An OAM mode converter based on fiber grating with ultra-wideband and a manufacturing method thereof

Abstract

The application provides an OAM mode converter based on a fiber grating ultra-wideband and a manufacturing method thereof, and relates to the technical field of photonics. The OAM mode converter comprises a first polarization controller, a mode stripper, a signal generator, a power amplifier and a second polarization controller. The first polarization controller is used for receiving an optical signal and adjusting a polarization state. The mode stripper is used for filtering the adjusted optical signal and only retaining a fundamental mode optical signal. The signal generator is used for generating an electrical signal. The power amplifier is used for power amplifying the electrical signal to obtain an amplified electrical signal. The second polarization controller is used for receiving the high-order mode optical signal and adjusting a relative amplitude and a phase to form an orbital angular momentum mode. The acoustic-optic converter is used for receiving the fundamental mode optical signal and converting the fundamental mode optical signal into a high-order mode optical signal through an acoustic-optic effect under the driving of the amplified electrical signal, and the acoustic-optic converter is realized by using a tapered etching four-mode graded fiber. The application realizes a working range with a bandwidth width far exceeding 1000 nm and high conversion efficiency.

Description

Technical Field

[0001] This invention relates to the field of photonics technology, and in particular to an ultrawideband OAM mode converter based on a fiber Bragg grating and its fabrication method. Background Technology

[0002] In recent years, research on fiber optic fields has increasingly focused on those with unique amplitudes, polarization patterns, and phases. Among these, vortex fields capable of generating orbital angular momentum have attracted the most attention.

[0003] Unlike common plane waves, which have a uniform wavefront and spatially uniform phase distribution, vortex beams are characterized by a wave vector that rotates along the optical axis to form a helical surface. Furthermore, the wavefront phase of a vortex beam is distributed in a helical shape, resulting in a non-uniform phase across the cross-section of the wave vector. This is expressed as exp(iℓφ), where φ is the angular component in cylindrical coordinates. This helical phase distribution results in vortex beams possessing orbital angular momentum. Each photon in a vortex beam has an orbital angular momentum of ℓh, where ℓ represents the topological charge and can only take any integer value. Then, i is the imaginary unit used to describe the complex form of the phase.

[0004] Besides possessing unique phase information, a prominent characteristic of orbital angular momentum is that the wavefront of a vortex beam is distributed in a helical pattern around the optical axis. Therefore, vortex beams have singularities and lack energy. These characteristics have led to the widespread application of vortex beams in areas such as particle micromanipulation and optical communication.

[0005] Work on the generation of optical vortices and orbital angular momentum is relatively mature, with numerous methods available. These methods can be broadly categorized into two types: one utilizes spatial structures, such as spiral phase plates, diffraction elements, spatial light modulators, and the superposition of special beams; the other involves generation within optical fibers. Compared to spatial generation, fiber-based vortex generation significantly simplifies the optical structure, employing methods such as phase-coupled element methods, fiber couplers, and fiber grating methods. Fiber grating methods can be further classified by fabrication method into chiral gratings, stress-twisted fiber gratings, point-by-point writing fiber gratings, mask-based fiber gratings, and acousto-induced fiber gratings. Generating optical vortices using gratings is currently a major research focus.

[0006] Fiber Bragg gratings (FBGs), a key technology for generating optical vortices in the fiber domain, can be further subdivided into various types based on fabrication processes, including chiral FBGs, stress-twisted FBGs, point-by-point writing FBGs, mask-based FBGs, and acousto-induced FBGs. Using FBGs to achieve optical vortex and orbital angular momentum (OAM) mode output offers advantages such as relatively simple structure, high stability, and high conversion efficiency. However, despite these advantages, a fundamental and pervasive fatal weakness limits almost all existing fiber-integrated OAM mode generators and converters: their notoriously narrow operating bandwidth. While current fiber dynamic OAM mode generation and conversion technologies possess high efficiency and purity, their extremely narrow operating bandwidth makes them unsuitable for wavelength division multiplexing (WDM) technology, directly limiting the potential for OAM capacity improvement. This bandwidth issue further restricts the development of optical information and optical sensing fields. Summary of the Invention

[0007] To address the shortcomings of the existing technologies, this invention utilizes a novel mechanism called equivalent continuous phase modulation acousto-optic interaction, implemented in a specially designed few-mode fiber (FMF), specifically a graded-index four-mode fiber (GIFMF). This FMF exhibits a parabolic refractive index distribution. Furthermore, the influencing factors of the dispersion turning point (DTP) are investigated. A fiber grating-based ultrawideband OAM mode converter and its fabrication method are proposed, aiming to achieve an operating bandwidth far exceeding 1000 nm with high mode conversion efficiency.

[0008] On one hand, this invention proposes an ultra-wideband OAM mode converter based on fiber Bragg gratings, the OAM mode converter comprising:

[0009] A first polarization controller is used to receive light signals from a light source and adjust the polarization state of the light signals; the output of the first polarization controller is connected to the input of a mode stripper.

[0010] The mode stripper is used to filter the adjusted optical signal, retaining only the fundamental mode LP01 optical signal; the output of the mode stripper is connected to the optical signal input of the acousto-optic converter.

[0011] A signal generator is used to generate sine and cosine electrical signals; the output of the signal generator is connected to the input of a power amplifier.

[0012] The power amplifier is used to amplify the sine and cosine electrical signals to obtain an amplified electrical signal; the output terminal of the power amplifier is connected to the electrical signal input terminal of the acousto-optic converter.

[0013] The acousto-optic converter is used to receive the fundamental mode LP01 optical signal and, driven by the amplified electrical signal, form a dynamic fiber optic grating through the acousto-optic effect to convert the fundamental mode LP01 optical signal into a higher-order mode LP11 optical signal; the output terminal of the acousto-optic converter is connected to the input terminal of the second polarization controller.

[0014] The second polarization controller is used to receive the higher-order mode LP11 optical signal and adjust the relative amplitude and phase of the higher-order mode LP11 optical signal to form an orbital angular momentum mode.

[0015] Furthermore, the acousto-optic converter includes: a piezoelectric ceramic, an aluminum conical and a conical etched four-mode graded fiber assembly;

[0016] The piezoelectric ceramic serves as the electrical signal input terminal of the acousto-optic converter; the piezoelectric ceramic is adhered to the bottom of the aluminum cone; the tip of the aluminum cone is adhered to the tapered etched region in the tapered etched four-mode graded fiber assembly; one end of the tapered etched four-mode graded fiber assembly is the optical signal input terminal of the acousto-optic converter; the other end of the tapered etched four-mode graded fiber assembly is the output terminal of the acousto-optic converter.

[0017] Furthermore, the acousto-optic converter also includes a copper plate, on which the piezoelectric ceramic is adhered.

[0018] Furthermore, the tapered etched four-mode graded fiber assembly includes: a first single-mode fiber, a tapered etched four-mode graded fiber, and a second single-mode fiber.

[0019] One end of the first single-mode fiber serves as the optical signal input end of the acousto-optic converter, and the other end is fused to one end of the tapered etched quadrature fiber; the other end of the tapered etched quadrature fiber is fused to one end of the second single-mode fiber; the other end of the second single-mode fiber serves as the output end of the acousto-optic converter.

[0020] Furthermore, the tapered etched four-mode graded fiber is sequentially divided into: a first cylindrical transition region, a tapered etched region, and a second cylindrical transition region;

[0021] The tapered etched region is the bare fiber structure obtained by tapering the quadrature fiber, and has a mirrored double-tapered profile. The first cylindrical transition region and the second cylindrical transition region are both unetched quadrature fibers, retaining the original diameter and coating of the quadrature fiber. The first cylindrical transition region is used for fusion splicing with the first single-mode fiber, and the second cylindrical transition region is used for fusion splicing with the second single-mode fiber.

[0022] The radial etching depth of the conical etching zone exhibits a Gaussian function distribution along the axial direction.

[0023] Furthermore, the operating wavelength ranges of the first polarization controller and the second polarization controller are the same.

[0024] On the other hand, this invention proposes a method for fabricating an ultrawideband OAM mode converter based on fiber Bragg gratings, which includes the following steps:

[0025] A section of four-mode graded fiber is selected, and the four-mode graded fiber is divided into a first cylindrical transition region, a tapered etched region, and a second cylindrical transition region according to a preset length.

[0026] The coating layer of the four-mode graded fiber in the tapered etched region is stripped off, and the four-mode graded fiber after the coating layer is stripped is subjected to tapered etch to form a tapered etched region.

[0027] A single-mode fiber is selected as the first single-mode fiber, and the first single-mode fiber is fused to the first cylindrical transition region; another single-mode fiber is selected as the second single-mode fiber, and the second single-mode fiber is fused to the second cylindrical transition region to form a single-mode-tapered four-mode graded fiber-single-mode structure, which serves as a tapered four-mode graded fiber assembly.

[0028] A copper plate, a piezoelectric ceramic, and an aluminum cone are selected. The copper plate and the piezoelectric ceramic are bonded together using UV adhesive. The other side of the piezoelectric ceramic is bonded to the bottom of the aluminum cone. The tip of the aluminum cone is then bonded to the cone-shaped corrosion area using UV-curing adhesive.

[0029] Select a signal generator and a power amplifier, connect the output of the signal generator to the input of the power amplifier, and connect the output of the power amplifier to the piezoelectric ceramic.

[0030] Select a mode stripper and connect the output of the mode stripper to the first single-mode fiber end of the single-mode-tapered four-mode graded fiber-single-mode structure.

[0031] Select a polarization controller as the first polarization controller, and connect the output of the first polarization controller to the input of the mode stripper.

[0032] Another polarization controller is selected as the second polarization controller, and the input end of the second polarization controller is connected to the second single-mode fiber end of the single-mode-tapered four-mode graded fiber-single-mode structure.

[0033] Furthermore, the method for the conical corrosion treatment is as follows:

[0034] The four-mode graded optical fiber is passed through a Teflon tube, which is supported by an adjustable bracket and sealed at both ends with UV-curable adhesive.

[0035] A corrosive liquid is injected through the central hole of the Teflon tube, causing the corrosive liquid to spread laterally along the Teflon tube and etch the four-mode graded optical fiber after the coating layer has been stripped.

[0036] The diffusion distribution of the corrosive liquid within the Teflon tube follows a Gaussian function distribution, causing the etching depth of the four-mode graded optical fiber after the coating layer is stripped to follow a Gaussian function distribution along the axial direction, forming a mirrored double-cone profile.

[0037] The Teflon tube is also provided with two circular holes to remove air bubbles generated during the corrosion process.

[0038] Furthermore, the corrosive liquid is a hydrofluoric acid solution with a mass fraction of 25%.

[0039] The beneficial effects of adopting the above technical solution are as follows:

[0040] This invention utilizes the dynamically tunable characteristics of acousto-induced fiber gratings to excite specific acoustic modes in specially designed few-mode fibers. This effectively decouples the eddy current conversion process from strict wavelength-dependent phase-matching constraints, thereby achieving the generation of ultra-wideband, high-efficiency OAM modes with dynamic reconfigurability. The fabricated mode converter achieves an operating bandwidth far exceeding 1000 nm. Solving the bandwidth problem provides a new solution to improving the information capacity of OAM in practical systems and offers technical support for the research of wide-bandwidth fiber lasers. The development of dynamically tunable ultra-wideband OAM converters is not merely an incremental improvement, but a key technology that is expected to drive the development of photonics and other information fields. Attached Figure Description

[0041] Figure 1 This is a structural diagram of an ultra-wideband OAM mode converter based on a fiber Bragg grating in this embodiment;

[0042] Figure 2 This is a partial structural diagram of the acousto-optic converter in this embodiment;

[0043] Figure 3 This is a diagram showing the internal structure of the tapered etched four-mode graded fiber in this embodiment;

[0044] Figure 4 This is a flowchart illustrating a method for fabricating an ultrawideband OAM mode converter based on a fiber Bragg grating in this embodiment.

[0045] Figure 5The transmission spectrum of the tapered etched fiber grating obtained in Example 2 is shown below.

[0046] Figure 6 This is a schematic diagram illustrating the operation of measuring mode intensity and OAM detection in Example 3;

[0047] Figure 7 The intensity distribution and spiral interference pattern of the generation mode measured in Example 3 are shown below; (a) is the spiral interference pattern corresponding to the 1500nm test light signal; (b) is the spiral interference pattern corresponding to the 1525nm test light signal; and (c) is the spiral interference pattern corresponding to the 1550nm test light signal.

[0048] In the figure: 1-piezoelectric ceramic, 2-aluminum cone, 3-conical etched four-mode graded fiber. Detailed Implementation

[0049] To facilitate understanding of this application, specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and embodiments. The following embodiments are illustrative of the invention but are not intended to limit its scope. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of this application.

[0050] Example 1:

[0051] This embodiment presents an ultra-wideband OAM mode converter based on a fiber Bragg grating, such as... Figure 1 As shown, the OAM mode converter includes:

[0052] A first polarization controller is used to receive light signals from a light source and adjust the polarization state of the light signals; the output of the first polarization controller is connected to the input of a mode stripper.

[0053] The mode stripper is used to filter the adjusted optical signal, retaining only the fundamental mode LP01 optical signal; the output of the mode stripper is connected to the optical signal input of the acousto-optic converter.

[0054] A signal generator is used to generate sine and cosine electrical signals; the output of the signal generator is connected to the input of a power amplifier.

[0055] The power amplifier is used to amplify the sine and cosine electrical signals to obtain an amplified electrical signal; the output terminal of the power amplifier is connected to the electrical signal input terminal of the acousto-optic converter.

[0056] The acousto-optic converter is used to receive the fundamental mode LP01 optical signal and, driven by the amplified electrical signal, form a dynamic fiber optic grating through the acousto-optic effect to convert the fundamental mode LP01 optical signal into a higher-order mode LP11 optical signal; the output end of the acousto-optic converter is connected to the input end of the second polarization controller.

[0057] The second polarization controller is used to receive the higher-order mode LP11 optical signal and adjust the relative amplitude and phase of the higher-order mode LP11 optical signal to form an orbital angular momentum mode.

[0058] In this embodiment, as Figure 1 As shown, a first polarization controller receives an optical signal from a light source and adjusts the polarization state of the optical signal to match the operating characteristics of the mode stripper and the acousto-optic converter; a mode stripper filters out unwanted higher-order mode components from the adjusted optical signal to ensure that only the fundamental mode LP01 optical signal is retained in the optical signal input to the acousto-optic converter; a signal generator generates sine and cosine electrical signals as the electrical signal source to drive the acousto-optic converter; a power amplifier amplifies the sine and cosine electrical signals to output an amplified electrical signal with sufficient power to drive the acousto-optic converter; the acousto-optic converter receives the fundamental mode LP01 optical signal and, driven by the amplified electrical signal, forms a dynamic fiber grating through the acousto-optic effect to convert the fundamental mode LP01 optical signal into a higher-order mode LP11 optical signal; a second polarization controller receives the higher-order mode LP11 optical signal, which contains two degenerate modes (LP11a and LP11b), and forms a high-purity orbital angular momentum mode by adjusting the relative amplitude and phase between the two degenerate modes. In order to monitor the spectral characteristics of the orbital angular momentum mode, this embodiment also connects a spectrometer to the output of the second polarization controller.

[0059] The acousto-optic converter includes: a piezoelectric ceramic 1, an aluminum cone 2, and a tapered four-mode graded fiber assembly.

[0060] like Figure 2 As shown, the piezoelectric ceramic serves as the electrical signal input terminal of the acousto-optic converter; the bottom of the aluminum cone is bonded to the piezoelectric ceramic; the tip of the aluminum cone is bonded to the tapered etched region in the tapered etched four-mode graded fiber assembly; one end of the tapered etched four-mode graded fiber assembly is the optical signal input terminal of the acousto-optic converter; the other end of the tapered etched four-mode graded fiber assembly is the output terminal of the acousto-optic converter.

[0061] The tapered etched quadmode graded fiber assembly includes: a first single-mode fiber, a tapered etched quadmode graded fiber 3, and a second single-mode fiber.

[0062] One end of the first single-mode fiber serves as the optical signal input end of the acousto-optic converter, and the other end is fused to one end of the tapered etched quadrature fiber; the other end of the tapered etched quadrature fiber is fused to one end of the second single-mode fiber; the other end of the second single-mode fiber serves as the output end of the acousto-optic converter.

[0063] In this embodiment, the mode conversion section uses a tapered etched four-mode graded fiber as the basic structure, such as... Figure 3 As shown, the internal structure of the four-mode graded fiber satisfies a unique parabolic refractive index distribution, giving it a unique advantage in terms of wide bandwidth. By solving the dispersion curve of the four-mode graded fiber corresponding to the tapered etched section, the relationship between the grating period and the resonant wavelength is determined using the phase-matching condition, and the dispersion inflection point is identified in the relationship diagram. Based on the dynamic tunability of the acousto-induced fiber grating, the operating wavelength is adjusted to near the dispersion inflection point, thereby achieving ultra-wide bandwidth mode switching.

[0064] Applying acousto-optic effects to a tapered, etched four-mode graded fiber assembly, such as... Figure 2 As shown, the basic working principle of fiber optic grating is as follows: the sine and cosine signals emitted by the signal generator are amplified by the power amplifier and transmitted to the piezoelectric ceramic. Then, due to the inverse piezoelectric effect of the piezoelectric ceramic, ultrasonic waves are generated. The ultrasonic waves are then converged to the tip of the aluminum cone (the part in contact with the bare fiber) to form a standing wave, which is then coupled into the optical fiber. Periodic microbending is formed inside the optical fiber, thereby forming a periodic change in refractive index along the axial direction, thus generating a fiber optic grating.

[0065] The acousto-optic converter also includes a copper plate, on which the piezoelectric ceramic is bonded.

[0066] Since piezoelectric ceramics vibrate to generate ultrasonic waves after receiving electrical signals of a certain frequency, in this embodiment, a copper plate is attached to the piezoelectric ceramic. The larger acoustic impedance of the copper plate is used to suppress the propagation of ultrasonic waves away from the aluminum cone, thereby allowing the ultrasonic energy to be more efficiently focused on the tip of the aluminum cone.

[0067] The tapered etched four-mode graded fiber 3 is sequentially divided into: a first cylindrical transition region, a tapered etched region, and a second cylindrical transition region.

[0068] The tapered etched region is the bare fiber structure obtained by tapering the four-mode graded fiber, and has a mirrored double-tapered profile; the first cylindrical transition region and the second cylindrical transition region are both unetched four-mode graded fibers, retaining the original diameter and coating of the four-mode graded fiber; the first cylindrical transition region is used for fusion splicing with the first single-mode fiber, and the second cylindrical transition region is used for fusion splicing with the second single-mode fiber.

[0069] The radial etching depth of the conical etching zone exhibits a Gaussian function distribution along the axial direction.

[0070] In this embodiment, the total length of the bare fiber structure is 12cm. The bare fiber structure has coated quadrature fiber on both sides, namely a first cylindrical transition region and a second cylindrical transition region. The cylindrical transition regions are only for facilitating fusion splicing; therefore, the lengths of the first and second cylindrical transition regions are determined according to actual fusion splicing requirements. The bare fiber structure of the quadrature fiber undergoes conical etching, with the radial etching depth assumed to follow a Gaussian function, causing the core diameter to gradually taper from both ends to the middle, resulting in a mirrored double-conical profile. Figure 3 As shown, the tapered etched four-mode graded-index fiber has a graded refractive index distribution, with its core center having a refractive index of [missing value]. The cladding refractive index is radial coordinates The refractive index at the point satisfies the maximum value from the axis. The cladding refractive index decreases smoothly radially to the core boundary. In the conical corrosion zone, the core radius gradually decreases from both ends to the middle along the axial direction, with the smallest core radius corresponding to the thinnest part, 58 micrometers (i.e., the middle part).

[0071] The first cylindrical transition region is fused with the first single-mode fiber, and the second cylindrical transition region is fused with the second single-mode fiber to obtain a tapered etched four-mode graded fiber assembly.

[0072] The first polarization controller and the second polarization controller have the same operating wavelength range.

[0073] In this embodiment, the first polarization controller, mode stripper, signal generator, power amplifier, and second polarization controller can all be conventional devices commonly used in this technical field. Those skilled in the art can select appropriate models and parameters according to actual needs, only requiring that the operating wavelength ranges of the first and second polarization controllers be consistent to ensure the matching of the optical signal path. The signal generator and power amplifier have the same operating frequency range and match the acoustic resonant frequency of the acousto-optic conversion structure.

[0074] Example 2:

[0075] This embodiment describes a method for fabricating an ultra-wideband OAM mode converter based on a fiber Bragg grating, such as... Figure 4 As shown, the method includes the following steps:

[0076] A section of four-mode graded fiber is selected, and the four-mode graded fiber is divided into a first cylindrical transition region, a conical etched region, and a second cylindrical transition region according to a preset length.

[0077] The coating layer of the four-mode graded fiber in the tapered etched region is stripped off, and the four-mode graded fiber after the coating layer is stripped is subjected to tapered etch to form a tapered etched region.

[0078] The method for the conical corrosion treatment is as follows:

[0079] The four-mode graded optical fiber is passed through a Teflon tube, which is supported by an adjustable bracket and sealed at both ends with UV-curable adhesive.

[0080] A corrosive liquid is injected through the central hole of the Teflon tube, causing the corrosive liquid to spread laterally along the Teflon tube and etch the four-mode graded optical fiber after the coating has been stripped.

[0081] The diffusion distribution of the corrosive liquid within the Teflon tube follows a Gaussian function distribution, causing the etching depth of the four-mode graded optical fiber after the coating layer is stripped to follow a Gaussian function distribution along the axial direction, forming a mirrored double-cone profile.

[0082] The Teflon tube is also provided with two circular holes to remove air bubbles generated during the corrosion process.

[0083] The corrosive liquid is a 25% hydrofluoric acid solution.

[0084] In this embodiment, the tapered etched quadrature fiber (GIFMF) is fabricated using a controllable fiber etching process. A 15cm long GIFMF fiber is selected, and a 12cm section in the middle is chosen as the tapered etching zone, where the coating is removed. The GIFMF fiber is guided through a Teflon tube, which is supported by an adjustable bracket secured by two clamps mounted on a triaxial platform and a fixed platform. Both ends of the Teflon tube are sealed with UV-curable adhesive to prevent corrosive liquid infiltration. A 25% hydrofluoric acid (HF) solution (i.e., the corrosive liquid) for fiber etching is introduced through the central hole of the Teflon tube, while two other circular holes on the tube help to effectively remove air bubbles during etching. This concentration ensures both a smooth etched fiber surface and relatively high etching efficiency. The injected hydrofluoric acid solution diffuses laterally over time, creating a spatially non-uniform etching profile with a Gaussian distribution; the central region has a longer etching time, while the edge regions have a shorter etching time. Therefore, tapered optical fibers can be formed.

[0085] It should be noted that the shape of the Teflon tube can be adjusted by optimizing the bending angle and implementing a step-by-step injection process, in which the volume of each injection, the number of injections, and the time interval between consecutive injections are precisely controlled.

[0086] A single-mode fiber is selected as the first single-mode fiber, and the first single-mode fiber is fused to the first cylindrical transition region; another single-mode fiber is selected as the second single-mode fiber, and the second single-mode fiber is fused to the second cylindrical transition region to form a single-mode-tapered four-mode graded fiber-single-mode structure, which serves as a tapered four-mode graded fiber assembly.

[0087] A copper plate, a piezoelectric ceramic, and an aluminum cone are selected. The copper plate and the piezoelectric ceramic are bonded together using UV adhesive. The other side of the piezoelectric ceramic is bonded to the bottom of the aluminum cone. Then, the tip of the aluminum cone is bonded to the cone-shaped corrosion area using UV-curing adhesive.

[0088] Select a signal generator and a power amplifier, connect the output of the signal generator to the input of the power amplifier, and connect the output of the power amplifier to the piezoelectric ceramic.

[0089] Select a mode stripper and connect the output of the mode stripper to the first single-mode fiber end of the single-mode-tapered four-mode graded fiber-single-mode structure.

[0090] A polarization controller is selected as the first polarization controller, and the output of the first polarization controller is connected to the input of the mode stripper.

[0091] Another polarization controller is selected as the second polarization controller, and the input end of the second polarization controller is connected to the second single-mode fiber end of the single-mode-tapered four-mode graded fiber-single-mode structure.

[0092] In this embodiment, an experimental verification of the aforementioned ultra-wideband OAM mode converter based on a fiber Bragg grating is performed, such as... Figure 1 As shown, the Amplified Spontaneous Emission (ASE) light source passes through a first polarization controller, then a mode stripper to separate unwanted higher-order modes, and then through a basic fiber grating (i.e., an acousto-optic converter) to convert the fundamental LP01 mode light into a higher-order LP11 mode. This light then passes through a second polarization controller to form a high-purity orbital angular momentum mode. The transmission spectrum of the experimentally obtained tapered etched fiber grating is shown in the figure. Figure 5 As shown.

[0093] Example 3:

[0094] To further verify the generated vortex mode, this embodiment conducts an experiment to measure the output optical field intensity and phase distribution of an ultra-wideband OAM mode converter based on a fiber optic grating proposed in Embodiment 1. The experimental setup is as follows: Figure 6 As shown, the experimental procedure is as follows:

[0095] The phase distribution measurement method for OAM mode involves analyzing the interference pattern between a single-wavelength laser beam emitted by the proposed optical vortex mode conversion device and a reference beam from a single-mode fiber (SMF). In addition to... Figure 1 In addition to the configuration shown for excitation and driving, this embodiment also incorporates other optical elements and instruments to realize this interferometric measurement method.

[0096] Specifically, this embodiment uses a tunable near-infrared laser with a wavelength range of 1480nm to 1640nm as the test light source, i.e. Figure 6 The supercontinuum light source shown splits its output light into two paths via an optical fiber coupler: one path serves as a reference beam, which passes sequentially through a phase modulator and a third polarization controller to form an extended Gaussian beam directly coupled to the SMF; the other path is input to the ultrawideband OAM mode converter based on the fiber grating to generate a signal beam carrying the OAM mode, which is then collimated by an objective lens before being output.

[0097] The reference beam and the collimated signal beam were combined using a non-polarizing beam splitter, and the resulting spiral interference pattern was captured by an InGaAs near-infrared CCD camera. The generation of the OAM mode was verified using interferometry, and the experimental results are as follows: Figure 7 As shown, the spiral interference pattern is clear, indicating that the OAM mode converter successfully generated a vortex beam carrying orbital angular momentum, verifying the correctness of its mode conversion function.

[0098] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope defined by the present invention.

Claims

1. An ultra-wideband OAM mode converter based on a fiber Bragg grating, characterized in that, The OAM mode converter includes: A first polarization controller is used to receive light signals from a light source and adjust the polarization state of the light signals; the output of the first polarization controller is connected to the input of a mode stripper. The mode stripper is used to filter the adjusted optical signal, retaining only the fundamental mode LP01 optical signal; the output of the mode stripper is connected to the optical signal input of the acousto-optic converter. A signal generator is used to generate sine and cosine electrical signals; the output of the signal generator is connected to the input of a power amplifier. The power amplifier is used to amplify the sine and cosine electrical signals to obtain an amplified electrical signal; the output terminal of the power amplifier is connected to the electrical signal input terminal of the acousto-optic converter. The acousto-optic converter is used to receive the fundamental mode LP01 optical signal and, driven by the amplified electrical signal, form a dynamic fiber optic grating through the acousto-optic effect to convert the fundamental mode LP01 optical signal into a higher-order mode LP11 optical signal; the output terminal of the acousto-optic converter is connected to the input terminal of the second polarization controller. The second polarization controller is used to receive the higher-order mode LP11 optical signal and adjust the relative amplitude and phase of the higher-order mode LP11 optical signal to form an orbital angular momentum mode.

2. The ultra-wideband OAM mode converter based on fiber Bragg grating according to claim 1, characterized in that, The acousto-optic converter includes: piezoelectric ceramic, aluminum conical and conical etched four-mode graded fiber optic assembly; The piezoelectric ceramic serves as the electrical signal input terminal of the acousto-optic converter; the piezoelectric ceramic is adhered to the bottom of the aluminum cone; the tip of the aluminum cone is adhered to the tapered etched region in the tapered etched four-mode graded fiber assembly; one end of the tapered etched four-mode graded fiber assembly is the optical signal input terminal of the acousto-optic converter; the other end of the tapered etched four-mode graded fiber assembly is the output terminal of the acousto-optic converter.

3. The ultra-wideband OAM mode converter based on fiber Bragg grating according to claim 2, characterized in that, The acousto-optic converter also includes a copper plate, on which the piezoelectric ceramic is bonded.

4. The ultra-wideband OAM mode converter based on fiber Bragg grating according to claim 2, characterized in that, The tapered etched four-mode graded fiber assembly includes: a first single-mode fiber, a tapered etched four-mode graded fiber, and a second single-mode fiber. One end of the first single-mode fiber serves as the optical signal input end of the acousto-optic converter, and the other end is fused to one end of the tapered etched quadrature fiber; the other end of the tapered etched quadrature fiber is fused to one end of the second single-mode fiber; the other end of the second single-mode fiber serves as the output end of the acousto-optic converter.

5. The ultra-wideband OAM mode converter based on fiber Bragg grating according to claim 4, characterized in that, The tapered etched four-mode graded fiber is divided into: a first cylindrical transition region, a tapered etched region, and a second cylindrical transition region. The tapered etched region is the bare fiber structure obtained by tapering the four-mode graded fiber, which has a mirrored double-tapered profile; the first cylindrical transition region and the second cylindrical transition region are both unetched four-mode graded fibers, retaining the original diameter and coating of the four-mode graded fiber. The first cylindrical transition region is used for fusion splicing with the first single-mode fiber, and the second cylindrical transition region is used for fusion splicing with the second single-mode fiber. The radial etching depth of the conical etching zone exhibits a Gaussian function distribution along the axial direction.

6. The ultra-wideband OAM mode converter based on fiber Bragg grating according to claim 1, characterized in that, The first polarization controller and the second polarization controller have the same operating wavelength range.

7. A method for fabricating an ultra-wideband OAM mode converter based on a fiber Bragg grating, used to prepare an ultra-wideband OAM mode converter based on a fiber Bragg grating as described in any one of claims 1-6, characterized in that, This method includes the following steps: A section of four-mode graded fiber is selected, and the four-mode graded fiber is divided into a first cylindrical transition region, a tapered etched region, and a second cylindrical transition region according to a preset length. The coating layer of the four-mode graded fiber in the tapered etched region is stripped off, and the four-mode graded fiber after the coating layer is stripped is subjected to tapered etch to form a tapered etched region. A single-mode fiber is selected as the first single-mode fiber, and the first single-mode fiber is fused to the first cylindrical transition region; another single-mode fiber is selected as the second single-mode fiber, and the second single-mode fiber is fused to the second cylindrical transition region to form a single-mode-tapered four-mode graded fiber-single-mode structure, which serves as a tapered four-mode graded fiber assembly. A copper plate, a piezoelectric ceramic, and an aluminum cone are selected. The copper plate and the piezoelectric ceramic are bonded together using UV adhesive. The other side of the piezoelectric ceramic is bonded to the bottom of the aluminum cone. The tip of the aluminum cone is then bonded to the cone-shaped corrosion area using UV-curing adhesive. Select a signal generator and a power amplifier, connect the output of the signal generator to the input of the power amplifier, and connect the output of the power amplifier to the piezoelectric ceramic. Select a mode stripper and connect the output of the mode stripper to the first single-mode fiber end of the single-mode-tapered four-mode graded fiber-single-mode structure. Select a polarization controller as the first polarization controller, and connect the output of the first polarization controller to the input of the mode stripper. Another polarization controller is selected as the second polarization controller, and the input end of the second polarization controller is connected to the second single-mode fiber end of the single-mode-tapered four-mode graded fiber-single-mode structure.

8. The method for fabricating an ultrawideband OAM mode converter based on a fiber Bragg grating according to claim 7, characterized in that, The method for the conical corrosion treatment is as follows: The four-mode graded optical fiber is passed through a Teflon tube, which is supported by an adjustable bracket and sealed at both ends with UV-curable adhesive. A corrosive liquid is injected through the central hole of the Teflon tube, causing the corrosive liquid to spread laterally along the Teflon tube and etch the four-mode graded optical fiber after the coating layer has been stripped. The diffusion distribution of the corrosive liquid within the Teflon tube follows a Gaussian function distribution, causing the etching depth of the four-mode graded optical fiber after the coating layer is stripped to follow a Gaussian function distribution along the axial direction, forming a mirrored double-cone profile. The Teflon tube is also provided with two circular holes to remove air bubbles generated during the corrosion process.

9. The method for fabricating an ultrawideband OAM mode converter based on a fiber Bragg grating according to claim 8, characterized in that, The corrosive liquid is a 25% hydrofluoric acid solution.

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