All-fiber perfect vortex beam generator and preparation method thereof

Abstract

The invention discloses an all-fiber perfect vortex beam generator and a preparation method thereof, and relates to the technical field of vortex light, the generator comprises a fiber substrate and a spiral cone lens phase plate attached to one end of the fiber substrate; the optical fiber substrate is obtained by welding a single-mode optical fiber and a beam expanding optical fiber; the beam expanding optical fiber is a coreless optical fiber; the spiral cone lens phase plate is attached to the end face of the coreless optical fiber in the optical fiber substrate; the spiral cone lens phase plate is formed by linearly superposing a spiral cone phase and a Fresnel diffraction lens phase. Based on the generator provided by the invention, a perfect vortex beam with a specific topological charge can be formed, and the problem that the spot size of the vortex beam is increased along with the increase of the topological charge in the prior art can be solved.

Description

Technical Field

[0001] The present application relates to the field of vortex light technology, and in particular to an all-fiber perfect vortex light beam generator and a preparation method thereof. Background Art

[0002] With the rapid development of information technology, people's demand for data transmission capacity and transmission rate is increasing. Traditional single-mode optical fiber communication systems have gradually been unable to meet this demand. Therefore, it is urgent to find new multiplexing technologies and light field control methods to improve the capacity and efficiency of optical fiber communication systems. Vortex beams have the characteristics of infinite topological charge and orthogonality between modes, which makes vortex beam mode division multiplexing based on photon orbital angular momentum a very potential communication expansion method. In theory, it can greatly increase the channel capacity and spectrum utilization of the communication system, providing a new idea for solving the communication capacity bottleneck problem.

[0003] In the prior art, there are two main ways to generate vortex beams: traditional optical elements and fiber mode conversion. The traditional way of generating vortex light can obtain high-quality vortex beams through spatial light modulators, spiral phase plates, etc., but the system structure is complex, usually requiring expensive equipment and complex optical path construction, and it is difficult to achieve miniaturization and integration, which is subject to certain limitations in practical applications. Optical fibers have the characteristics of miniaturization and easy integration, and are used to realize the structural basis for the generation of vortex beams. There are many existing small optical fiber devices based on optical fibers that can directly generate vortex beams, such as fiber gratings, fiber end face microstructure devices, etc. However, the existing fiber vortex light generation technology is limited by the inherent characteristic that the spot size of the vortex beam increases with the increase of topological charge. Summary of the invention

[0004] The purpose of the present application is to provide an all-fiber perfect vortex beam generator and a preparation method thereof, which can overcome the problem in the prior art that the vortex beam spot size increases with the increase of topological charge.

[0005] To achieve the above objectives, this application provides the following solutions:

[0006] In a first aspect, the present application provides an all-fiber perfect vortex beam generator, comprising:

[0007] An optical fiber base and a spiral cone lens phase plate attached to one end of the optical fiber base; the optical fiber base is obtained by fusion splicing a single-mode optical fiber and a beam expanding optical fiber; the beam expanding optical fiber is a coreless optical fiber; the spiral cone lens phase plate is attached to the end face of the coreless optical fiber in the optical fiber base; the spiral cone lens phase plate is formed by linearly superimposing a spiral cone phase and a Fresnel diffraction lens phase.

[0008] Optionally, the spiral cone lens phase plate is prepared on the end face of the coreless optical fiber by two-photon polymerization.

[0009] Optionally, the spiral cone lens phase plate is a circular structure adapted to the optical fiber base structure.

[0010] Optionally, the optical fiber substrate is embedded into the spiral cone lens phase plate using optical fiber embedding technology.

[0011] Optionally, the length of the coreless optical fiber is a length corresponding to the incident light beam just filling the entire end face of the coreless optical fiber.

[0012] In a second aspect, the present application provides a method for preparing an all-fiber perfect vortex beam generator based on the above, comprising:

[0013] Obtaining optical fiber substrate preparation materials and spiral cone lens phase plate; the optical fiber substrate preparation materials include single-mode optical fiber and coreless optical fiber;

[0014] Use the optical fiber three-dimensional refractive index tester to measure the refractive index distribution of coreless optical fiber;

[0015] Using FDTD algorithm, a connection model between single-mode fiber and coreless fiber is constructed;

[0016] A Gaussian base mode beam with a beam waist of 10 μm is input at the end of a single-mode optical fiber, and the optimal length of the coreless optical fiber is determined with a perfect matching layer as a boundary condition; the optimal length of the coreless optical fiber is the length of the coreless optical fiber corresponding to the light field just expanding to the edge of the optical fiber;

[0017] The coreless optical fiber is fused with the single-mode optical fiber to form an optical fiber substrate to be cut;

[0018] According to the optimal length of the coreless optical fiber, cutting the coreless optical fiber portion in the optical fiber substrate to be cut to obtain an optical fiber substrate;

[0019] Photoresist is coated on the end face of the coreless optical fiber on the optical fiber substrate, and two-photon polymerization technology is used to polymerize the three-dimensional structure of the spiral cone lens phase plate on the end face of the coreless optical fiber to obtain an all-fiber perfect vortex beam generator.

[0020] Optionally, before obtaining the optical fiber substrate preparation material and the spiral cone lens phase plate, the method further includes:

[0021] According to the formula h = λ0J l (ksin(α)r)mod(lφ+n1πr 2 / λ0f,2π) / (2πn1*(n2-n1)), determine the thickness distribution of the spiral cone lens phase plate;

[0022] Where n2 is the refractive index of the lens material, (r, φ) is the polar coordinate of the plane where the phase plate is located, l is the topological charge, and J l(*) is the l-th order first kind Bessel function, k is the wave vector in vacuum, α represents the cone angle parameter in the Bessel function, (ρ, θ) are the polar coordinates of the target light field plane, n1 is the ambient refractive index, λ0 is the wavelength of light in vacuum, and f is the focal length of the Fresnel lens.

[0023] Optionally, photoresist is coated on the coreless optical fiber end face of the optical fiber substrate, and two-photon polymerization technology is used to polymerize a three-dimensional structure of a spiral cone lens phase plate on the coreless optical fiber end face to obtain an all-fiber perfect vortex beam generator, which specifically includes:

[0024] Coating a predetermined photoresist material on the end face of the coreless optical fiber in the optical fiber substrate;

[0025] The optical fiber substrate coated with photoresist is placed on the working table of the two-photon polymerization device through a clamp;

[0026] Inputting or setting a three-dimensional model of a spiral cone lens phase plate in a two-photon polymerization device; the three-dimensional model is determined according to a selected photoresist refractive index and a thickness distribution formula of the spiral cone lens phase plate;

[0027] Use a focused ultra-strong laser beam to perform point-by-point or layer-by-layer scanning exposure on the photoresist;

[0028] Controlling the scanning path and exposure time of the laser to aggregate the three-dimensional structure of the spiral cone lens phase plate;

[0029] placing the polymerized photoresist structure into a developer for development to remove unpolymerized photoresist;

[0030] The residues after development were removed using a cleaning solution to obtain an all-fiber perfect vortex beam generator.

[0031] Optionally, placing the polymerized photoresist structure in a developer for development to remove unpolymerized photoresist, specifically comprising:

[0032] The polymerized photoresist structure is immersed and developed by using an organic solvent, and then immersed and cleaned in an isopropanol solution to remove the unpolymerized photoresist.

[0033] Optionally, the organic solvent includes propylene glycol methyl ether acetate.

[0034] According to the specific embodiments provided in this application, this application discloses the following technical effects:

[0035] The present application provides an all-fiber perfect vortex beam generator and a preparation method thereof, the generator comprising an optical fiber substrate and a spiral cone lens phase plate attached to one end of the optical fiber substrate; the optical fiber substrate is obtained by fusion splicing a single-mode optical fiber and a beam expanding optical fiber; the beam expanding optical fiber is a coreless optical fiber; the spiral cone lens phase plate is attached to the end face of the coreless optical fiber in the optical fiber substrate; the spiral cone lens phase plate is formed by linear superposition of the spiral cone phase and the Fresnel diffraction lens phase. The generator adopts a spiral cone lens phase plate, and realizes precise control of the light beam by linear superposition of the spiral cone phase and the Fresnel diffraction lens phase. This enables the light beam to form a perfect vortex beam with a specific topological charge after passing through the phase plate, and the spot size is no longer affected by the expansion of the topological charge. Therefore, the all-fiber perfect vortex beam generator provided by the present application realizes effective control of the vortex beam spot size. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0037] Figure 1 A schematic diagram of the structure of an all-fiber perfect vortex beam generator provided in one embodiment of the present application;

[0038] Figure 2 A schematic diagram of the propagation path of light after it is coupled from a single-mode optical fiber into a coreless optical fiber according to an embodiment of the present application;

[0039] Figure 3 A schematic diagram of the phase composition principle of the phase plate of the conical lens provided in one embodiment of the present application;

[0040] Figure 4 The first row of images provided in one embodiment of the present application is a phase diagram of a spiral cone lens corresponding to a cone angle parameter of 0.1;

[0041] Figure 5 The first row of images provided for an embodiment of the present application is an optical micrograph of a spiral cone lens phase plate corresponding to a cone angle parameter of 0.1;

[0042] Figure 6 The focal plane light field distribution of the all-fiber perfect vortex light generator under different topological loads provided in one embodiment of the present application and the light field distribution diagram after coaxial interference with a Gaussian beam;

[0043] Figure 7The focal plane light field distribution of the all-fiber perfect vortex light generator with different cone angle parameters provided in one embodiment of the present application and the light field distribution diagram after coaxial interference with the Gaussian beam;

[0044] Figure 8 An optical micrograph of an all-fiber perfect vortex beam generator provided in one embodiment of the present application. DETAILED DESCRIPTION

[0045] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0046] In order to make the above-mentioned objects, features and advantages of the present application more obvious and easy to understand, the present application is further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0047] In an exemplary embodiment, Figure 1 As shown, an all-fiber perfect vortex beam generator is provided, comprising:

[0048] An optical fiber base and a spiral cone lens phase plate attached to one end of the optical fiber base; the optical fiber base is obtained by fusion splicing a single-mode optical fiber and a beam expanding optical fiber; the beam expanding optical fiber is a coreless optical fiber; the spiral cone lens phase plate is attached to the end face of the coreless optical fiber in the optical fiber base; the spiral cone lens phase plate is formed by linearly superimposing a spiral cone phase and a Fresnel diffraction lens phase.

[0049] Specifically, Figure 1 As shown, the all-fiber perfect vortex beam generator is composed of ordinary single-mode optical fiber, beam expanding optical fiber, and spiral cone lens phase plate. The beam expanding optical fiber is a coreless optical fiber, and a section of coreless optical fiber is fused to the single-mode optical fiber as the optical fiber base of the generator; the spiral cone lens phase plate is composed of the linear superposition of the spiral cone phase and the Fresnel diffraction lens phase; the spiral cone lens phase plate is prepared on the end face of the coreless optical fiber by two-photon polymerization; wherein, the spiral cone lens phase plate is designed to be a circular structure slightly larger than the end face of the optical fiber. In order to ensure the firmness of the adhesion between the phase plate and the optical fiber, the phase plate is polymerized on the end face of the coreless optical fiber in a manner of being embedded in the optical fiber.

[0050] Among them, in this embodiment, the spiral cone lens phase plate is prepared on the end face of the coreless optical fiber by two-photon polymerization; the spiral cone lens phase plate is a circular structure adapted to the optical fiber base structure; the optical fiber base adopts optical fiber embedding technology to embed the spiral cone lens phase plate; the coreless optical fiber is a pure quartz glass rod; the length of the single-mode optical fiber can be any length, and the length of the coreless optical fiber is the length corresponding to the incident light beam just filling the entire coreless optical fiber end face. For example, when the single-mode optical fiber end is connected to a pump light source with a wavelength of 1064nm, the coreless optical fiber with a length of 800μm can just expand the fundamental mode light beam in the single-mode optical fiber to the entire optical fiber end face, and still maintain the Gaussian mode distribution.

[0051] Embodiment 2

[0052] This embodiment provides a method for preparing an all-fiber perfect vortex beam generator based on the above, comprising:

[0053] Step 1: Obtain optical fiber substrate preparation materials and spiral cone lens phase plate; the optical fiber substrate preparation materials include single-mode optical fiber and coreless optical fiber;

[0054] Step 2: Use a fiber 3D refractive index tester to measure the refractive index distribution of the coreless optical fiber;

[0055] Step 3: Use the FDTD algorithm to construct a connection model between single-mode optical fiber and coreless optical fiber;

[0056] Step 4: Input a Gaussian base mode beam with a beam waist of 10 μm at the end of a single-mode optical fiber, and determine the optimal length of the coreless optical fiber with a perfect matching layer as a boundary condition; the optimal length of the coreless optical fiber is the length of the coreless optical fiber corresponding to the light field just expanding to the edge of the optical fiber;

[0057] Step 5: Fusion-splice the coreless optical fiber with the single-mode optical fiber to form an optical fiber substrate to be cut;

[0058] Step 6: Cutting the coreless optical fiber portion in the optical fiber substrate to be cut according to the optimal length of the coreless optical fiber to obtain an optical fiber substrate;

[0059] Step 7: Coat photoresist on the coreless optical fiber end face of the optical fiber substrate, and use two-photon polymerization technology to polymerize the three-dimensional structure of the spiral cone lens phase plate on the coreless optical fiber end face to obtain an all-fiber perfect vortex beam generator.

[0060] When executing step 1-step 2, the specific steps may be as follows:

[0061] For spiral cone lens phase plate, the larger the geometric size, the larger the focal length adjustment range that can be obtained. The core diameter of ordinary single-mode optical fiber is 8-12μm, and the energy of transmitted light is mainly distributed in the core, which is too small for the actual working range of spiral cone lens phase plate. Due to the existence of beam focusing diffraction limit, it is even impossible to form a perfect vortex light field in the focal plane.

[0062] Therefore, this embodiment uses a coreless optical fiber with a diameter that matches that of an ordinary single-mode optical fiber to expand and collimate the fundamental mode light beam emitted from the single-mode optical fiber. The selected coreless optical fiber is a pure quartz glass rod, which does not have the core structure of a traditional optical fiber to limit the propagation path of the light beam. When the light in the single-mode optical fiber enters the coreless optical fiber, since there is no strong constraint between the core and the cladding, the light is diffused and propagated in a gradually increasing cross-section inside the optical fiber, achieving the effect of beam expansion, such as Figure 2 shown.

[0063] When executing step 3 to step 6, the specific steps may be as follows:

[0064] Since the size of the coreless fiber is limited, in order to ensure that the light beam is evenly expanded in the coreless fiber until the cross section just covers the surface of the fiber, the length of the coreless fiber needs to be determined.

[0065] In this embodiment, the appropriate length of the coreless optical fiber is determined by using finite difference time domain (FDTD) simulation calculation and experimental measurement methods.

[0066] First, a fiber 3D refractive index tester is used to measure the refractive index distribution of the coreless fiber. Then, a model is established in the FDTD algorithm based on the refractive index distribution of the coreless fiber and its geometric structure parameters, and a connection model between the single-mode fiber and the coreless fiber is constructed. A high-base mode beam with a beam waist of 10μm is input at the end of the single-mode fiber, and the propagation of the light field in the fiber is recorded through a two-dimensional axial monitor with a perfect matching layer as the boundary condition, so as to determine the length of the coreless fiber where the light field is just expanded to the edge of the fiber.

[0067] Next, this embodiment directly records the light field distribution of the coreless fiber end face by fusing coreless fibers of different lengths at the fiber end face, and connecting a pump light source with a wavelength of 1064nm at the single-mode fiber end. A microscope with a CCD camera is used to directly record the light field distribution of the coreless fiber end face. In a specific case, coreless fibers of lengths of 100μm, 200μm, 300μm, 400μm, 500μm, 600μm, 700μm, 800μm, 900μm, and 1000μm at intervals of 100μm are fused at the rear end of the single-mode fiber. After connecting to the light source, the light field distribution of the coreless fiber end face is recorded to determine the corresponding length at which the light intensity just fills the entire fiber end face. The experimental results show that the 800μm coreless fiber can expand the fundamental mode beam in the single-mode fiber to the entire fiber end face while maintaining the Gaussian mode distribution.

[0068] Therefore, the optical fiber substrate in this embodiment is formed by fusing a 50 cm single-mode optical fiber to an 800 μm coreless optical fiber. To ensure the cleanliness of the optical fiber end face, the optical fiber is cleaned with an organic solvent, and then ultrasonic cleaning is used to effectively remove impurity particles on the optical fiber end face. For example, the optical fiber is immersed in an acetone solution for ultrasonic cleaning for 10-15 minutes, and then cleaned with an ethanol solution to remove acetone residues. After drying in a clean environment, the next step of device preparation is carried out.

[0069] During step 7, the specific steps are as follows:

[0070] Step 701: coating a predetermined photoresist material on the end face of the coreless optical fiber in the optical fiber substrate;

[0071] Step 702: placing the optical fiber substrate coated with photoresist on a workbench of a two-photon polymerization device through a fixture;

[0072] Step 703: inputting or setting a three-dimensional model of the spiral cone lens phase plate in the two-photon polymerization device; the three-dimensional model is determined according to the selected photoresist refractive index and the thickness distribution formula of the spiral cone lens phase plate;

[0073] Step 704: using a focused ultra-strong laser beam to perform point-by-point or layer-by-layer scanning exposure on the photoresist;

[0074] Step 705: Control the scanning path and exposure time of the laser to polymerize the three-dimensional structure of the spiral cone lens phase plate;

[0075] Step 706: placing the polymerized photoresist structure into a developer for development to remove unpolymerized photoresist;

[0076] Step 707: Use a cleaning solution to remove the residue after development to obtain an all-fiber perfect vortex beam generator.

[0077] Specifically, the spiral cone lens phase plate is prepared on the end face of the optical fiber by two-photon polymerization technology. The thickness distribution of the lens is closely related to the selection of the lens material. Therefore, before processing, an appropriate photoresist must be selected. The photoresist should be sensitive to the two-photon polymerization process, be able to undergo polymerization reaction under the action of two-photon absorption, and the refractive index after polymerization should be higher than the refractive index of the device environment. Based on the refractive index of the selected photoresist and the thickness distribution formula, the three-dimensional model of the spiral cone lens phase plate can be determined. Then, the end face of the coreless optical fiber is coated with an appropriate photoresist material and fixed on the workbench of the two-photon polymerization device using a clamp. According to the three-dimensional model of the spiral cone lens phase plate, the photoresist is scanned and exposed point by point or layer by layer using a focused ultra-strong laser beam. At the laser focus, the photoresist undergoes two-photon absorption and triggers a polymerization reaction, thereby forming a solid polymer structure. By precisely controlling the scanning path and exposure time of the laser, the three-dimensional structure of the spiral cone lens phase plate can be accurately polymerized. After development, the spiral cone lens phase plate is cleaned with a developer to obtain a stable solid structure. The choice of developer depends on the type of photoresist, and the development time needs to be optimized and adjusted according to factors such as the type of photoresist, polymer thickness, exposure dose, and developer concentration. The choice of cleaning solution depends on the characteristics of the photoresist and developer, and organic solvents or deionized water can usually be used.

[0078] Specifically, the photoresist selected in this embodiment is IP-dip photoresist, and the refractive index of the material after polymerization corresponding to the design wavelength is 1.54. The polymerized device is immersed and developed in propylene glycol methyl ether acetate (PGMEA) organic solvent for 20 minutes and then immersed and cleaned in isopropyl alcohol solution for 2 minutes.

[0079] like Figure 5 As shown, they correspond to Figure 8 Optical micrograph of the spiral conical phase plate on the fiber end face of the design diagram. Figure 8 This is an optical micrograph of an all-fiber perfect vortex light generator with a topological charge of 5, a cone angle parameter of 0.1, and a focal length of 100 μm. Figure 5 The first row of images are optical micrographs of the spiral cone lens phase plate corresponding to the cone angle parameter of 0.1, and the topological charges are l=1, -5 and 10 respectively; the second row of images are optical micrographs of the spiral cone lens phase plate corresponding to the topological charge of 5, and the cone angle parameters are 0.1, 0.08, and 0.06 respectively.

[0080] In addition, in some embodiments, before obtaining the optical fiber substrate preparation material and the spiral cone lens phase plate, the following steps may also be included:

[0081] According to the formula h = λ0J l (ksin(α)r)mod(lφ+n1πr 2 / λ0f,2π) / (2πn1*(n2-n1)), determine the thickness of the spiral cone lens phase plate;

[0082] Where n2 is the refractive index of the lens material, (r, φ) is the polar coordinate of the plane where the phase plate is located, l is the topological charge, and J l (*) is the l-th order first kind Bessel function, k is the wave vector in vacuum, α represents the cone angle parameter in the Bessel function, (ρ, θ) are the polar coordinates of the target light field plane, n1 is the ambient refractive index, λ0 is the wavelength of light in vacuum, and f is the focal length of the Fresnel lens.

[0083] Specifically, the spiral cone lens phase plate is designed before obtaining the optical fiber substrate preparation material and the spiral cone lens phase plate.

[0084] Specifically, the complex amplitude distribution of a perfect vortex beam can be expressed by multiplying the Dirac function δ(ρ-ρ0) and the spiral phase function. At the same time, it has been proven that the perfect vortex light can be obtained by Fourier transform of the Bessel beam B(r,φ). Therefore, we have:

[0085] B(r,φ)=J l (ksin(α)r)exp(ilφ) (1)

[0086] E(ρ,θ)=δ(ρ-ρ0)exp(ilθ) (2)

[0087] Where (r, φ) is the polar coordinate of the plane where the phase plate is located, l is the topological charge, and J l (*) is the l-th order first kind Bessel function, k is the wave vector in vacuum, α represents the cone angle parameter in the Bessel function, and (ρ,θ) are the polar coordinates of the target light field plane.

[0088] According to the Bessel beam expression, its phase presents a spiral periodic change in the angular direction and a periodic change related to the cone angle parameter α in the radial direction. In the actual process, it corresponds to a spiral cone phase plate.

[0089] Therefore, the spiral cone lens phase plate can be formed by linearly superimposing the spiral cone phase and the Fresnel diffraction lens phase, such as Figure 3 As shown. When the fundamental mode Gaussian light passes through the phase plate, diffraction occurs and a perfect vortex light field is obtained in the focusing plane. The complex amplitude distribution corresponding to the plane where the spiral cone lens phase plate is located is:

[0090] E0(r,φ)=J l (ksin(α)r)exp(ilφ)exp(-in1πr 2 / λ0f), (3)

[0091] n1 is the ambient refractive index, λ0 is the wavelength of light in vacuum, and f is the focal length of the Fresnel lens. According to its complex amplitude expression, the phase structure of the spiral cone lens phase plate is as follows: Figure 1 As shown. When the fundamental mode beam diffracts after passing through the spiral cone lens phase plate, a bright ring is obtained on the focal plane that is independent of the topological charge. According to the diffraction of the light field, the radius D of the focal plane ring is:

[0092] D=2fsin(α) (4)

[0093] In the case of a fixed focal plane, the size of the ring can be arbitrarily controlled by controlling the cone angle parameter α. As can be seen from formulas (1)-(4), the perfect vortex beam in this embodiment can be controlled by three parts, namely, the amplitude control represented by the cone angle parameter, which mainly controls the ring radius of the perfect vortex beam; the phase control represented by the topological charge number is used for the order control of the perfect vortex light; and finally, the spherical phase control represented by the focal length is used for the position control of the focal plane of the perfect vortex light. In the design process of the spiral cone lens phase plate, the required phase plate focal length f is first determined according to the actual application process. By adjusting the topological charge l in the spiral cone lens phase formula, the spiral cone lens phase plate can convert the fundamental mode Gaussian beam emitted from the optical fiber base into a perfect vortex beam of different orders at the focal plane; by adjusting the cone angle parameter α, the annular light field radius of the perfect vortex beam in the focal plane can be arbitrarily controlled.

[0094] In practice, the spiral cone lens phase plate provides corresponding phase delay to the transmitted light beam through the difference between the material refractive index and the ambient refractive index and the thickness h at different positions to achieve light field control. According to formula (3), the thickness distribution h of the spiral cone lens phase plate corresponds to the phase distribution in the following relationship:

[0095] h=λ0J l (ksin(α)r)mod(lφ+n1πr 2 / λ0f,2π) / (2πn1*(n2-n1)) (5)

[0096] Wherein, n2 is the refractive index of the lens material. According to the thickness distribution, the present embodiment can obtain a three-dimensional model of the spiral cone lens phase plate and determine information such as the shape, taper, and height of the spiral.

[0097] In the implementation case, in order to obtain the maximum aperture, the maximum working range of the spiral cone lens phase plate matches the diameter of the optical fiber, so the spiral cone lens phase plate is designed as a circular structure with a diameter of 125μm. The parameter settings of the spiral cone lens phase plate corresponding to obtaining a perfect vortex light field with a diameter of 20μm at the focal plane of 100μm are: cone angle parameter α = 0.1, extension lens focal length f = 100μm, such as Figure 4As shown in the first row, the phase diagrams are designed with topological charges of 1, -5, and 10. The corresponding spiral cone lens phase plate setting for obtaining vortex light fields of different diameters and orders of 5 at the focal plane of 100 μm is: topological charge number l = 5, lens focal length f = 100 μm, as shown in Figure 4 As shown in the second row, the cone angle parameters are 0.1, 0.08, and 0.06, respectively, corresponding to focal plane light field diameters of 20 μm, 16 μm, and 12 μm.

[0098] In some other embodiments, in order to optimize the all-fiber perfect vortex light generator, its optical performance needs to be tested. The implementation case of this embodiment uses a dual-path interference method to test the coaxial interference pattern of the generator's output light beam and the Gaussian beam in the focal plane to achieve the determination of the topological charge. At the same time, the perfect vortex light excitation characteristics and size-adjustable characteristics of the generator are analyzed by testing the intensity distribution of the output light beam of the generator with different parameters in the focal plane. According to the test results, the scanning frequency of two-photon polymerization, laser power and other preparation parameters are optimized and adjusted to improve the performance and quality of the device. In the implementation case, according to the light field distribution of the perfect vortex light generator in the focal plane under the six designed parameters and the coaxial interference pattern with the Gaussian beam, as shown in FIG. Figure 6 , Figure 7 As shown in the figure. Under the same cone angle parameter, the annular light field distribution in the focal plane does not change with the change of topological charge; the radius of the light field can be controlled by adjusting the cone angle parameter of the spiral cone lens.

[0099] In summary, this application has the following technical effects:

[0100] (1) The design method proposed in this application is an all-fiber perfect vortex light generator. The optical fiber has good flexibility and can be bent to a certain extent without affecting light transmission. This enables the optical fiber-based perfect vortex optical device to adapt to various complex spatial environments and equipment layouts, which is conducive to its miniaturized and integrated application in optical communication systems.

[0101] (2) The all-fiber perfect vortex beam generator proposed in this application has the characteristic of adjustable spot size, that is, the annular spot size on the focal plane can be adjusted by changing the cone angle parameter of the spiral cone lens to meet the requirements of spot size in different application scenarios. For example, in the field of super-resolution imaging, a smaller vortex light spot can be generated to improve the imaging resolution; in optical tweezers technology, the spot size can be controlled as needed to achieve effective manipulation of particles of different sizes.

[0102] (3) The appropriate spot size can make the vortex light interact better with the target material, improving the efficiency and accuracy of the interaction between light and matter. For example, in the field of laser processing, the vortex light spot size can be adjusted to match the processing area to achieve more accurate material removal, modification and other processing operations.

[0103] (4) The spot position of the generator proposed in this application can be controlled by the focusing parameters of the spiral cone lens phase plate. This makes the fiber perfect vortex light generator more flexible when integrated with other optical elements or systems, and can be optimized according to the overall requirements of the system to improve system performance.

[0104] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0105] This article uses specific examples to illustrate the principles and implementation methods of this application. The description of the above embodiments is only used to help understand the method and core ideas of this application. At the same time, for those skilled in the art, according to the ideas of this application, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting this application.

Claims

1. An all-fiber perfect vortex beam generator, characterized in that: It comprises an optical fiber base and a spiral cone lens phase plate attached to one end of the optical fiber base; the optical fiber base is obtained by fusion splicing a single-mode optical fiber and a beam-expanding optical fiber; the beam-expanding optical fiber is a coreless optical fiber; the spiral cone lens phase plate is attached to the end face of the coreless optical fiber in the optical fiber base; the spiral cone lens phase plate is formed by linearly superimposing the spiral cone phase and the Fresnel diffraction lens phase.

2. The all-fiber perfect vortex beam generator according to claim 1, characterized in that: The spiral cone lens phase plate is prepared on the end face of the coreless optical fiber by two-photon polymerization.

3. The all-fiber perfect vortex beam generator according to claim 1, characterized in that: The spiral cone lens phase plate is a circular structure adapted to the optical fiber base structure.

4. The all-fiber perfect vortex beam generator according to claim 1, characterized in that: The optical fiber substrate adopts the optical fiber embedding technology and is embedded in the spiral cone lens phase plate.

5. The all-fiber perfect vortex beam generator according to claim 1, characterized in that: The length of the coreless optical fiber is the length corresponding to the incident light beam just filling the entire end face of the coreless optical fiber.

6. A method for preparing an all-fiber perfect vortex beam generator according to any one of claims 1 to 5, characterized in that: include: Obtaining optical fiber substrate preparation materials and spiral cone lens phase plate; The optical fiber substrate preparation material includes single-mode optical fiber and coreless optical fiber; Use the optical fiber three-dimensional refractive index tester to measure the refractive index distribution of coreless optical fiber; Using FDTD algorithm, a connection model between single-mode fiber and coreless fiber is constructed; A Gaussian base mode beam with a beam waist of 10 μm is input at the end of a single-mode optical fiber, and the optimal length of the coreless optical fiber is determined with a perfect matching layer as a boundary condition; the optimal length of the coreless optical fiber is the length of the coreless optical fiber corresponding to the light field just expanding to the edge of the optical fiber; The coreless optical fiber is fused with the single-mode optical fiber to form an optical fiber substrate to be cut; According to the optimal length of the coreless optical fiber, cutting the coreless optical fiber portion in the optical fiber substrate to be cut to obtain an optical fiber substrate; Photoresist is coated on the end face of the coreless optical fiber on the optical fiber substrate, and two-photon polymerization technology is used to polymerize the three-dimensional structure of the spiral cone lens phase plate on the end face of the coreless optical fiber to obtain an all-fiber perfect vortex beam generator.

7. The preparation method according to claim 6, characterized in that: Before obtaining the optical fiber substrate preparation materials and the spiral cone lens phase plate, it also includes: According to the formula h = λ0J l (ksin(α)r)mod(lφ+n1πr 2 / λ0f,2π) / (2πn1*(n2-n1)), determine the thickness distribution of the spiral cone lens phase plate; Where n2 is the refractive index of the lens material, (r, φ) is the polar coordinate of the plane where the phase plate is located, l is the topological charge, and J l (*) is the l-th order first kind Bessel function, k is the wave vector in vacuum, α represents the cone angle parameter in the Bessel function, (ρ, θ) are the polar coordinates of the target light field plane, n1 is the ambient refractive index, λ0 is the wavelength of light in vacuum, and f is the focal length of the Fresnel lens.

8. The preparation method according to claim 7, characterized in that: The coreless optical fiber end face of the optical fiber substrate is coated with photoresist, and the three-dimensional structure of the spiral cone lens phase plate is polymerized on the coreless optical fiber end face by two-photon polymerization technology to obtain an all-fiber perfect vortex beam generator, which specifically includes: Coating a predetermined photoresist material on the end face of the coreless optical fiber in the optical fiber substrate; The optical fiber substrate coated with photoresist is placed on the working table of the two-photon polymerization device through a clamp; Inputting or setting a three-dimensional model of a spiral cone lens phase plate in a two-photon polymerization device; the three-dimensional model is determined according to a selected photoresist refractive index and a thickness distribution formula of the spiral cone lens phase plate; Use a focused ultra-strong laser beam to perform point-by-point or layer-by-layer scanning exposure on the photoresist; Controlling the scanning path and exposure time of the laser to aggregate the three-dimensional structure of the spiral cone lens phase plate; placing the polymerized photoresist structure into a developer for development to remove unpolymerized photoresist; The residues after development were removed using a cleaning solution to obtain an all-fiber perfect vortex beam generator.

9. The preparation method according to claim 8, characterized in that: The polymerized photoresist structure is placed in a developer for development to remove unpolymerized photoresist, specifically including: The polymerized photoresist structure is immersed and developed by using an organic solvent, and then immersed and cleaned in an isopropanol solution to remove the unpolymerized photoresist.

10. The preparation method according to claim 9, characterized in that: The organic solvent includes propylene glycol methyl ether acetate.

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