A radial high-order orbital angular momentum modulation method based on twisted optical fiber
By using a two-dimensional fiber model based on COMSOL and a twisted fiber sample prepared by a Fujikura fusion splicer, combined with observational optical path verification, effective control of radial high-order OAM was achieved, solving the problem of insufficient radial high-order OAM control in existing technologies, reducing experimental costs and improving the success rate.
Patent Information
- Application Number
- CN202311695522.3
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Filing Date
- 2023-12-12
- Publication Date
- 2026-11-03
- Estimated Expiration
- 2043-12-12
AI Technical Summary
Existing technologies have not thoroughly studied methods for controlling higher-order radial orbital angular momentum, and there is a lack of guidance on setting the fiber torsion rate during experiments, leading to increased time and cost.
Numerical simplification calculations were performed using a two-dimensional fiber model based on COMSOL. Combined with mode coupling theory and phase matching conditions, experimental parameter settings were guided by theoretical simulation. Torsified fiber samples were prepared using the optimized arc discharge technology of the Fujikura FSM-100P+ fusion splicer, and then verified by observation and interference experiments.
This study achieved efficient control of radial high-order OAM in twisted optical fibers, reducing experimental costs, improving sample fabrication success rate, and expanding the application scope of radial high-order OAM control.
Smart Images
Figure CN117761894B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of optical fiber communication technology, specifically relating to a method for controlling radial high-order orbital angular momentum based on torsion optical fiber. Background Technology
[0002] In 1992, Allen et al. discovered that vortex beams contain OAM (orbital angular momentum), and beams carrying OAM have attracted a great deal of attention.
[0003] Currently, OAM beams can be generated and stably transmitted using optical fibers or space optical devices. Compared to space optical devices, optical fibers provide a stable transmission medium for OAM beams, reducing external interference. Helical twisted fiber gratings have proven to be a good choice for generating OAM beams; by periodically modulating the fiber's refractive index, highly efficient fiber OAM mode generators can be produced. The generation of OAM modes in optical fibers has been experimentally demonstrated in twisted photonic crystal fibers, twisted single-mode fibers, twisted hollow-core anti-resonant fibers, and twisted few-mode toroidal fibers. Compared to the aforementioned fibers, elliptical fibers have a core-double-symmetric structure, which can maintain linearly or circularly polarized light. By introducing twist to periodically modulate the refractive index of elliptical fibers, helical twisted elliptical fibers can generate OAM2 or OAM4 modes. However, most existing studies only consider increasing the angular order of OAM, and have not explored radially higher-order OAM in depth. Radially higher-order OAM can provide research ideas for additional dimensions of OAM multiplexing; therefore, it is necessary to study radially higher-order OAM. Summary of the Invention
[0004] The main objective of this invention is to overcome the shortcomings and deficiencies of the prior art and to propose a radial high-order orbital angular momentum control method based on torsion optical fiber.
[0005] To achieve the above objectives, the present invention adopts the following technical solution:
[0006] A method for controlling radial high-order orbital angular momentum based on torsion optical fiber includes the following steps:
[0007] Numerical simplification calculation based on the COMSOL torsion model is performed by replacing the three-dimensional fiber model with a two-dimensional fiber model. The relative permittivity matrix of the two-dimensional torsion fiber model is calculated, and then the coupling coefficient of the torsion fiber mode is calculated using matrix calculation methods.
[0008] By combining mode coupling theory and phase matching conditions, the transformation relationship between the core mode and the cladding mode in the theoretical simulation model and the corresponding torsional rate are obtained.
[0009] The torsion rate, as simulated theoretically, guides the parameter settings for the experimental torsion fiber sample, thus enabling the fabrication of torsion fiber samples.
[0010] The twisted fiber sample was placed in the constructed experimental observation optical path. The radial order was verified by observing the output light spot, and the order of the OAM angular direction was verified by conducting an interference experiment.
[0011] Compared with the prior art, the present invention has the following advantages and beneficial effects:
[0012] 1. The method of the present invention uses the simulated torsion rate obtained by theoretical simulation to guide the setting of torsion parameters in the experiment, roughly determines the fiber torsion rate required for the actual experiment, avoids blindly trying to adjust the fiber torsion rate during the experiment, and thus greatly reduces the time cost of the experiment.
[0013] 2. This invention employs an optimized arc discharge technology based on the Fujikura FSM-100P+ welding machine to prepare the sample, which to a certain extent avoids the inhomogeneity of the twisted sample, improves the success rate of sample preparation, and reduces experimental costs.
[0014] 3. The method of the present invention realizes the control of radial higher-order OAM of different claddings at 1550nm in twisted optical fibers; the method can not only be used to control radial higher-order OAM in twisted elliptical optical fibers, but also to twist other types of optical fibers to achieve control of radial higher-order OAM. Attached Figure Description
[0015] Figure 1 This is a flowchart of the method of the present invention;
[0016] Figure 2 This is a schematic diagram of the optimized arc discharge technology fabrication process based on the Fujikura FSM-100P+ welding machine;
[0017] Figure 3 This is a schematic diagram of the optical path used in the experimental observation.
[0018] Figure 4 This is a comparison diagram of the input modes, output results, and interference results of different radial high-order OAMs for left-handed spiral twisted elliptical fibers in the embodiment;
[0019] Figure 5 This is a comparison chart of the torsion rate between theoretical simulation and experimental results in the embodiments. Detailed Implementation
[0020] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto.
[0021] like Figure 1 As shown, this invention provides a method for controlling radial high-order orbital angular momentum based on torsion optical fiber, comprising the following steps:
[0022] Numerical simplification based on the COMSOL torsion model involves replacing the three-dimensional fiber model with a two-dimensional fiber model. The relative permittivity matrix of the two-dimensional torsion fiber model is calculated, and then matrix calculations are used to calculate the coupling coefficient of the torsion fiber mode. Specifically:
[0023] The optical fiber model under actual conditions is drawn using CAD software, and the simulation is made to fit the actual conditions as closely as possible. The model drawn in the CAD software is then imported into COMSOL, where the torsion rate and helical coordinate system are introduced. By setting the torsion matrix, the two-dimensional optical fiber torsion model is used to replace the three-dimensional optical fiber torsion model.
[0024] By deriving the refractive index distribution of a two-dimensional untwisted fiber model and combining it with the torsion matrix, the relative permittivity matrix of the two-dimensional torsion fiber model is calculated using a formula. The fiber mode coupling coefficient is then calculated using matrix calculation methods. The two-dimensional untwisted fiber model is the simulation model without torsion, while the two-dimensional torsion fiber model is the simulation model that incorporates torsion and a helical coordinate system.
[0025] The torsion matrix is:
[0026]
[0027] The formula for calculating the relative permittivity is:
[0028] ε' r =ε r T -1 =n 2 T -1
[0029] The formula for calculating the fiber mode coupling coefficient is:
[0030]
[0031] Where ξ1 and ξ2 are helical coordinate systems, α is the torsion required for fiber conversion, n is the refractive index of the fiber, ω is the angular frequency, ε0 is the dielectric constant in vacuum, I is the identity matrix, and E m and E n The electric field distributions of the untwisted fiber model and the twisted fiber model are shown below, P. m and P n These represent the power values for the untwisted fiber model and the twisted fiber model, respectively.
[0032] By combining mode coupling theory and phase matching conditions, the transformation relationship between the core mode and the cladding mode in the theoretical simulation model, as well as the corresponding torsional rate, are obtained, as follows:
[0033] By simulating the torsion model, the numerical characteristics (such as mode propagation constants) of the core mode and cladding mode are derived.
[0034] Based on the angular momentum matching condition and the phase matching condition, the conversion relationship between the core mode and the cladding mode, as well as the torsional rate required for the conversion, are derived.
[0035] The torsion rate, as simulated theoretically, guides the parameter settings for experimental torsion fiber samples, enabling the fabrication of high-precision torsion fiber samples. Specifically:
[0036] Based on the torsion rate obtained from theoretical simulation, the actual process parameters for manufacturing torsion fiber samples were set. During manufacturing, the torsion rate of the fiber was controlled by controlling the rotation speed of the left rotary motor and the movement speed of the two left and right translation motors inside the fusion splicer, based on the arc discharge technology of the Fujikura FSM-100P+ fusion splicer. The straightness of the torsion fiber sample was achieved by maintaining a certain speed difference between the two left and right translation motors and controlling the distance between the control electrodes.
[0037] The twisted fiber sample was placed in the constructed experimental observation optical path. The radial order was verified by observing the output light spot, and the order of the OAM angular direction was verified by conducting an interference experiment.
[0038] The experimental observation optical path specifically includes:
[0039] Tunable lasers, erbium-doped fiber amplifiers, fiber couplers, collimators, 40x objectives, 20x objectives, single-mode fibers, optical attenuators, polarizers, spherical lenses, beam splitters, and CCD detectors;
[0040] During the experiment, the input Gaussian light was emitted by a tunable laser. After being amplified by an erbium-doped fiber amplifier, the light was split into two beams by a fiber coupler: the main optical path and the secondary optical path.
[0041] The main optical path is the optical path for the experimental sample, used to observe the sample output results. It is sequentially set with a collimator, a 40x objective lens, a twisted fiber sample, and a 20x objective lens.
[0042] The secondary optical path serves as a reference optical path, used to interfere with the output of the main optical path. It consists of a polarizer, a single-mode fiber, an optical attenuator, a collimator, and a spherical lens arranged sequentially. The polarizer converts the output beam into linear polarization; the optical attenuator controls the power of the reference beam to obtain a clear interference pattern; and the spherical lens converts the plane wave into a spherical wave.
[0043] The light from the main and secondary optical paths is converged by a beam splitter onto the photosensitive area of the CCD detector, and the CCD detector observes the output light spot and the interference pattern of the generated OAM mode.
[0044] During the experiment, a certain length of untwisted fiber can be kept at the input front end of the twisted fiber sample to ensure that Gaussian light is input into the twisted fiber sample.
[0045] The radial order was verified by observing the output light spot, and the angular order of the OAM was verified by conducting an interference experiment. Specifically:
[0046] The optical path was observed in the experiment to see if the output spot of the twisted fiber sample at 1550nm matched the radial higher order of the simulation results. If it matched, the torsion rate corresponding to other radial higher order OAMs was calculated based on the difference range between the simulation and experimental torsion rates, and an interference experiment was conducted to verify the order of the OAM in the angular direction. If it did not match, the twisted fiber sample was remanufactured after fine-tuning the torsion rate, and then it was observed whether it matched the radial higher order of the simulation results.
[0047] Example
[0048] The following example illustrates the conversion from the core fundamental mode to a higher-order radial OAM in a doubly symmetric twisted elliptical fiber. 2n The present invention will be further described in detail using the model as an example.
[0049] Using the cross-section of an elliptical optical fiber as a reference, a geometric model of the fiber was drawn using AutoCAD. The DXF file generated by AutoCAD was then imported into the full-vector finite element modeling software COMSOL to simulate the modal characteristics of the fiber under experimental conditions. By introducing a helical coordinate system, the consistency of material properties in the fiber was maintained after the coordinate transformation was helically twisted.
[0050] In this embodiment, the elliptical fiber is twisted in the molten state to form a doubly symmetrical twisted fiber. According to the angular momentum matching condition of formula (1), it can be known that the doubly symmetrical twisted elliptical fiber can convert the Gaussian beam input to the fiber core into OAM. 2n or OAM 4n According to the phase matching condition in formula (2), the propagation constants of the fundamental mode and the radial higher-order mode can be obtained in the untorsed model in COMSOL, thereby calculating the torsion rate required to match the corresponding higher-order mode.
[0051] AM1-AM2±N=0(1)
[0052] β1-β2+Nα=0 (2)
[0053] Where AM1 and AM2 are the angular momentum corresponding to the core mode and cladding mode, respectively; β1 and β2 are the propagation constants corresponding to the core mode and cladding mode, respectively; N is the symmetry of the optical fiber; and α is the twist rate required for optical fiber conversion.
[0054] Taking a left-handed helix as an example, such as... Figure 2As shown, twisted optical fiber samples were prepared using an optimized arc discharge technology based on the Fujikura fusion splicer FSM-100P+.
[0055] When the power of the electrode discharge brings the optical fiber to a molten state, gravity will cause the fiber to sag naturally. To eliminate the impact of this sag on the quality of the finished product, the two translation motors on the left and right need to maintain a certain speed difference, such as V. l >V r The fiber is then shifted to the left, while the left-side rotary motor rotates at a certain speed θ. The twist rate of the twisted sample fiber can be expressed by the formula α=(θ×π / 180) / V. l Calculations, such as achieving a torsion sample with a torsion rate of 12.08 rad / mm, can be performed by setting V... l =0.052mm / s, θ =0.036° / ms; the length L of the twisted sample can be controlled by controlling the motor's working time T, and L can be expressed by the equation L = V l *T is obtained.
[0056] like Figure 3 The diagram shows the experimental optical path for observing the light spot and interference generated by the twisted fiber sample. During the experiment, the input Gaussian light was emitted from a tunable laser. After being amplified by an erbium-doped fiber amplifier, the light was split into two beams by a 90:10 fiber coupler. The upper part is the main optical path. The 90% beam split by the coupler passes through a collimator and a 40x objective lens, and is input into the twisted elliptical fiber sample to generate different radial high-order OAM modes. It is then collimated by a 20x objective lens and directed to the beam splitter. The lower part is the reference optical path. A polarizer converts the output beam into linear polarization, an attenuator controls the power of the reference beam to obtain a clear interference pattern, and a spherical lens converts the plane wave into a spherical wave. The two beams from the main and secondary optical paths are converged by the beam splitter and strike the photosensitive area of the CCD detector, allowing the CCD detector to observe the output light spot and the interference pattern of the generated OAM modes.
[0057] like Figure 4 The figure shows a comparison of the input modes, output results, and interference results of different radial higher-order OAM modes for left-handed helical twisted elliptical fibers. By adjusting the twist rate, it was observed that the twisted samples with different twist rates produced their respective radial higher-order modes. To verify the characteristics of the helical phase, the output mode was interfered with the Gaussian light of the reference optical path. From the vortex fringes of the interference results, it can be concluded that a second-order OAM mode was successfully generated at a wavelength of 1550 nm.
[0058] like Figure 5The figure shows a comparison of the torsion rates from theoretical simulation and experimental results. It can be observed that there is a certain difference between the experimental and simulated torsion rates, but the differences between the experimental and simulation results show a high degree of consistency. Furthermore, the trends of the torsion rate differences in adjacent radial higher orders are very similar. Therefore, this difference and the adjacent trends can be used to guide the experimental setup using the simulation results.
[0059] It should also be noted that, in this specification, terms such as "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.
[0060] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily 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 invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
Claims
1. A method for controlling radial high-order orbital angular momentum based on torsion optical fiber, characterized in that, Includes the following steps: Numerical simplification calculation based on the COMSOL torsion model is performed by replacing the three-dimensional fiber model with a two-dimensional fiber model. The relative permittivity matrix of the two-dimensional torsion fiber model is calculated, and then the coupling coefficient of the torsion fiber mode is calculated using matrix calculation methods. By combining mode coupling theory and phase matching conditions, the transformation relationship between the core mode and the cladding mode in the theoretical simulation model and the corresponding torsional rate are obtained. The torsion rate, as simulated theoretically, guides the parameter settings for experimental torsion fiber samples, thus enabling the fabrication of torsion fiber samples. The twisted fiber sample was placed in the constructed experimental observation optical path. The radial order was verified by observing the output light spot, and the order of the OAM angular direction was verified by conducting an interference experiment. Specifically: The optical path was observed experimentally to see if the output spot of the twisted fiber sample at 1550nm matched the radial higher order of the simulation results. If it matched, the twist rate corresponding to other radial higher order OAMs was calculated based on the difference range between the simulation and experimental twist rates, and an interference experiment was conducted to verify the order of the OAM in the angular direction. If it did not match, the twisted fiber sample was remanufactured after fine-tuning the twist rate, and then it was observed whether it matched the radial higher order of the simulation results. The twisted optical fiber is a twisted elliptical optical fiber.
2. The radial high-order orbital angular momentum control method based on torsion optical fiber according to claim 1, characterized in that, Numerical simplification based on the COMSOL torsion model involves replacing the three-dimensional fiber model with a two-dimensional fiber model. The relative permittivity matrix of the two-dimensional torsion fiber model is calculated, and then the coupling coefficient of the torsion fiber mode is calculated using matrix computation methods. Specifically: A fiber optic model based on actual conditions is drawn using CAD software to maximize the simulation's fit to real-world conditions. The model drawn in CAD software is then imported into COMSOL, where torsion rate and helical coordinates are introduced. By setting the torsion matrix, a two-dimensional fiber optic torsion model is used to replace the three-dimensional fiber optic torsion model. By deriving the refractive index distribution of a two-dimensional untwisted fiber model and combining it with the torsion matrix, the relative permittivity matrix of the two-dimensional torsion fiber model is calculated using a formula. The fiber mode coupling coefficient is then calculated using matrix calculation methods. The two-dimensional untwisted fiber model is the simulation model without torsion, while the two-dimensional torsion fiber model is the simulation model with torsion and helical coordinates. The torsion matrix is: The formula for calculating the relative permittivity is: The formula for calculating the fiber mode coupling coefficient is: Where ξ1 and ξ2 are helical coordinates, α is the twist rate required for fiber conversion, n is the refractive index of the fiber, ω is the angular frequency, ε0 is the dielectric constant in vacuum, and I is the identity matrix. and E n The electric field distributions of the untwisted fiber model and the twisted fiber model are shown below, P. m and P n These represent the power values for the untwisted fiber model and the twisted fiber model, respectively.
3. The radial high-order orbital angular momentum control method based on torsion optical fiber according to claim 1, characterized in that, By combining mode coupling theory and phase matching conditions, the transformation relationship between the core mode and the cladding mode in the theoretical simulation model, as well as the corresponding torsional rate, are obtained, as follows: By simulating the torsion model, the numerical characteristics of the core mode and cladding mode are derived. Based on the angular momentum matching condition and the phase matching condition, the conversion relationship between the core mode and the cladding mode, as well as the torsional rate required for the conversion, are derived. The angular momentum matching condition is: AM1-AM2±N=0 The phase matching condition is: β1-β2+Nα=0 Where AM1 and AM2 are the angular momentum orders corresponding to the core mode and cladding mode, respectively; β1 and β2 are the propagation constants corresponding to the core mode and cladding mode, respectively; N is the symmetry of the optical fiber; and α is the twist rate required for optical fiber conversion.
4. The radial high-order orbital angular momentum control method based on torsion optical fiber according to claim 1, characterized in that, The fabrication of twisted optical fiber samples is specifically as follows: Based on the torsion rate obtained from theoretical simulation, the actual process parameters for manufacturing torsion fiber samples were set. During manufacturing, the torsion rate of the fiber was controlled by controlling the rotation speed of the left rotary motor and the movement speed of the two left and right translation motors inside the fusion splicer, based on the arc discharge technology of the Fujikura FSM-100P+ fusion splicer. The straightness of the torsion fiber sample was achieved by maintaining a certain speed difference between the two left and right translation motors and controlling the distance between the control electrodes.
5. The radial high-order orbital angular momentum control method based on torsion fiber according to claim 1, characterized in that, The experimental observation optical path specifically includes: Tunable lasers, erbium-doped fiber amplifiers, fiber couplers, collimators, 40x objectives, 20x objectives, single-mode fibers, optical attenuators, polarizers, spherical lenses, beam splitters, and CCD detectors; During the experiment, the input Gaussian light was emitted by a tunable laser. After being amplified by an erbium-doped fiber amplifier, the light was split into two beams by a fiber coupler: the main optical path and the secondary optical path. The main optical path is the optical path for the experimental sample, used to observe the sample output results. It is sequentially set with a collimator, a 40x objective lens, a twisted fiber sample, and a 20x objective lens. The secondary optical path serves as a reference optical path, used to interfere with the output of the main optical path. It consists of a polarizer, a single-mode fiber, an optical attenuator, a collimator, and a spherical lens arranged sequentially. The polarizer converts the output beam into linear polarization; the optical attenuator controls the power of the reference beam to obtain a clear interference pattern; and the spherical lens converts the plane wave into a spherical wave. The light from the main and secondary optical paths is converged by a beam splitter onto the photosensitive area of the CCD detector, and the CCD detector observes the output light spot and the interference pattern of the generated OAM mode.
6. The radial high-order orbital angular momentum control method based on torsion optical fiber according to claim 5, characterized in that, During the experiment, a certain length of untwisted fiber can be kept at the input front end of the twisted fiber sample to ensure that Gaussian light is input into the twisted fiber sample.