A dual-polarization frequency-doubling orbital angular momentum mode generator

Abstract

The present invention discloses a dual-polarization frequency-doubled orbital angular momentum mode generating device, which is processed in a lithium niobate crystal using laser processing technology. The processed lithium niobate crystal includes two regions; the first region is a non-processed region, which converts the fundamental frequency ordinary light into the frequency-doubled extraordinary light based on the principle of birefringence phase matching; the second region is a fork-shaped grating array, and due to the change in the refractive index of the region, the frequency-doubled extraordinary light is linearly holographically diffracted, and the orbital angular momentum is loaded at a high order; in addition, the change in the second-order nonlinear coefficient of the region also performs nonlinear holographic diffraction on the fundamental frequency extraordinary light based on the principle of quasi-phase matching, and generates the frequency-doubled extraordinary light with orbital angular momentum at a high order. The present invention is based on the principles of birefringence phase matching and quasi-phase matching, combined with linear holographic diffraction and nonlinear holographic diffraction, so that both polarizations of the fundamental frequency ordinary light and the extraordinary light can generate efficient frequency-doubled orbital angular momentum modes.

Description

Technical Field

[0001] The present invention relates to the technical field of nonlinear light field control, in particular to the generation of dual-polarization frequency-doubled orbital angular momentum modes in lithium niobate crystals, and specifically to a dual-polarization frequency-doubled orbital angular momentum mode generating device. Background Art

[0002] Photon orbital angular momentum (OAM) has the characteristics of spirally distributed phase, infinite dimension and orthogonality. Since it was verified by Allen et al. in 1992, it has been widely used in light manipulation, optical communication, quantum, imaging, detection and optical encryption. The important application of OAM mode in the field of optics has promoted the research on its generation method. In addition to the commonly used spiral phase plate, q-plate, computational holography, π / 2 mode converter, etc., there are also various micro-integrated OAM generation devices. However, these methods are limited to the linear field. If you want to efficiently generate OAM mode at a new frequency, the traditional method requires two devices: first, frequency conversion is performed through nonlinear materials, and then OAM is loaded through linear devices. In order to save cost and space, a device is needed to perform nonlinear light field control and realize beam shaping while nonlinear conversion. Two-dimensional nonlinear photonic crystals and metasurfaces can realize nonlinear light field control, but the efficiency is low. The three-dimensional nonlinear photonic crystals that have emerged in recent years are not only designed in the vertical propagation direction, but also in the vertical direction. (2) The light field can be distributed and regulated, and at the same time, the periodic structure can be considered in the propagation direction to achieve quasi-phase matching, improve the nonlinear conversion efficiency, and realize efficient nonlinear beam shaping of extraordinary light (e-light for short).

[0003] Femtosecond laser direct writing technology provides a flexible way to prepare three-dimensional nonlinear photonic crystals. It can not only erase the second-order nonlinear coefficient χ at any position inside the crystal, but also (2) By utilizing the birefringence phase matching of lithium niobate crystals, the fundamental frequency ordinary light (ordinary light, referred to as o-light) is firstly efficiently nonlinearly converted to generate double frequency e-light, and then the double frequency e-light is linearly diffracted based on the change of the structural refractive index, which can also achieve efficient nonlinear light field control. Summary of the invention

[0004] In view of this, in order to solve the above-mentioned problems in the prior art, the present invention proposes a dual-polarization frequency-doubled orbital angular momentum mode generating device, which can simultaneously realize birefringence phase matching and quasi-phase matching by switching the polarization direction of the fundamental wave, and combines linear holographic diffraction and nonlinear holographic diffraction to make both o-light and e-light produce efficient frequency-doubled OAM mode, ultimately realizing the generation of efficient dual-polarization frequency-doubled OAM mode.

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

[0006] In a first aspect, the present invention provides a dual-polarization frequency-doubled orbital angular momentum mode generating device, which is processed in a lithium niobate crystal using laser processing technology, which not only changes the second-order nonlinear technology of the lithium niobate crystal, but also changes the refractive index of the lithium niobate crystal. The processed lithium niobate crystal includes two regions, which are a first region and a second region arranged in sequence from left to right;

[0007] The first region is a non-processing region, and based on the birefringence phase matching principle, the fundamental frequency ordinary light is converted into the doubled frequency extraordinary light;

[0008] The second region is a fork-shaped grating array. Due to the change in the refractive index of this region, linear holographic diffraction is performed on the doubled extraordinary light, and orbital angular momentum is loaded at a high order. In addition, the change in the second-order nonlinear coefficient of this region also performs nonlinear holographic diffraction on the fundamental frequency extraordinary light based on the quasi-phase matching principle, and generates doubled extraordinary light with orbital angular momentum at a high order.

[0009] Preferably, the processed lithium niobate crystal includes two regions, represented as:

[0010]

[0011] in, is the azimuth angle of the xz plane, l0 is the topological charge of the OAM mode, x, y, z are the three azimuth values ​​of the rectangular coordinate system, and on the yz plane, the spatial frequency G y =G z =2π / Λ, where Λ is the grating period; T is the binarization function, for any value Z, it is expressed as:

[0012]

[0013] Laser processing technology causes the change of the second-order nonlinear coefficient of the crystal Δχ (2) and the refractive index change Δn, the linear and nonlinear fork grating arrays are expressed as:

[0014] n(x,y,z)=n0-Δnf(x,y,z) (3)

[0015]

[0016] where n0 and Represents the original refractive index and nonlinear coefficient of lithium niobate crystal.

[0017] Preferably, when the fundamental wave is incident on the crystal along the y direction and polarized along the z direction, the fundamental wave is ordinary light, i.e., o light, and passes through the non-processed area of ​​the lithium niobate crystal to generate frequency-doubled extraordinary light, i.e., e light, and the birefringence phase matching condition satisfied is:

[0018] k 2ω (e) = k ω (o)+k ω (o) (5)

[0019] where k 2ω (e), k ω (o) are the wave vectors of the doubled frequency e-light and the fundamental frequency o-light, respectively. From formula (5), we can see that the doubled frequency light passing through the non-processed area is e-light; then it undergoes linear holographic diffraction through the processed area, generating an orbital angular momentum beam at a higher order;

[0020] When the fundamental wave is incident on the crystal along the y direction and polarized along the x direction, the fundamental wave is e-light. The change of the second-order nonlinear coefficient directly performs nonlinear holographic diffraction on the fundamental wave, and an OAM frequency-doubled beam is emitted at a higher order, while satisfying the quasi-phase matching condition:

[0021] k 2ω (e) = k ω (e)+k ω (e)+G m,n (6)

[0022] where k ω (e) is the fundamental frequency e light, G m,n The reciprocal lattice vector required for the quasi-phase matching condition is expressed as:

[0023] G m,n =mG y +nG z (7)

[0024] Where m and n are arbitrary integers. From formula (6), it can be seen that the frequency-doubled OAM light emitted by quasi-phase matching is still e-light.

[0025] Preferably, the laser processing technology is femtosecond laser processing technology.

[0026] In a second aspect, the present invention provides a system for preparing a dual-polarization frequency-doubling orbital angular momentum mode generating device, which is used to prepare the dual-polarization frequency-doubling orbital angular momentum mode generating device, comprising 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 pinhole, a gradient attenuation plate, an objective lens, a CCD, a three-dimensional nano-translation stage and a computer;

[0027] The first laser emits laser along the x direction, the first half-wave plate and the first polarization beam splitter control the laser power, the second half-wave plate changes the polarization of the laser to be along the z direction, the first lens, the pinhole and the second lens are used to shape the light beam, the laser power is continuously changed through the gradient attenuation plate, and the energy compensation of laser processing in depth is realized. After the laser passes through the objective lens, it is focused on the magnesium-doped lithium niobate crystal, and the reflected light beam is collected by the CCD to observe the processing status in real time; the lithium niobate crystal is placed on a three-dimensional nano translation stage, and the stroke of the three-dimensional nano translation stage is controlled by a computer, so that the laser is engraved along the x direction to form a fork grating, and multiple layers of the same grating are processed along the y direction to form a fork grating array.

[0028] Preferably, the system for preparing the dual-polarization frequency-doubled orbital angular momentum mode generating device further comprises 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.

[0029] Preferably, the preparation system of the dual-polarization frequency-doubled orbital angular momentum mode generating device also includes a dichroic mirror, which is arranged between the second lens and the objective lens, and is used to reflect the laser emitted by the second lens to the objective lens, and at the same time transmit the light beam reflected by the lithium niobate crystal to the CCD for collection.

[0030] Preferably, the first laser emits 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.

[0031] Preferably, the focal length of the first lens and the second lens is 75 mm.

[0032] In a third aspect, the present invention provides a dual-polarization frequency-doubling orbital angular momentum mode generating system, comprising the dual-polarization frequency-doubling orbital angular momentum mode generating device, and also comprising 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;

[0033] The second laser emits laser along the x direction, and the energy of the laser is regulated by the third half-wave plate and the second polarization beam splitter, and the polarization direction of the fundamental wave is changed by the fourth half-wave plate, and then it is focused by the third lens onto the dual-polarization frequency-doubled orbital angular momentum mode generating device, and the frequency-doubled signal is observed on the receiving screen.

[0034] Compared with the prior art, the beneficial effects of the present invention include at least:

[0035] The present invention processes a three-dimensional fork grating array in a lithium niobate crystal, based on the principles of birefringence phase matching and quasi-phase matching, combined with linear holographic diffraction and nonlinear holographic diffraction, so that both o-light and e-light produce efficient frequency-doubled OAM modes, and ultimately achieves the generation of efficient dual-polarization frequency-doubled orbital angular momentum modes. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0037] Figure 1 It is a design principle diagram of the dual-polarization frequency-doubling orbital angular momentum mode generating device (nonlinear photonic crystal) of the present invention;

[0038] Figure 2 It is a structural diagram of the preparation system of the dual-polarization frequency-doubling orbital angular momentum mode generating device of the present invention (processing of nonlinear photonic crystal);

[0039] Figure 3 It is a structural diagram of the dual-polarization frequency-doubling orbital angular momentum mode generation system of the present invention;

[0040] Figure 4 : This is the light field diagram of the frequency-doubled light, where (a) the birefringence phase matches the frequency-doubled light spot; (b) the quasi-phase matches the frequency-doubled light spot. DETAILED DESCRIPTION

[0041] In order to make the above-mentioned purposes, features and advantages of the present invention more obvious and easy to understand, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be pointed out that the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0042] Example 1

[0043] The present invention discloses a dual-polarization frequency-doubled orbital angular momentum mode generating device, which is processed in a lithium niobate crystal by using a femtosecond laser processing technology, which not only changes the second-order nonlinear technology of the lithium niobate crystal, but also changes the refractive index of the lithium niobate crystal. The structure of the processed lithium niobate crystal mainly includes two regions, namely, a first region and a second region arranged from left to right; the first region is a non-processing region, which can convert the baseband ordinary light into the frequency-doubled extraordinary light based on the principle of birefringence phase matching; the second region is a fork-shaped grating array, and since the refractive index of the region changes, the frequency-doubled extraordinary light can be linearly holographically diffracted, and the orbital angular momentum is loaded at a high order. In addition, the change of the second-order nonlinear coefficient of the region can also perform nonlinear holographic diffraction on the baseband extraordinary light based on the quasi-phase matching principle, and generate the frequency-doubled extraordinary light with orbital angular momentum at a high order. This technology processes a three-dimensional fork grating array in a lithium niobate crystal. Based on the principles of birefringence phase matching and quasi-phase matching, it combines linear holographic diffraction and nonlinear holographic diffraction to enable both fundamental frequency ordinary light and extraordinary light polarizations to produce efficient frequency-doubled orbital angular momentum modes.

[0044] Specifically, Figure 1 As shown, this embodiment provides a dual-polarization frequency-doubling orbital angular momentum mode generating device, such as Figure 1 As shown in (a), the structure of the processed lithium niobate crystal mainly includes two regions, which can be expressed as:

[0045]

[0046] in, is the azimuth angle of the xz plane, l0 is the topological charge of the OAM mode, x, y, z are the three azimuth values ​​of the rectangular coordinate system, and on the yz plane, the spatial frequency G y =G z =2π / Λ, where Λ is the grating period. T is a binary function, which can be expressed as:

[0047]

[0048] Femtosecond laser processing can cause a change in the second-order nonlinear coefficient of the crystal Δχ (2) and the refractive index change Δn, the linear and nonlinear fork grating arrays can be expressed as:

[0049] n(x,y,z)=n0-Δnf(x,y,z) (3)

[0050]

[0051] where n0 and Represents the original refractive index and nonlinear coefficient of lithium niobate crystal.

[0052] When the fundamental wave is incident on the crystal along the y direction and polarized along the z direction (optical axis direction), the fundamental wave is ordinary light (ordinary light, referred to as o light), which passes through the non-processed area of ​​the lithium niobate crystal and generates frequency-doubled extraordinary light (ordinary light, referred to as e light), such as Figure 1 As shown in (b), the birefringence-like phase matching condition is:

[0053] k 2ω (e) = k ω (o)+k ω (o) (5)

[0054] where k 2ω (e), k ω (o) are the wave vectors of the doubled frequency e-light and the fundamental frequency o-light, respectively. From formula (5), it can be seen that the doubled frequency light passing through the non-processed area is e-light. Then, linear holographic diffraction occurs in the processed area, generating an orbital angular momentum beam at a higher order.

[0055] When the fundamental wave is incident on the crystal along the y direction and polarized along the x direction, the fundamental wave is e-light. The change of the second-order nonlinear coefficient directly performs nonlinear holographic diffraction on the fundamental wave, and an OAM frequency-doubled beam is emitted at a higher order, satisfying Figure 1 (c) shows the quasi-phase matching condition:

[0056] k 2ω (e) = k ω (e)+k ω (e)+G m,n (6)

[0057] where k ω (e) is the fundamental frequency e light, G m,n The reciprocal lattice vector required for quasi-phase matching conditions can be expressed as:

[0058] G m,n =mG y +nG z (7)

[0059] Where m and n can be any integers. It can be seen from formula (6) that the frequency-doubled OAM light emitted by quasi-phase matching is still e-light.

[0060] Example 2

[0061] like Figure 2As shown, the present invention provides a preparation system for a dual-polarization frequency-doubling orbital angular momentum mode generating device, which is used to prepare the dual-polarization frequency-doubling orbital angular momentum mode generating 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 pinhole, a gradient attenuation plate, an objective lens, a CCD, a three-dimensional nano translation stage and a computer.

[0062] The dual-polarization frequency-doubling orbital angular momentum mode generating device is processed in a 1.5 mm (x) × 2.5 mm (y) × 1 mm (z) lithium niobate crystal, such as Figure 2 As shown in (a), the first laser emits a laser with a wavelength of 800nm, a pulse width of 34fs, and a repetition frequency of 1kHz along the x direction. The shutter controls the switch of the laser, the first half-wave plate and the first polarization beam splitter control the power of the laser, the second half-wave plate changes the polarization of the laser to the z direction, the first lens with a focal length of 75mm, the pinhole and the second lens with a focal length of 75mm are used to shape the beam, the laser power is continuously changed through the gradient attenuation plate, the energy compensation of laser processing in depth is realized, and the processed structural lines can be made more uniform, the laser passes through the objective lens with a numerical aperture of 0.75, and is focused on a 5% magnesium-doped lithium niobate crystal, the reflected beam is collected by the CCD, and the processing status is observed in real time. The lithium niobate crystal is placed on a three-dimensional nano-translation stage. The stroke of the three-dimensional nano-translation stage is controlled by a computer (Labview software) to make the laser scribe along the x-direction. The energy of the laser processing changes from 240nJ to 200nJ between 120μm and 20μm below the crystal surface. The length of the line is 100μm, forming a fork-shaped grating. 15 layers of the same grating are processed along the y-direction to form a fork-shaped grating array. The period of the y-axis and z-axis is Λ=6.25μm. The prepared structure is shown in FIG. Figure 2 (b).

[0063] Specifically, the preparation system of the dual-polarization frequency-doubled orbital angular momentum mode generating device also includes a dichroic mirror, which is arranged between the second lens and the objective lens, and is used to reflect the laser emitted by the second lens to the objective lens, and at the same time transmit the light beam reflected by the lithium niobate crystal to the CCD for collection.

[0064] Example 3

[0065] like Figure 3 As shown, the present invention provides a dual-polarization frequency-doubling orbital angular momentum mode generating system, including the dual-polarization frequency-doubling orbital angular momentum mode generating device, and also including 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.

[0066] The second laser emits a laser with a pulse width of 140fs and a repetition frequency of 80MHz 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, and then the laser is focused by a third lens with a focal length of 75mm to hit the dual-polarization frequency-doubled orbital angular momentum mode generating device, and the frequency-doubled signal is observed on a receiving screen.

[0067] The experimental results are as follows Figure 4 As shown: The wavelength is 1030nm, when the polarization is in the z direction, the frequency-doubled light spot is as follows: Figure 4 As shown in (a), the normalized frequency doubling conversion efficiency of the first-level OAM mode is measured to be 2×10 -3 W -1 When the polarization changes to the x direction, the reciprocal lattice vector used for quasi-phase matching is G 1,1 , the emitted frequency-doubled light spot is as follows Figure 4 As shown in (b), the normalized frequency doubling conversion efficiency of the first-level OAM mode is measured to be 1.2×10 -5 W -1 .

[0068] The present invention can simultaneously realize birefringence phase matching and quasi-phase matching by switching the polarization direction of the fundamental wave, and combine linear holographic diffraction and nonlinear holographic diffraction to make both o-light and e-light produce efficient frequency-doubled OAM mode, and finally realize the generation of efficient dual-polarization frequency-doubled orbital angular momentum mode.

[0069] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A dual-polarization frequency-doubling orbital angular momentum mode generating device, characterized in that: The device is processed in a lithium niobate crystal by using laser processing technology, which not only changes the second-order nonlinear technology of the lithium niobate crystal, but also changes the refractive index of the lithium niobate crystal. The processed lithium niobate crystal includes two regions, which are a first region and a second region arranged in sequence from left to right. The first region is a non-processed region, and based on the principle of birefringence phase matching, the fundamental frequency ordinary light is converted into the doubled frequency extraordinary light; when the fundamental wave is incident on the crystal along the y direction and the polarization is along the z direction, the fundamental wave is ordinary light, and passes through the non-processed region of the lithium niobate crystal to generate the doubled frequency extraordinary light; The second region is a fork-shaped grating array. Due to the change in the refractive index of the region, the frequency-doubled extraordinary light is linearly holographically diffracted, and the orbital angular momentum is loaded at a high order. In addition, the change of the second-order nonlinear coefficient in this area also performs nonlinear holographic diffraction on the fundamental frequency extraordinary light based on the quasi-phase matching principle. When the fundamental wave is incident on the crystal along the y direction and polarized along the x direction, the fundamental wave is the extraordinary light. The change of the second-order nonlinear coefficient directly performs nonlinear holographic diffraction on the fundamental wave, generating frequency-doubled extraordinary light with orbital angular momentum at higher orders.

2. The dual-polarization frequency-doubling orbital angular momentum mode generating device according to claim 1, characterized in that: The processed lithium niobate crystal consists of two regions, represented as: in, is the azimuth angle of the xz plane, l0 is the topological charge of the OAM mode, x, y, z are the three azimuth values ​​of the rectangular coordinate system, and on the yz plane, the spatial frequency G y =G z =2π / Λ, where Λ is the grating period; T is the binarization function, for any value Z, it is expressed as: Laser processing technology causes the change of the second-order nonlinear coefficient of the crystal Δχ (2) and the refractive index change Δn, the linear and nonlinear fork grating arrays are expressed as: n(x,y,z)=n0-Δnf(x,y,z) (3) where n0 and Represents the original refractive index and nonlinear coefficient of lithium niobate crystal.

3. The dual-polarization frequency-doubling 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 ordinary light, o light, which passes through the non-processed area of ​​the lithium niobate crystal and generates frequency-doubled extraordinary light, e light. The birefringence phase matching condition is: k 2ω (e)=k ω (o)+k ω (o) (5) where k 2ω (e), k ω (o) are the wave vectors of the doubled frequency e-light and the fundamental frequency o-light, respectively. From formula (5), we can see that the doubled frequency light passing through the non-processed area is e-light; then it undergoes linear holographic diffraction through the processed area, generating an orbital angular momentum beam at a higher order; When the fundamental wave is incident on the crystal along the y direction and polarized along the x direction, the fundamental wave is e-light. The change of the second-order nonlinear coefficient directly performs nonlinear holographic diffraction on the fundamental wave, and an OAM frequency-doubled beam is emitted at a higher order, while satisfying the quasi-phase matching condition: k 2ω (e)=k ω (e)+k ω (e)+G m,n (6) where k ω (e) is the fundamental frequency e light, G m,n The reciprocal lattice vector required for the quasi-phase matching condition is expressed as: G m,n =mG y +nG z (7) Where m and n are arbitrary integers; It can be seen from formula (6) that the frequency-doubled OAM light emitted by quasi-phase matching is still e-light.

4. The dual-polarization frequency-doubling orbital angular momentum mode generating device according to claim 1, characterized in that: The laser processing technology is a femtosecond laser processing technology.

5. A system for preparing a dual-polarization frequency-doubling orbital angular momentum mode generating device, used for preparing the dual-polarization frequency-doubling orbital angular momentum mode generating device as claimed in any one of claims 1 to 4, 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 pinhole, a gradient attenuation plate, an objective lens, a CCD, a three-dimensional nano translation stage and a computer; The first laser emits laser along the x direction, the first half-wave plate and the first polarization beam splitter control the laser power, the second half-wave plate changes the polarization of the laser to be along the z direction, the first lens, the pinhole and the second lens are used to shape the light beam, the laser power is continuously changed through the gradient attenuation plate, and the energy compensation of laser processing in depth is realized. After the laser passes through the objective lens, it is focused on the magnesium-doped lithium niobate crystal, and the reflected light beam is collected by the CCD to observe the processing status in real time; the lithium niobate crystal is placed on a three-dimensional nano translation stage, and the stroke of the three-dimensional nano translation stage is controlled by a computer, so that the laser is engraved along the x direction to form a fork grating, and multiple layers of the same grating are processed along the y direction to form a fork grating array.

6. The system for preparing the dual-polarization frequency-doubling orbital angular momentum mode generating device according to claim 5, characterized in that: The manufacturing system of the dual-polarization frequency-doubled orbital angular momentum mode generating device further comprises 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.

7. The system for preparing the dual-polarization frequency-doubling orbital angular momentum mode generating device according to claim 5, characterized in that: The preparation system of the dual-polarization frequency-doubled orbital angular momentum mode generating device also includes a dichroic mirror, which is arranged between the second lens and the objective lens and is used to reflect the laser emitted by the second lens to the objective lens, and at the same time transmit the light beam reflected by the lithium niobate crystal to the CCD for collection.

8. The system for preparing the dual-polarization frequency-doubling orbital angular momentum mode generating device according to claim 5, characterized in that: The first laser emits a 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 system for preparing the dual-polarization frequency-doubling orbital angular momentum mode generating device according to claim 5, characterized in that: The focal length of the first lens and the second lens is 75 mm.

10. A dual-polarization frequency-doubling orbital angular momentum mode generation system, characterized in that: A dual-polarization frequency-doubling orbital angular momentum mode generating device as claimed in any one of claims 1 to 4, further comprising 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, and the energy of the laser is regulated by the third half-wave plate and the second polarization beam splitter, and the polarization direction of the fundamental wave is changed by the fourth half-wave plate, and then it is focused by the third lens onto the dual-polarization frequency-doubled orbital angular momentum mode generating device, and the frequency-doubled signal is observed on the receiving screen.

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