THE DRAWINGS
[0017] Figure 1 is a longitudinal cross-sectional view of an OAM mode converter of an embodiment; Figure 2 is a schematic cross-sectional view of a single-ring seven-cladding tube antiresonant hollow-core optical fiber used in the embodiment; Figure 3 is a schematic diagram of an arc discharge device used in the embodiment; Figure 4 is a test result diagram of verifying the torsional antiresonant hollow core fiber excitation OAM beam in the embodiment; Figure 5 is a test optical path provided in the embodiment; Figure 6is a graph showing the polarization insensitivity test results of the OAM light beam in the embodiment; Figure 7 3 is a test result diagram of the output polarization controllable characteristics of the OAM light beam in the embodiment. DETAILED DESCRIPTION
[0018] The present invention is further described in detail below in conjunction with embodiments and drawings, but the embodiments of the present invention are not limited thereto.
[0019] Example like Figure 1 As shown, the present invention, an OAM mode modulator based on a torsional antiresonant hollow core fiber, comprises a torsional antiresonant hollow core fiber; The twisted antiresonant hollow core fiber is provided with a uniform and periodic spiral twist structure in the axial direction, which is used to make the incident light undergo a uniform and periodic spiral refractive index modulation, thereby coupling the incident light to a higher-order mode and carrying OAM, generating an OAM beam. Since the OAM beam is generated by a uniform and periodic spiral structure, it is little affected by the polarization direction of the electric field, so different input polarization states can excite high-order OAM modes.
[0020] The OAM mode modulator of this embodiment is an all-fiber device, which can directly excite the OAM mode through a uniform and periodic spiral structure, does not require additional control devices, is directly compatible with the optical fiber communication system, is easy to prepare, and has low cost.
[0021] In this embodiment, the twisted anti-resonant hollow-core optical fiber is specifically a single-ring seven-cladding tube anti-resonant hollow-core optical fiber, such as Figure 2 The figure shows a cross-sectional schematic diagram of the optical fiber, which realizes antiresonance effect and suppresses mode coupling through a seven-cladding tube microstructure. The torsional antiresonance hollow core optical fiber includes a cladding, a geometric microstructure and an air core; the cladding provides support for the geometric microstructure, the geometric microstructure realizes antiresonance effect and suppresses mode coupling, and light is transmitted in the air core.
[0022] In another embodiment, a method for preparing a polarization-insensitive and polarization-controllable OAM mode modulator based on a torsional antiresonant hollow core fiber is also provided, comprising: S1. Use finite element simulation software to calculate the optical properties of the anti-resonant hollow core fiber and collect electric field data of different modes of the anti-resonant hollow core fiber; In this embodiment, based on the cross section of the antiresonant hollow core fiber, the optical characteristics thereof are calculated using finite element software and the electric field data of the antiresonant hollow core fiber supported mode calculated by the finite element software are collected.
[0023] S2. Determine the order of the excited OAM mode based on the topological charge matching theory, and provide the target mode for calculating the mode coupling coefficient; specifically: The order of the excited OAM mode is determined according to the topological charge matching formula: l_p+l_q±m=0; Among them, l_p and l_q represent the topological charges carried by modes p and q respectively, m represents the geometric symmetry of the antiresonant hollow core fiber, and + and - represent whether the handedness of the torsional antiresonant hollow core fiber is left or right. Specifically, for example, when m=1, for a right-handed torsional antiresonant hollow core fiber, the Gaussian base mode with l_p=0 can be converted into an OAM mode with l_q=1.
[0024] Since the manufacturing error of the single-ring seven-cladding tube structure antiresonant hollow-core optical fiber destroys its seven-fold symmetry, m can be regarded as 1 in this embodiment, and the absolute value of the topological charge of the OAM beam excited by the sample of this embodiment is 1.
[0025] S3. Calculate the mode coupling coefficient of the fundamental mode coupling to the higher-order OAM mode based on the mode coupling theory and the torsion model. In this embodiment, the specific formula is: ; in, and represents the angular frequency and the dielectric constant of vacuum; N and N q Indicates the change in the refractive index of the material before and after twisting; E p and E q denote the electric field distribution of mode p and mode q respectively; the superscript * denotes conjugation; H p represents the magnetic field distribution of mode p; u z Represents the unit vector in the z direction; N is obtained through the torsion matrix, which is expressed as: ; Where α represents the torsion rate applied to the optical fiber, based on which the Cartesian coordinates can be Convert to spiral coordinates On this basis, the mode coupling coefficient is calculated by the data collected by the finite element software, and the efficiency of mode conversion can be evaluated by the mode coupling coefficient.
[0026] S4. Setting parameters of the torsional antiresonant optical fiber sample during the preparation process according to the mode coupling theory; specifically: According to mode coupling theory: ; ; Among them, P a (z) and P b (z) represents the normalized power variation of mode a and mode b along the z-axis direction, represents the maximum coupling efficiency; represents the difference in propagation constants between mode a and mode b under the influence of torsion rate α; , is the mode coupling coefficient; z represents the length of the prepared torsional antiresonant fiber sample; according to the mode coupling theory, a reasonable setting of the torsion rate α can make the maximum coupling efficiency F=1, and setting an appropriate length z can ensure that the mode conversion efficiency reaches the maximum coupling efficiency.
[0027] Based on the mode coupling theory and the twist model, the mode coupling coefficient of the fundamental mode coupling to the first-order OAM mode is calculated. The order of magnitude of the mode coupling coefficient is 10 cubed, which indicates that the first-order OAM can be generated at a smaller twist rate. A smaller twist rate can maintain the microstructure of the antiresonant hollow-core fiber to a great extent and ensure a uniform periodic spiral structure.
[0028] S5, using an improved arc discharge method to twist the anti-resonant hollow core optical fiber; in this embodiment, Figure 3 As shown, the arc discharge device includes: a pair of electrodes, two rotating motors and two horizontal displacement motors; The electrodes are used to discharge to form a heating field to keep the antiresonant hollow core optical fiber in a molten state; the two horizontal displacement motors are used to make the antiresonant hollow core optical fiber pass through the arc discharge heating field at a uniform speed; the rotary motor is used to twist the antiresonant hollow core optical fiber in a molten state in the heating field; When the optical fiber is in a molten state, gravity will cause it to naturally bend downward. A differential speed is set between the left and right horizontal displacement motors to eliminate the bending of the antiresonant hollow-core optical fiber caused by gravity and torsional stress in the molten state; the speed of one of the horizontal displacement motors is V2, and the speed of the other horizontal displacement motor is V1. The speeds of the horizontal displacement motors are set so that V2 is greater than V1. In this embodiment, a left-handed torsional antiresonant hollow-core optical fiber is prepared.
[0029] The height of both ends of the antiresonant hollow-core fiber is controlled by the V-groove to keep the left and right sides aligned horizontally, avoiding eccentricity during the twisting process and preventing uneven samples caused by eccentric twisting; Control the distance between the fixture and the V-groove to improve the eccentric torsion caused by the mismatch between the large core diameter fiber and the fixture, which leads to the fiber tilting.
[0030] When twisting the antiresonant hollow core fiber, the electrode spacing is controlled to form a larger heating field, ensuring that the large diameter fiber is in a uniform molten state, ensuring the uniformity of the sample and maintaining the geometric microstructure of the antiresonant hollow core fiber.
[0031] When the optical fiber is in a molten state, the rotary motor is set to twist the antiresonant hollow core optical fiber at a speed of Ω, and the twist rate α applied to the optical fiber is calculated by the following formula: α=(V2*π / 180) / Ω; The working time T of the rotating motor is adjusted to control the length L of the prepared torsional antiresonant hollow-core optical fiber, L=V2*T, and finally the torsional antiresonant hollow-core optical fiber is prepared. like Figure 4 As shown, it is verified that the OAM beam excited by the OAM mode modulator of this embodiment is excited by a uniform and periodic spiral structure. When the length of the antiresonant hollow core fiber is short, the transmission of the high-order mode can be effectively maintained. In order to ensure that the OAM beam output by the torsion sample is generated by fundamental mode conversion, control is applied at the coupling end to ensure that the input is a high-base mode and the output is maintained as a high-base mode to avoid misaligned excitation of high-order modes. On this basis, it can be ensured that the OAM beam generated by the torsion antiresonant hollow core fiber OAM mode modulator is due to the uniform and periodic refractive index modulation applied by its axial spiral structure, thereby converting the high-base mode into the OAM mode.
[0032] like Figure 5 The figure shows the test optical path used to test the mode field distribution, polarization characteristics and interference fringes of the OAM beam excited by the torsional antiresonant hollow core fiber sample. The input Gaussian light is generated by a tunable laser, and after being released by an erbium-doped fiber amplifier, it is split into two beams by a 90:10 fiber coupler.
[0033] Among them, the upper part is the main light path, and 90% of the light beam separated by the coupler is converted into spatial light through a collimator. First, add polarizer 1 to the spatial light path to control the polarization state of the Gaussian beam. Secondly, add a 1 / 2 wave plate to polarizer 1 to generate a Gaussian beam with any linear polarization direction. Finally, add a 1 / 4 wave plate after the 1 / 2 wave plate to convert the linearly polarized light into left circular polarization (LCP) and right circular polarization (RCP). These three devices can form a Gaussian beam with any polarization direction. The Gaussian beam is coupled into the antiresonant hollow core fiber through a 40x objective lens. In the untwisted region, the input light is transmitted in the form of a Gaussian base mode, and then input into the twisted antiresonant hollow core fiber region as a Gaussian base mode. The light beam output from the twisted antiresonant hollow core fiber is collimated by a 20x objective lens, then the stray light is filtered out by an aperture, and then the polarization characteristics of the output beam are verified by polarizer 2, and finally directly irradiated on the CCD camera through a beam splitter. The lower part is the reference light path. The polarization controller controls the polarization state of the output light of the reference light path. The attenuator is used to control the intensity of the reference beam. The reference light is converted into a spherical wave using a spherical lens, and finally directly irradiated onto the CCD camera through a beam splitter. When the reference light path is shielded, the mode field distribution of the output light beam of the main light path can be directly observed using a CCD camera, and the polarization controllable characteristics of the output light beam of the main light path can be tested through polarizer 1, 1 / 2 wave plate, 1 / 4 wave plate and polarizer 2. When the output light of the reference light path and the main light path passes through the beam splitter coaxially, interference occurs. The interference fringes can be directly observed through a CCD camera to determine whether the output light of the main light path is an OAM beam and determine its order, and the polarization insensitivity of the output light beam of the main light path can be tested by controlling polarizer 1, 1 / 2 wave plate and 1 / 4 wave plate.
[0034] like Figure 6 The figure shows the test results of the polarization insensitivity of the OAM beam excited at a wavelength of 1562nm. By controlling the polarizer 1, 1 / 2 wave plate and 1 / 4 wave plate, the conversion of any polarization state can be achieved. The input polarization is controlled to linear polarization 0°, linear polarization 45°, linear polarization 90°, linear polarization 135°, LCP and RCP, and the reference light path is shielded. Through CCD observation, it is found that under different input polarization states, the mode field distribution of the output beam of the main light path has no obvious change. The main light path and the reference light path are made to interfere coaxially, and through CCD observation, it is found that the interference fringes formed under different input polarization states are all single spiral counterclockwise rotating fringes.
[0035] like Figure 7As shown in the figure, it is a test result diagram of the output polarization controllable characteristics of the OAM beam excited at a wavelength of 1562nm. Shield the reference light path, and by controlling polarizer 1, 1 / 2 wave plate, 1 / 4 wave plate and polarizer 2, it is possible to input a beam of arbitrary polarization state while verifying the polarization state of the output beam of the main light path. The input Gaussian beam polarization is controlled to be linearly polarized a and linearly polarized b, where linear polarization a and linear polarization b are perpendicular to each other. By rotating polarizer 2 at intervals of 90°, it can be seen that the light spot on the CCD camera changes periodically from light to dark at intervals of 90°. The above phenomenon shows that when a linearly polarized Gaussian beam is input, the torsion antiresonant hollow core fiber outputs a linearly polarized OAM beam. When the input Gaussian beam polarization is controlled to be circularly polarized light, where LCP and RCP are two circularly polarized lights with completely opposite spin directions, and polarizer 2 is rotated at intervals of 90°, it can be seen that the light spot on the CCD camera does not change significantly. When a circularly polarized Gaussian beam is input, the torsion antiresonant hollow core fiber outputs a circularly polarized OAM beam. In summary, the OAM mode modulator of this embodiment is a device that can control the output polarization by controlling the input polarization, that is, it has a polarization controllable characteristic.
[0036] It should also be noted that in this specification, terms such as "comprises", "includes" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also includes other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of more restrictions, an element defined by the sentence "comprises a ..." does not exclude the presence of other identical elements in the process, method, article or device including the element.
[0037] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.
Claims
1. An OAM mode modulator based on torsional antiresonant hollow core fiber, characterized in that: including torsional antiresonant hollow core optical fibers; The twisted antiresonant hollow core optical fiber is provided with a uniform and periodic spiral twist structure in the axial direction, which is used to make the incident light undergo a uniform and periodic spiral refractive index modulation, thereby coupling the incident light to a higher-order mode and carrying OAM, thereby generating an OAM beam.
2. The OAM mode modulator according to claim 1, characterized in that: The twisted antiresonant hollow core optical fiber is a single-ring seven-cladding tube antiresonant hollow core optical fiber.
3. The OAM mode modulator according to claim 1, characterized in that: The torsional antiresonant hollow core optical fiber includes a cladding, a geometric microstructure and an air core; The cladding provides support for the geometric microstructure, which realizes the anti-resonance effect and suppresses mode coupling, and transmits light in the air core.
4. A method for preparing an OAM mode modulator based on a torsional antiresonant hollow core fiber, characterized in that: include: Finite element simulation software is used to calculate the optical properties of antiresonant hollow core fibers and collect electric field data of different modes of antiresonant hollow core fibers; The order of the excited OAM mode is determined based on the topological charge matching theory, providing the target mode for calculating the mode coupling coefficient; The mode coupling coefficients of the fundamental mode coupling to the higher-order OAM modes are calculated based on the mode coupling theory and the torsion model. The parameters of the torsional antiresonant fiber sample are set according to the mode coupling theory during the preparation process; The antiresonant hollow core optical fiber was twisted using an improved arc discharge method.
5. The preparation method according to claim 4, characterized in that: The order of the excited OAM mode is determined based on the topological charge matching theory, specifically: The order of the excited OAM mode is determined according to the topological charge matching formula: l_p+l_q±m=0; Wherein, l_p and l_q represent the topological charges carried by modes p and q, respectively, m represents the geometric symmetry of the antiresonant hollow-core fiber, and + and - represent the handedness of the torsional antiresonant hollow-core fiber to the left and right, respectively.
6. The preparation method according to claim 4, characterized in that: The mode coupling coefficient of the fundamental mode coupling to the higher-order OAM mode is calculated based on the mode coupling theory and the torsion model. The specific formula is: ； in, and represents the angular frequency and the dielectric constant of vacuum; N and N q Indicates the change in the refractive index of the material before and after twisting; E p and E q denote the electric field distribution of mode p and mode q respectively; the superscript * denotes conjugation; H p represents the magnetic field distribution of mode p; u z Represents the unit vector in the z direction; N is obtained by twisting the matrix, the twist matrix T -1 It is expressed as: ； Where α represents the torsion rate applied to the optical fiber, based on which the Cartesian coordinates Convert to spiral coordinates On this basis, the mode coupling coefficient is calculated through the data collected by the finite element software, and the efficiency of mode conversion is evaluated through the mode coupling coefficient.
7. The preparation method according to claim 6, characterized in that: According to the mode coupling theory, the parameters of the torsional antiresonant fiber sample are set during the preparation process, specifically: According to mode coupling theory: ； ； Among them, P a (z) and P b (z) represents the normalized power variation of mode a and mode b along the z-axis direction, represents the maximum coupling efficiency; represents the difference in propagation constants between mode a and mode b under the influence of torsion rate α; , is the mode coupling coefficient; z represents the length of the prepared torsional antiresonant fiber sample; according to the mode coupling theory, the torsion rate α is set so that the maximum coupling efficiency F=1 and the length z is set to ensure that the mode conversion efficiency reaches the maximum coupling efficiency.
8. The preparation method according to claim 4, characterized in that: The invention adopts an improved arc discharge method to twist the anti-resonant hollow core optical fiber, comprising: A pair of electrodes, two rotation motors and two horizontal displacement motors are provided; The electrodes are used to discharge to form a heating field to keep the antiresonant hollow core optical fiber in a molten state; the two horizontal displacement motors are used to make the antiresonant hollow core optical fiber pass through the arc discharge heating field at a uniform speed; the rotary motor is used to twist the antiresonant hollow core optical fiber in a molten state in the heating field; When the optical fiber is in a molten state, a differential speed is set between the left and right horizontal displacement motors to eliminate the bending phenomenon of the anti-resonant hollow-core optical fiber caused by gravity and torsional stress in the molten state; the speed of one horizontal displacement motor is V2, and the speed of the other horizontal displacement motor is V1, and the speed of the horizontal displacement motor is set so that V2 is greater than V1; The antiresonant hollow-core fiber is controlled by V-grooves to keep both sides in a horizontal position to prevent eccentric twisting from causing uneven samples. Control the distance between the fixture and the V-groove to improve the eccentric torsion caused by the mismatch between the large core diameter fiber and the fixture, which leads to the fiber tilting.
9. The preparation method according to claim 8, characterized in that: When twisting the antiresonant hollow core fiber, the electrode spacing is controlled to form a larger heating field, ensuring that the large diameter fiber is in a uniform molten state, ensuring the uniformity of the sample and maintaining the geometric microstructure of the antiresonant hollow core fiber.
10. The preparation method according to claim 8, characterized in that: When the optical fiber is in a molten state, the rotary motor is set to twist the antiresonant hollow core optical fiber at a speed of Ω, and the twist rate α applied to the optical fiber is calculated by the following formula: α=(V2*π / 180) / Ω; The working time T of the rotating motor is adjusted to control the length L of the prepared torsional antiresonant hollow-core optical fiber, L=V2*T, and finally the torsional antiresonant hollow-core optical fiber is prepared.