A dual-frequency array-distributed orbital angular momentum mode generation device

Abstract

The present invention discloses a dual-frequency array-distributed orbital angular momentum mode generating device, which is processed by using laser processing technology to change the refractive index of a lithium niobate crystal. The processed lithium niobate crystal is divided into three regions; the first region is an unprocessed region, and the fundamental frequency light is converted into second harmonic light by using the birefringent phase matching of the lithium niobate crystal; the refractive index of the lithium niobate crystal is changed in the second region and the third region by using laser processing technology, and the fundamental frequency light and the second harmonic light are linearly diffracted; the second region is a fork-shaped grating array, which linearly diffracts the fundamental frequency light and the second harmonic light in the z direction and loads orbital angular momentum at non-zero orders; the third region is a periodic grating array, which linearly diffracts the fundamental frequency light and the second harmonic light in the x direction, so that the orbital angular momentum modes are distributed in an array. The present invention can simultaneously expand the fundamental wave and the second harmonic orbital angular momentum modes from one-dimensional distribution to two-dimensional, increasing the modulation channels of the optical field.

Description

Technical Field

[0001] The present invention relates to the technical field of optical field control, in particular to the control of the array distribution of linear and non-linear orbital angular momentum modes in lithium niobate crystals, and specifically to a dual-frequency array distribution orbital angular momentum mode generation device. Background Art

[0002] The photon orbital angular momentum (Orbital angular momentum, abbreviated as OAM) has characteristics such as a helically distributed phase, infinite dimensions, and orthogonality. Since it was verified by Allen et al. in 1992, it has been widely used in fields such as optical manipulation, optical communication, quantum, imaging, detection, and optical encryption. The important applications of OAM modes in the optical field have promoted the research on their generation methods. In addition to the commonly used spiral phase plates, q-plates, computer-generated holograms, π / 2 mode converters, etc., there are also various micro-integrated OAM generation devices. If one wants to generate an OAM mode at a new frequency, the traditional method requires two devices: first, frequency conversion is performed through a non-linear medium, and then an OAM is loaded through a linear device. In order to save cost and space, a direct method has emerged, which realizes beam shaping in one step while performing non-linear conversion. The three-dimensional non-linear photonic crystal not only designs the second-order non-linear coefficient χ in the vertical propagation direction (2) to control the optical field distribution, but also can consider the periodic structure in the propagation direction for quasi-phase matching of the fundamental wave. Compared with metal metasurfaces and two-dimensional non-linear photonic crystals, the non-linear efficiency of three-dimensional non-linear photonic crystals has been greatly improved.

[0003] The femtosecond laser direct writing technology provides a flexible way for the preparation of three-dimensional non-linear photonic crystals. It can not only erase the second-order non-linear coefficient at any position inside the crystal, but also change the refractive index, with the advantages of low thermal effect and high resolution. Ideally, the second-order non-linear coefficient in the non-linear photonic crystal is erased. However, under the current processing conditions, the second-order non-linear coefficient can only be erased by about 20%, which is much lower than the non-linear conversion efficiency under complete erasure or polarization conditions that satisfy the quasi-phase matching condition. Summary of the Invention

[0004] In view of this, in order to solve the above problems in the prior art, the present invention proposes a dual-frequency array distribution orbital angular momentum mode generation device, which processes a three-dimensional grating structure in a lithium niobate crystal. First, the first-kind birefringence phase matching of the lithium niobate crystal itself is used for efficient frequency doubling conversion, and then the optical field of the frequency-doubled beam is controlled based on the change in the refractive index of the grating structure. Based on the flexibility of laser processing, a fork-shaped grating array combined with a periodic grating array with opposite fringe directions is designed to form a three-dimensional volume grating structure, which can expand the OAM mode in one dimension to two dimensions, increasing the non-linear optical field modulation dimension and control channels.

[0005] The present invention solves the above problems through the following technical means:

[0006] In a first aspect, the present invention provides a dual-frequency array-distributed orbital angular momentum mode generation device, which is characterized in that the device is processed by using laser processing technology to change the refractive index of a lithium niobate crystal. The processed lithium niobate crystal includes three regions, namely a first region, a second region, and a third region arranged in sequence from left to right;

[0007] The first region is an unprocessed region, and the fundamental frequency light is converted into second harmonic light by using the birefringent phase matching of the lithium niobate crystal;

[0008] The refractive index of the lithium niobate crystal in the second region and the third region is changed by using laser processing technology, and linear diffraction is performed on the fundamental frequency light and the second harmonic light;

[0009] The second region is a fork-shaped grating array, which performs linear diffraction on the fundamental frequency light and the second harmonic light in the z direction and loads orbital angular momentum at non-zero orders;

[0010] The third region is a periodic grating array, which performs linear diffraction on the fundamental frequency light and the second harmonic light in the x direction, so that the orbital angular momentum modes are distributed in an array.

[0011] Preferably, the processed lithium niobate crystal includes three regions, which is expressed by the formula:

[0012]

[0013] where is the azimuth angle in the x-z plane, l0 is the topological charge number of the OAM mode, and the spatial frequency G x = 2π / Λ x 、G y = 2π / Λ y 、G z = 2π / Λ z ,Λ x 、Λ y 、Λ y are the grating periods in the x, y, and z directions in the rectangular coordinate system respectively; T is a binarization function, and for any value Z, it is expressed as:

[0014]

[0015] Laser processing can cause a change Δn in the refractive index of the crystal, and the linear fork-shaped grating array is expressed as:

[0016] n(x, y, z) = n0 - Δnf(x, y, z) (3)

[0017] where n0 represents the original refractive index of the lithium niobate crystal.

[0018] Preferably, when the fundamental wave is incident on the crystal along the y direction and polarized along the z direction, the fundamental wave is an ordinary light, i.e., o light. After passing through the unprocessed area of the lithium niobate crystal, a second-harmonic extraordinary light, i.e., e light, is generated. The quasi-birefringent phase-matching condition satisfied is as follows:

[0019] k 2ω (e) = k ω (o) + k ω (o) (4)

[0020] where k 2ω (e) and k ω (o) are the wave vectors of the second-harmonic e light and the fundamental o light respectively. It can be seen from formula (4) that the second-harmonic light passing through the unprocessed area is e light. Then, linear diffraction occurs through the laser processing area. First, it passes through the fork grating array and linear diffraction occurs along the z direction, generating OAM modes with a topological charge of 1 at the positive and negative first orders, and the zero order is still the Gaussian mode. Then, it passes through the periodic grating array and linear diffraction occurs along the x direction, and the second-harmonic light spot is expanded to a two-dimensional plane, generating multiple OAM modes with a topological charge of 1. Among them, (0, 1), (1, 1), (0, -1), (1, -1), (-1, -1), (-1, 1) are OAM modes with a topological charge of 1, and the positions of (0, 0), (1, 0), (-1, 0) are Gaussian modes. In addition, the change in refractive index can also perform the same linear diffraction on the fundamental wave at the same time, realizing the array distribution of the dual-frequency OAM modes.

[0021] Preferably, the laser processing technology is femtosecond laser processing technology.

[0022] In a second aspect, the present invention provides a preparation system for a dual-frequency array-distributed orbital angular momentum mode generation device for preparing the dual-frequency array-distributed orbital angular momentum mode generation device, including a first laser, a first half-wave plate, a second half-wave plate, a first polarization beam splitter, a first lens, a second lens, a small hole, a gradient attenuator, an objective lens, a CCD, a three-dimensional nano translation stage, and a computer;

[0023] The first laser emits laser light along the x direction. The power of the laser is controlled by a first half-wave plate and a first polarization beam splitter. The polarization of the laser is changed to be along the z direction by a second half-wave plate. The beam is shaped by a first lens, a small hole, and a second lens. Energy compensation in the depth direction for laser processing is performed by a gradient attenuator. After the laser passes through the objective lens, it is focused on a magnesium-doped lithium niobate crystal. The reflected beam is collected by a CCD to observe the processing state in real time. The lithium niobate crystal is placed on a three-dimensional nano translation stage. The travel of the three-dimensional nano translation stage is controlled by a computer, so that the laser scribes lines along the x direction to form a fork-shaped grating, and multiple layers of the same grating are processed along the y direction to form a fork-shaped grating array. Then, the laser scribes lines along the z direction to obtain a periodic grating, and multiple layers of the same grating are processed along the y direction to form a periodic grating array.

[0024] Preferably, the preparation system of the dual-frequency array-distributed orbital angular momentum mode generating device further includes a shutter, which is arranged between the first laser and the first half-wave plate and is used to control the opening and closing of the laser.

[0025] Preferably, the preparation system of the dual-frequency array-distributed orbital angular momentum mode generating device further includes a dichroic mirror, which is arranged between the second lens and the objective lens and is used to reflect the laser light emitted from the second lens to the objective lens, and at the same time transmit the beam reflected by the lithium niobate crystal to the CCD for collection.

[0026] Preferably, the first laser is used to emit laser light with a wavelength of 800 nm, a pulse width of 34 fs, and a repetition frequency of 1 kHz along the x direction.

[0027] Preferably, the focal lengths of the first lens and the second lens are 75 mm.

[0028] In a third aspect, the present invention provides a dual-frequency array-distributed orbital angular momentum mode generating system, which includes the dual-frequency array-distributed orbital angular momentum mode generating device, and further includes a second laser, a third half-wave plate, a second polarization beam splitter, a fourth half-wave plate, a third lens, and a receiving screen;

[0029] The second laser emits laser light along the x direction. The energy of the laser is regulated by the third half-wave plate and the second polarization beam splitter. The polarization direction of the laser is changed by the fourth half-wave plate so that the polarization is along the z direction, and then it is focused on the dual-frequency array-distributed orbital angular momentum mode generating device through the third lens, and the second harmonic signal is observed on the receiving screen.

[0030] Compared with the prior art, the beneficial effects of the present invention at least include:

[0031] The present invention processes a three-dimensional grating structure in a lithium niobate crystal. First, efficient second harmonic generation conversion is carried out using the type-I birefringence phase matching of the lithium niobate crystal itself, and then the optical field of the second harmonic beam is regulated based on the refractive index change of the grating structure. Based on the flexibility of laser processing, a fork-shaped grating array combined with a periodic grating array with perpendicular fringe directions is designed to form a three-dimensional volume grating structure, which can make the fundamental wave and the second harmonic beam undergo linear diffraction simultaneously, generating an array-distributed orbital angular momentum mode; at the same time, the fundamental wave and the second harmonic orbital angular momentum modes are extended from one-dimensional distribution to two-dimensional, increasing the modulation channels of the optical field. Brief Description of the Drawings

[0032] To more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0033] Figure 1 It is the design principle of the dual-frequency array-distributed orbital angular momentum mode generation device (three-dimensional nonlinear photonic crystal) of the present invention; where (a) crystal design; (b) birefringence phase matching; (c) linear diffraction of the fundamental frequency light and the second harmonic light.

[0034] Figure 2 It is the structural diagram of the preparation system of the dual-frequency array-distributed orbital angular momentum mode generation device of the present invention (nonlinear photonic crystal processing); where (a) processing optical path; (b) processing structural diagram.

[0035] Figure 3 It is the structural diagram of the dual-frequency array-distributed orbital angular momentum mode generation system of the present invention;

[0036] Figure 4 It is the fundamental frequency and second harmonic optical fields of the present invention. Where (a) fundamental frequency light spot; (b) second harmonic light spot. Detailed Description of the Embodiments

[0037] To make the above objects, features, and advantages of the present invention more obvious and understandable, the technical solutions of the present invention will be described in detail below in conjunction with the drawings and specific embodiments. It should be noted that the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art without creative efforts based on the embodiments of the present invention belong to the scope of protection of the present invention.

[0038] Embodiment 1

[0039] The present invention discloses a dual-frequency array-distributed orbital angular momentum mode generation device, which is processed by using femtosecond laser processing technology to change the refractive index of a lithium niobate crystal. After processing, the lithium niobate crystal is divided into three regions, namely a first region, a second region, and a third region arranged in sequence from left to right; the first region is an unprocessed region, which mainly uses the birefringent phase matching of the lithium niobate crystal to convert the fundamental frequency light into second harmonic light; the second region and the third region mainly use femtosecond laser processing technology to change the refractive index of the lithium niobate crystal and perform linear diffraction on the fundamental frequency light and the second harmonic light. The second region is a fork-shaped grating array, which can perform linear diffraction of the fundamental frequency light and the second harmonic light in the z direction and load orbital angular momentum at non-zero orders; the third region is a periodic grating array, which can perform linear diffraction of the fundamental frequency light and the second harmonic light in the x direction to make the orbital angular momentum mode be array-distributed. The present invention can simultaneously expand the fundamental wave and the second harmonic orbital angular momentum modes from one-dimensional distribution to two-dimensional, increasing the modulation channels of the optical field.

[0040] Specifically, as Figure 1 shown, this embodiment provides a dual-frequency array-distributed orbital angular momentum mode generation device. As Figure 1 (a) shows, this device is composed of a processed lithium niobate crystal and is mainly divided into three regions. The first region is an unprocessed region, the second region is a fork-shaped grating array, and the third region is a periodic grating array, which can be expressed by the formula:

[0041]

[0042] where is the azimuth angle in the x-z plane, l0 is the topological charge number of the OAM mode, and the spatial frequencies G x = 2π / Λ x 、G y = 2π / Λ y 、G z = 2π / Λ z , Λ x 、Λ y 、Λ y are the grating periods in the x, y, and z directions in the rectangular coordinate system respectively. T is a binarization function, which can be expressed for any value Z as:

[0043]

[0044] Femtosecond laser processing can cause a change Δn in the refractive index of the crystal. The linear fork-shaped grating array can be expressed as:

[0045] n(x, y, z) = n0 - Δnf(x, y, z) (3)

[0046] where n0 represents the original refractive index of the lithium niobate crystal.

[0047] When the fundamental wave is incident on the crystal along the y direction and polarized along the z direction (the optical axis direction), the fundamental wave is an ordinary light (abbreviated as o - light). After passing through the unprocessed area of the lithium niobate crystal, a second - harmonic - generated extraordinary light (abbreviated as e - light) is generated. As shown in Figure 1 (b), the quasi - birefringent phase - matching condition satisfied is:

[0048] k 2ω (e)=k ω (o)+k ω (o)(4)

[0049] Where k 2ω (e) and k ω (o) are the wave vectors of the second - harmonic - generated e - light and the fundamental o - light respectively. It can be seen from formula (4) that the second - harmonic - generated light passing through the unprocessed area is e - light. Then, linear diffraction occurs through the laser - processed area, as shown in Figure 1 (c). First, through the fork - shaped grating array, linear diffraction occurs along z, generating OAM modes with a topological charge of 1 at the positive and negative first orders, and the zero - order is still the Gaussian mode. Then, through the periodic grating array, linear diffraction occurs along the x direction, and the second - harmonic - generated light spot is expanded to a two - dimensional plane, generating multiple OAM modes with a topological charge of 1. Among them, (0, 1), (1, 1), (0, - 1), (1, - 1), (- 1, - 1), (- 1, 1) are OAM modes with a topological charge of 1, and the positions of (0, 0), (1, 0), (- 1, 0) are Gaussian modes. In addition, the change in refractive index can also perform the same linear diffraction on the fundamental wave simultaneously, realizing the array distribution of the dual - frequency OAM modes.

[0050] Embodiment 2

[0051] As shown in Figure 2 , the present invention provides a preparation system for a dual - frequency array - distributed orbital angular momentum mode generation device for preparing the dual - frequency array - distributed orbital angular momentum mode generation device, including a first laser, a shutter, a first half - wave plate, a second half - wave plate, a first polarization beam splitter, a first lens, a second lens, a small hole, a gradient attenuator, an objective lens, a CCD, a three - dimensional nano - translation stage, and a computer;

[0052] The dual - frequency array - distributed orbital angular momentum mode generation device is processed in a 1.5mm (x)×2.5mm (y)×1mm (z) lithium niobate crystal, as shown in Figure 2As shown in (a), the first laser emits a laser beam with a wavelength of 800 nm, a pulse width of 34 fs, and a repetition rate of 1 kHz along the x direction. The on / off of the laser is controlled by a shutter, the power of the laser is controlled by a first half-wave plate and a first polarization beam splitter, the polarization of the laser is changed to be along the z direction by a second half-wave plate, the beam is shaped by a first lens with a focal length of 75 mm, a small hole, and a second lens with a focal length of 75 mm, and the energy compensation in the depth direction of the laser processing is performed by a gradient attenuator, which can make the processed structural lines more uniform. After passing through a first objective lens with a numerical aperture of 0.75, the laser is focused on a 5% magnesium-doped lithium niobate crystal, and the reflected beam is collected by a CCD to observe the processing state in real time. The lithium niobate crystal is placed on a three-dimensional nano translation stage, and the travel of the three-dimensional nano translation stage is controlled by a computer (Labview software) to make the laser scribe lines along the x direction. The laser processing energy changes from 245 nJ to 204 nJ in the range from 120 μm to 20 μm below the crystal surface. The length of the line is 100 μm, a fork-shaped grating is formed along the z direction, and 15 identical gratings are processed along the y direction to form a fork-shaped grating array. The periods Λ in the y direction and the z direction y = Λ z = 3.35 μm. Then, the laser scribes lines along the z direction. The length of the line is 100 μm, the laser processing energy changes from 225 nJ to 115 nJ in the range from 120 μm to 20 μm below the crystal surface. A periodic grating is obtained along the x direction, and 10 identical gratings are processed along the y direction to form a periodic grating array. The periods Λ in the x direction and the y direction x = Λ y = 5 μm. The processed structures in the x-z plane and the y-z plane are as shown in Figure 2 (b).

[0053] Specifically, the preparation system of the dual-frequency array-distributed orbital angular momentum mode generation device further includes a dichroic mirror, which is arranged between the second lens and the objective lens and is used to reflect the laser beam emitted by the second lens to the objective lens, and at the same time transmit the beam reflected by the lithium niobate crystal to the CCD for collection.

[0054] Example 3

[0055] As shown in Figure 3 , the present invention provides a dual-frequency array-distributed orbital angular momentum mode generation system, which includes the dual-frequency array-distributed orbital angular momentum mode generation device, and also includes a second laser, a third half-wave plate, a second polarization beam splitter, a fourth half-wave plate, a third lens, and a receiving screen;

[0056] The second laser emits laser with a pulse width of 140 fs and a repetition frequency of 80 MHz along the x direction. The energy of the laser is regulated by a third half-wave plate and a second polarization beam splitter. The polarization direction of the fundamental wave is changed by a fourth half-wave plate to make the polarization along the z direction, and then it is focused onto the dual-frequency array-distributed orbital angular momentum mode generating device through a third lens with a focal length of 75 mm, and the second-harmonic signal is observed on the receiving screen.

[0057] The emitted fundamental wave and second-harmonic optical field are as Figure 4 shown. Compared with the Figure 1 (c) theoretical optical field shown, due to the platform for placing the crystal blocking the downward diffracted light spot, only the light spots at positions (0, 1), (1, 1), (0, -1), (1, -1), (0, 0), and (1, 0) are observed on the observation screen. Among them, (0, 1), (1, 1), (0, -1), and (1, -1) are OAM modes, and the positions (0, 0) and (1, 0) are Gaussian modes. For easy observation, the fundamental wave is first modulated to the visible band. When the fundamental wave with a wavelength of 760 nm and a power of 0.34 W is incident on the crystal, the light spot is as Figure 4 (a) shown. After measurement, the light spot powers at positions (0, 0), (1, 0), (0, 1), and (1, 1) are 146 mW, 16.4 mW, 6.8 mW, and 1.8 mW respectively. When the fundamental wave with a wavelength of 1030 nm and a power of 78 mW is switched and incident on the crystal, satisfying the birefringent phase matching condition, the second-harmonic light spot is as Figure 4 (b) shown. After measurement, the light spot powers at positions (0, 0), (1, 0), (0, 1), and (1, 1) are 580 μW, 30 μW, 16.9 μW, and 3 μW respectively, and the normalized second-harmonic generation efficiencies are 9.5×10 -2 W -1 , 0.5×10 -2 W -1 , 0.3×10 -2 W -1 , 0.05×10 -2 W -1 .

[0058] A dual-frequency array-distributed orbital angular momentum mode generating device disclosed by the present invention can enable the fundamental wave and the second-harmonic light beam to simultaneously undergo linear diffraction to generate an array-distributed orbital angular momentum mode. The present invention can simultaneously expand the one-dimensional distribution of the fundamental wave and the second-harmonic orbital angular momentum mode to two dimensions, increasing the modulation channels of the optical field.

[0059] The above-described embodiments merely represent several implementation manners of the present invention. The description thereof is relatively specific and detailed, but it should not be construed as a limitation on the scope of the patent for the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several variations and improvements can still be made, and these all fall within the protection scope of the present invention. Therefore, the protection scope of the patent for the present invention shall be subject to the appended claims.

Claims

1. A dual-frequency array-distributed orbital angular momentum mode generation device, characterized in that The device is processed by using laser processing technology to change the refractive index of the lithium niobate crystal. The processed lithium niobate crystal includes three regions, namely a first region, a second region, and a third region arranged in sequence from left to right; The first region is an unprocessed region, and the fundamental frequency light is converted into second harmonic light by using the birefringent phase matching of the lithium niobate crystal; In the second region and the third region, the refractive index of the lithium niobate crystal is changed by using laser processing technology, and linear diffraction of the fundamental frequency light and the second harmonic light is performed; The second region is a fork-shaped grating array, which performs linear diffraction of the fundamental frequency light and the second harmonic light in the z direction and loads orbital angular momentum at non-zero orders; The third region is a periodic grating array, which performs linear diffraction of the fundamental frequency light and the second harmonic light in the x direction, so that the orbital angular momentum modes are distributed in an array; When the fundamental wave is incident on the crystal along the y direction and polarized along the z direction, the fundamental wave is an ordinary light. After passing through the unprocessed region of the lithium niobate crystal, a second harmonic extraordinary light is generated; then linear diffraction occurs through the laser processing region. First, it passes through the fork-shaped grating array and undergoes linear diffraction along the z direction; then it passes through the periodic grating array and undergoes linear diffraction along the x direction, and the second harmonic light spot is expanded to a two-dimensional plane; the change in refractive index can also perform the same linear diffraction on the fundamental wave at the same time, realizing the array distribution of the dual-frequency OAM mode.

2. The dual-frequency array distribution orbital angular momentum mode generation device according to claim 1, wherein The processed lithium niobate crystal includes three regions, which is expressed by the formula: Among them, is the azimuth angle of the x-z plane, l0 is the topological charge number of the OAM mode, and the spatial frequency G x = 2π / Λ x 、G y = 2π / Λ y 、G z = 2π / Λ z , Λ x 、Λ y 、Λ y are the grating periods in the x, y, and z directions in the rectangular coordinate system respectively; T is a binarization function, and for any value Z, it is expressed as: Laser processing can cause a change in the refractive index of the crystal Δn, and the linear fork-shaped grating array is expressed as: n(x,y,z) = n0 - Δnf(x,y,z) (3) where n0 represents the original refractive index of the lithium niobate crystal.

3. The dual-frequency array distribution orbital angular momentum mode generating device according to claim 2, characterized in that When the fundamental wave is incident on the crystal along the y direction and polarized along the z direction, the fundamental wave is an ordinary light, that is, an o light. After passing through the unprocessed region of the lithium niobate crystal, a second harmonic extraordinary light, that is, an e light, is generated. The quasi-birefringent phase matching condition satisfied is: k 2ω (e) = k ω (o) + k ω (o) (4) where k 2ω (e), k ω (o) are the wave vectors of the second-harmonic e-light and the fundamental o-light respectively. It can be seen from formula (4) that the second-harmonic light passing through the non-processing area is e-light, and then linear diffraction occurs through the laser processing area. First, it passes through the fork grating array and undergoes linear diffraction along z, generating an OAM mode with a topological charge of 1 at the positive and negative first orders, and the zero order remains a Gaussian mode. Then, it passes through the periodic grating array and undergoes linear diffraction along the x direction, and the second-harmonic light spot is extended to a two-dimensional plane, generating multiple OAM modes with a topological charge of 1. Among them, (0, 1), (1, 1), (0, -1), (1, -1), (-1, -1), (-1, 1) are OAM modes with a topological charge of 1, and the positions of (0, 0), (1, 0), (-1, 0) are Gaussian modes.

4. The dual-frequency array distribution orbital angular momentum mode generation device according to claim 1, wherein The laser processing technology is femtosecond laser processing technology.

5. A preparation system for a dual-frequency array-distributed orbital angular momentum mode generation device, which is used to prepare the dual-frequency array-distributed orbital angular momentum mode generation device as described in any one of claims 1-4, and is characterized in that It includes a first laser, a first half-wave plate, a second half-wave plate, a first polarization beam splitter, a first lens, a second lens, a small hole, a gradient attenuator, an objective lens, a CCD, a three-dimensional nano translation stage, and a computer; The first laser emits laser light along the x direction. The power of the laser is controlled by the first half-wave plate and the first polarization beam splitter. The polarization of the laser is changed to be along the z direction by the second half-wave plate. The beam is shaped by the first lens, the small hole, and the second lens. The energy compensation in the depth direction of the laser processing is performed by the gradient attenuator. After the laser passes through the objective lens, it is focused on the magnesium-doped lithium niobate crystal, and the reflected beam is collected by the CCD to observe the processing state in real time; the lithium niobate crystal is placed on the three-dimensional nano translation stage, and the travel of the three-dimensional nano translation stage is controlled by the computer, so that the laser scribes lines along the x direction to form a fork-shaped grating, and multiple layers of the same grating are processed along the y direction to form a fork-shaped grating array; then the laser scribes lines along the z direction to obtain a periodic grating, and multiple layers of the same grating are processed along the y direction to form a periodic grating array.

6. The preparation system of the dual-frequency array distribution orbital angular momentum mode generation device according to claim 5, characterized in that The preparation system of the dual-frequency array-distributed orbital angular momentum mode generation device further includes a shutter, which is arranged between the first laser and the first half-wave plate and is used to control the turning on and off of the laser.

7. The preparation system of the dual-frequency array distribution orbital angular momentum mode generation device according to claim 5, characterized in that, The preparation system of the dual-frequency array-distributed orbital angular momentum mode generation device further includes a dichroic mirror, which is arranged between the second lens and the objective lens and is used to reflect the laser emitted from the second lens to the objective lens, and at the same time transmit the beam reflected by the lithium niobate crystal to the CCD for collection.

8. The preparation system of the dual-frequency array distribution orbital angular momentum mode generation device according to claim 5, characterized in that, The first laser is used to emit laser with a wavelength of 800 nm, a pulse width of 34 fs, and a repetition frequency of 1 kHz along the x direction.

9. The preparation system of the dual-frequency array distribution orbital angular momentum mode generation device according to claim 5, characterized in that The focal lengths of the first lens and the second lens are 75 mm.

10. A dual-frequency array-distributed orbital angular momentum mode generation system, comprising the dual-frequency array-distributed orbital angular momentum mode generation device according to any one of claims 1-4, characterized in that, It further includes a second laser, a third half-wave plate, a second polarization beam splitter, a fourth half-wave plate, a third lens, and a receiving screen; The second laser emits laser along the x direction, regulates the energy of the laser through the third half-wave plate and the second polarization beam splitter, changes the polarization direction of the laser through the fourth half-wave plate to make the polarization along the z direction, and then focuses it onto the dual-frequency array-distributed orbital angular momentum mode generation device through the third lens, and observes the second harmonic signal on the receiving screen.

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