Orbital angular momentum up-conversion detection device and method based on optical superlattice structure

Abstract

This invention relates to an orbital angular momentum upconversion detection device and method based on an optical superlattice structure. The optical superlattice structure contains a pre-defined two-dimensional spatially distributed ferroelectric domain structure. This ferroelectric domain structure satisfies the quasi-phase-matching condition of the nonlinear frequency conversion process and pre-modulates the complex amplitude distribution of the optical field participating in the frequency conversion. This allows the topological charge information of the incident vortex beam to be directly mapped into a specific fringe pattern on the cross-section of the output frequency-doubled beam during the frequency conversion process. Thus, the orbital angular momentum topological charge number carried by the incident vortex beam is determined based on the spatial distribution characteristics of the upconversion output optical field. This invention integrates frequency upconversion and orbital angular momentum detection into the same nonlinear crystal, which not only reduces the complexity of the detection system and improves integration, but also improves the nonlinear conversion efficiency by optimizing the ferroelectric domain structure.

Description

Technical Field

[0001] This invention relates to the fields of ferroelectric domain engineering, nonlinear optics and orbital angular momentum beam detection technology, and specifically to orbital angular momentum upconversion detection devices and methods based on optical superlattice structures. Background Technology

[0002] Vortex beams carrying orbital angular momentum (OAM) have gradually attracted researchers' interest due to their novel phase distribution and physical properties. OAM has proven to be an outstanding optical tool for applications such as optical communication, particle manipulation, and imaging. Since photon-carried OAM possesses an infinite number of eigenstates, theoretically, vortex beams carrying OAM have an infinite-dimensional space for loading information, thus greatly increasing the capacity of laser-loaded information. Therefore, accurate measurement of OAM is of great significance. For the detection of OAM in the mid-to-far-infrared band, where conventional detectors cannot respond, a nonlinear frequency conversion method is often used to convert the beam to the visible light band before detection.

[0003] For precise measurement of OAM, several research groups have proposed a method based on the singularity beam astigmatism transformation theory. Experimentally, they utilize cylindrical lenses to obtain dark fringes on the focal plane that characterize the OAM carried by vortex beams ("Determination of topological charges of polychromatic optical vortices," published in *Opticsexpress*, Vol. 17, No. 26, p. 23374, 2009). Another group has designed two graded-period gratings using the second-phase diffraction factor, enabling precise measurement of the incident beam's OAM by observing the diffraction pattern under far-field diffraction ("Measuring OAM states of light beams with gradually-changing-period gratings," published in *Optics Letters*, Vol. 40, No. 4, p. 562, 2015). For detecting OAM in mid- and far-infrared vortex beams, it is necessary to use an infrared detector or a nonlinear crystal to convert the mid- and far-infrared vortex beams to the visible light band before detection. Using an infrared detector requires... arrive Operating at low temperatures is expensive; while converting the mid-infrared band to the visible light band by frequency conversion and then using a series of astigmatic transformation elements such as tilting lenses to achieve OAM detection results in a detection device that is too complex and bulky, making it difficult to integrate and unable to meet the needs of scientific research and industrial production. Summary of the Invention

[0004] The purpose of this invention is to address the problems of complex detection systems, low integration, high cost, and separate implementation of frequency conversion and mode analysis in existing mid- and far-infrared vortex beam orbital angular momentum detection methods. This invention provides an orbital angular momentum upconversion detection device and method based on an optical superlattice structure. By constructing a pre-defined two-dimensional spatially distributed ferroelectric domain structure in a nonlinear crystal, the incident vortex beam simultaneously completes spatial mode mapping during frequency upconversion, thereby determining the topological charge of orbital angular momentum carried by the incident vortex beam based on the spatial distribution characteristics of the upconversion output light field.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: an orbital angular momentum upconversion detection device based on an optical superlattice structure, defining the x-direction as the direction perpendicular to the beam propagation direction, defining the y-direction as the beam propagation direction, and sequentially including a spatial light modulator along the light propagation direction. The system comprises an optical superlattice structure and a filter. A fork-shaped grating hologram is loaded within the spatial light modulator. Initial linearly polarized light enters the spatial light modulator to generate a vortex beam, which then passes through… The system performs spatial filtering, then passes through a half-wave plate and a third lens in sequence, and then along... The light is incident on the optical superlattice structure in a specific direction, and after the fundamental frequency beam is filtered out by a filter, it is collected by a CCD camera.

[0006] The optical superlattice structure is provided with a ferroelectric domain structure with a preset two-dimensional spatial distribution. The ferroelectric domain structure is used to satisfy the quasi-phase matching condition of the nonlinear frequency conversion process and to preset modulate the complex amplitude distribution of the optical field participating in the frequency conversion, so that the topological charge information of the incident vortex beam is directly mapped into a specific stripe pattern on the cross section of the output frequency-doubled beam during the frequency conversion process.

[0007] Preferably, the The system includes, in sequence along the direction of light propagation, a first lens, a pinhole aperture, a second lens, a first reflecting mirror, and a second reflecting mirror.

[0008] Preferably, the two-dimensional spatially distributed ferroelectric domain structure exhibits a periodically gradually varying distribution in the x-direction, providing an equivalent second-order phase modulation to achieve preset modulation of the complex amplitude distribution of the optical field;

[0009] It has a fixed periodic distribution in the y-direction, providing transverse reciprocal lattice vector components to satisfy the quasi-phase matching condition of the non-collinear frequency doubling process.

[0010] Preferably, the spatial distribution of the second-order nonlinear coefficients corresponding to the two-dimensional spatially distributed ferroelectric domain structure satisfies:

[0011] ;

[0012] in, It is a second-order nonlinear magnetic susceptibility. yes direction The position is at the polarization period, It is a description The chirp of the rate of change of directional polarization period. yes Directional polarization period, It is a constant, and , Duty cycle;

[0013] The two-dimensional spatially distributed ferroelectric domain structure has positive and negative first-order reciprocal lattice vectors in the x and y directions, respectively. , .

[0014] Preferably, within the optical superlattice structure The non-collinear angle between the fundamental frequency light and the second harmonic light The quasi-phase matching condition is:

[0015] ;

[0016] in, and These are the wave vectors of the frequency-doubled beam and the fundamental frequency beam, respectively.

[0017] This invention also provides a method for detecting orbital angular momentum upconversion based on an optical superlattice structure. Employing the aforementioned orbital angular momentum upconversion detection device based on an optical superlattice structure, the method includes the following steps:

[0018] Provide wavelength for Waist corset And a Gaussian beam with horizontal polarization; and after being incident on a spatial light modulator loaded with a fork-shaped phase grating hologram, a vortex beam is generated;

[0019] The vortex beam undergoes a Fourier transform through the first lens, moving from the real space to the frequency domain. After unwanted spatial modes are filtered out on the frequency spectrum using a pinhole aperture, it undergoes another Fourier transform through the second lens, returning from the frequency domain to the real space. After passing through the first mirror, the second mirror, and the half-wave plate, it is focused by the third lens.

[0020] After focusing, in the ferroelectric crystal coordinate system Position along The beam is incident in a specific direction onto the optical superlattice structure; wherein, the beam is... Nonlinear diffraction occurs in the direction; simultaneously The inverse lattice vector of the direction will compensate for the longitudinal phase mismatch;

[0021] Under the condition of quasi-phase matching, after the fundamental frequency beam is filtered out by the filter at the output port of the ferroelectric crystal, it is collected by a CCD camera to obtain the frequency-doubled beam after nonlinear diffraction (non-collinear, the frequency-doubled beam and the incident fundamental frequency beam have different propagation directions). The number of OAMs carried by the fundamental frequency beam is obtained by the orientation and number of dark fringes of the cross-sectional spot of the frequency-doubled beam.

[0022] Preferably, the two-dimensional spatially distributed ferroelectric domain structure is constructed by the interference pattern of a planar nonlinear polarized wave and a spherical secondary wave, and the distribution of the corresponding second-order nonlinear coefficients in the xy-plane space satisfies a preset phase modulation law, so as to simultaneously achieve quasi-phase matching in the frequency doubling process and preset modulation of the complex amplitude distribution of the optical field.

[0023] Preferably, the spatial distribution of the second-order nonlinear coefficients corresponding to the two-dimensional spatially distributed ferroelectric domain structure satisfies:

[0024] ;

[0025] in, It is a second-order nonlinear magnetic susceptibility. and These are the wave vectors of the frequency-doubled beam and the fundamental frequency beam, respectively. Let be the radius of curvature, and .

[0026] This invention also provides a method for detecting orbital angular momentum upconversion based on an optical superlattice structure. Employing the aforementioned orbital angular momentum upconversion detection device based on an optical superlattice structure, the method includes the following steps:

[0027] Provide wavelength for Waist corset And a Gaussian beam with horizontal polarization; and after being incident on a spatial light modulator loaded with a fork-shaped phase grating hologram, a vortex beam is generated;

[0028] The vortex beam undergoes a Fourier transform through the first lens, moving from the real space to the frequency domain. After unwanted spatial modes are filtered out on the frequency spectrum using a pinhole aperture, it undergoes another Fourier transform through the second lens, returning from the frequency domain to the real space. After passing through the first mirror, the second mirror, and the half-wave plate, it is focused by the third lens.

[0029] After focusing, in the ferroelectric crystal coordinate system Position along The beam is incident in a specific direction onto the optical superlattice structure; wherein, the beam is... The inverse lattice vector of the direction will compensate for the longitudinal phase mismatch;

[0030] Under the condition of quasi-phase matching, after the fundamental frequency beam is filtered out by the filter at the output port of the ferroelectric crystal, it is collected by a CCD camera to obtain the frequency-doubled beam after nonlinear diffraction (non-collinear, the frequency-doubled beam and the incident fundamental frequency beam have different propagation directions). The number of OAMs carried by the fundamental frequency beam is obtained by the orientation and number of dark fringes of the cross-sectional spot of the frequency-doubled beam.

[0031] Beneficial effects: This invention integrates frequency upconversion and orbital angular momentum detection into the same nonlinear crystal, which not only reduces the complexity of the detection system and improves the integration, but also improves the nonlinear conversion efficiency by optimizing the ferroelectric domain structure. In the chirped structure, the reciprocal lattice vector in the x-direction changes continuously with the spatial coordinates, enabling astigmatic mode transformation and detection of vortex beam OAM while performing nonlinear frequency transformation.

[0032] This invention also employs a nonlinear cylindrical lens structure to more rigorously satisfy the phase-matching condition, thereby supporting a longer effective interaction length (device length up to 5.5 mm). Simultaneously, due to the near-collinear propagation of the frequency doubling process, there is a larger spatial overlap between the fundamental and frequency-doubled light, significantly enhancing the nonlinear interaction. Real-world testing demonstrates a frequency doubling conversion efficiency of approximately 0.8%, representing an improvement of about two orders of magnitude compared to chirped structures. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the orbital angular momentum upconversion detector based on an optical superlattice structure in this invention.

[0034] Figure 2 This is a schematic diagram of the chirped superlattice structure in this invention and its phase matching method during the frequency doubling process.

[0035] Figure 3 This is a frequency-doubled light pattern generated by nonlinear diffraction of vortex light with different topological charges in a chirped superlattice sample, as experimentally measured in this invention.

[0036] Figure 4 This is a schematic diagram of the nonlinear cylindrical lens superlattice structure in this invention and its phase matching method in the frequency doubling process.

[0037] Figure 5 This invention presents frequency-doubled light spot patterns generated by nonlinear diffraction of vortex light with different topological charges within a nonlinear cylindrical lens superlattice sample. Detailed Implementation

[0038] To make the objectives and advantages of this invention clearer, the invention will be specifically described below with reference to embodiments. It should be understood that the following text is merely used to describe one or more specific embodiments of the invention and does not strictly limit the scope of protection specifically claimed by the invention.

[0039] like Figure 1 As shown, this is an orbital angular momentum upconversion detector based on an optical superlattice structure, including a spatial light modulator 1, a first lens 2, a pinhole aperture 3, a second lens 4, a first reflector 5, a second reflector 6, a half-wave plate 7, a third lens 8, an optical superlattice structure 9, and a filter 10. In this embodiment, the optical superlattice structure 9 uses a 0.5 mm thick z-cut ferroelectric crystal of the same composition. The ferroelectric crystal can be any one of lithium tantalate, lithium niobate, magnesium-doped lithium niobate, and KTP. A preset two-dimensional spatially distributed ferroelectric domain structure is constructed in the optical superlattice structure 9 and configured to satisfy the quasi-phase matching of the nonlinear frequency conversion process and the preset modulation of the complex amplitude distribution of the light field participating in the conversion. The modulation causes the topological charge information of the incident vortex beam to be directly mapped into a specific fringe pattern on the cross section of the output frequency-doubled beam during the frequency upconversion process.

[0040] During operation, the wavelength emitted by the laser is Waist corset And the polarization is horizontal polarization ( A Gaussian beam, incident on a spatial light modulator 1 loaded with a fork-shaped phase grating hologram, produces a vortex beam:

[0041] ;

[0042] In the formula, Here are the parameters for the cylindrical coordinate system, where... Represents the transverse radial coordinate perpendicular to the direction of beam propagation. Represents azimuth coordinates. Represents the axial coordinate along the direction of beam propagation. It represents the generalized Laguerre polynomial. They are respectively The beam waist width, wavefront radius of curvature, Gouy phase shift, and wave vector at a point (where z is not a fixed constant but a position coordinate along the propagation direction). These correspond to the angular and radial indices of the LG beam, respectively. Represents the set of integers. Represents the set of non-negative integers. Also known as the topological charge number, it represents the charge carried by each photon in this mode. OAM, is the reduced Planck constant.

[0043] The vortex beam undergoes a Fourier transform through the first lens 2, entering the frequency domain from the real space. After the unwanted spatial modes are filtered out on the spectral surface using the pinhole aperture 3, it undergoes another Fourier transform through the second lens 4, returning from the frequency domain to the real space. After passing through the first reflector 5, the second reflector 6, and the half-wave plate 7, it is focused by the third lens 8 and enters the ferroelectric crystal modulated by the second-order nonlinear coefficient. After the fundamental frequency beam is filtered out by the filter 10, the nonlinearly diffracted frequency-doubled beam can be obtained by collecting it at the output end using a CCD camera. The number of OAMs carried by the fundamental frequency beam can be obtained by the orientation and number of dark fringes in the cross-sectional spot of the frequency-doubled beam.

[0044] Example 1: In this embodiment of the orbital angular momentum upconversion detector, the ferroelectric crystal is selected as lithium tantalate crystal. The two-dimensional spatially distributed ferroelectric domain structure of the lithium tantalate crystal is a chirped structure. The chirped structure has a periodically gradually varying distribution in the x-direction, providing equivalent quadratic phase modulation to achieve preset modulation of the complex amplitude distribution of the optical field; the chirped structure has a fixed periodic distribution in the y-direction, providing transverse reciprocal lattice vector components to satisfy the quasi-phase matching condition of the non-collinear frequency doubling process; wherein, along The periodic structure with gradually changing direction provides reciprocal lattice vectors that, after Taylor series expansion, contain... The relevant secondary phase factor; first, initial linearly polarized light is incident on a spatial light modulator loaded with a fork-shaped grating hologram to generate a vortex beam; subsequently, after being focused by a lens, the beam follows... The light is incident in a specific direction onto a lithium tantalate crystal with a chirped structure; within the lithium tantalate crystal, The reciprocal lattice vectors provided by the directional periodic structure can satisfy the phase-matching condition in the non-collinear harmonic case; at the same time, due to The secondary phase factor introduced by the direction is consistent with the astigmatic phase provided by the cylindrical lens, thus enabling astigmatic mode transformation; while performing nonlinear frequency transformation, it can also detect vortex beam OAM.

[0045] In this embodiment, the spatial distribution of the second-order nonlinear coefficients corresponding to the chirped structure satisfies:

[0046] ;

[0047] in, It is a second-order nonlinear magnetic susceptibility. yes direction The position is at the polarization period, It is a description The chirp of the rate of change of directional polarization period. yes Directional polarization period, It is a constant, and , Duty cycle;

[0048] Lithium tantalate crystal design polarization structure such as Figure 2 As shown in (a); the positive and negative first-order reciprocal lattice vectors of the chirped structure in the x and y directions are respectively ,

[0049] vortex beam passes After the system performs spatial filtering, among which, The system mainly refers to a first lens 2, a pinhole aperture 3 positioned in the Fourier plane, and a second lens 4. The distance between the first lens 2 and the pinhole aperture 3 is the focal length of the first lens, and the distance between the second lens 4 and the pinhole aperture 3 is the focal length of the second lens 4, thus forming a Fourier filter imaging system. In the lithium tantalate crystal coordinate system... Position along The beam is incident on a lithium tantalate crystal in a specific direction; during the second harmonic generation process, due to the effect of the transverse phase matching condition, the beam... The direction produces nonlinear diffraction; simultaneously, The inverse lattice vector in the direction will compensate for the longitudinal phase mismatch in lithium tantalate crystals. The non-collinear angle between the fundamental frequency light and the second harmonic light is ,like Figure 2 As shown in (b);

[0050] In this embodiment, the phase matching condition is:

[0051] ;

[0052] In the formula, and These are the wave vectors of the frequency-doubled beam and the fundamental beam, respectively. Under phase-matching conditions, after placing a filter at the crystal exit port to remove the fundamental beam, the nonlinearly diffracted frequency-doubled beam can be collected using a CCD camera.

[0053] Based on Example 1, in one specific embodiment: during the experiment, the laser source was a pulsed source with a wavelength of 1342nm, a pulse width of 50ns, and a repetition rate of 10kHz; the waist of the Gaussian beam emitted by the laser was 2mm; the second-order nonlinear coefficient parameters after modulation by the lithium tantalate crystal were... The duty cycle D is set to 0.5; the chirped superlattice structure is as follows: Figure 2 As shown in (a), the phase matching method for the frequency doubling process is as follows: Figure 2 As shown in (b).

[0054] After incident on the spatial light modulator, the resulting topological charge is: The vortex beam (i.e., LG mode), the vortex beam passes through After spatial filtering, the light is focused onto the lithium tantalate crystal. After passing through filter 10, the diffracted light field collected by the CCD camera is as follows: Figure 3 As shown, it can be seen that the number of topological charges carried by the mid-to-far-infrared band vortex field before nonlinear frequency transformation can be obtained by the number and orientation of the dark fringes in the diffraction field.

[0055] Example 2: To meet the phase-matching condition, the sample length is typically designed to be short (approximately 0.5 mm). Simultaneously, because this structure corresponds to a non-collinear frequency doubling process, the overlap of the optical fields between the fundamental and harmonic waves is small, thus limiting the intensity of the nonlinear interaction. Furthermore, as the topological charge increases, the intelligibility of the interference fringes gradually decreases. To achieve effective detection of high topological charge vortex beams, it is necessary to reduce the center period gradient factor n, which will result in the domain structure size in some regions being less than 10. m*10 m significantly increases the processing difficulty; therefore, this embodiment designs a nonlinear cylindrical lens structure, specifically as follows:

[0056] In this embodiment of the orbital angular momentum upconversion detector, the ferroelectric crystal selected is lithium tantalate crystal. The two-dimensional spatial distribution of ferroelectric domains of the lithium tantalate crystal is a nonlinear cylindrical lens structure. The nonlinear cylindrical lens structure is constructed by the interference pattern of the planar nonlinear polarized wave and the spherical secondary wave. The distribution of the second-order nonlinear coefficients corresponding to the nonlinear cylindrical lens structure in the xy plane space satisfies the preset phase modulation law, so as to simultaneously realize the quasi-phase matching in the frequency doubling process and the preset modulation of the complex amplitude distribution of the optical field.

[0057] In this embodiment, the spatial distribution of the second-order nonlinear coefficients corresponding to the nonlinear cylindrical lens structure satisfies:

[0058] ;

[0059] in, Let be the radius of curvature, and satisfy Under these conditions, the first-order reciprocal lattice vector of the second-order nonlinear coefficient corresponding to the nonlinear cylindrical lens structure can be approximately expressed as a form containing a transverse quadratic phase modulation term, thereby achieving quasi-phase matching while generating frequency-doubled beams. The focusing effect of direction.

[0060] When the incident light is a vortex fundamental frequency beam carrying orbital angular momentum, it generates frequency-doubled light after being acted upon by a nonlinear cylindrical lens structure. Due to the transverse phase modulation introduced by the nonlinear cylindrical lens structure, the frequency-doubled output light field will undergo mode conversion, thereby forming a frequency-doubled light spot with bright and dark stripe distribution characteristics.

[0061] The number of fringes in the frequency-doubled beam is related to the absolute value of the topological charge of the incident vortex beam, and the number of fringes is twice the absolute value of the topological charge; the tilt direction of the fringes in the frequency-doubled beam is related to the sign of the topological charge. Based on this, the magnitude and sign of the orbital angular momentum topological charge of the incident vortex beam can be determined according to the number of fringes and their tilt direction in the frequency-doubled beam.

[0062] Based on Example 2, in one specific embodiment: during the experiment, the laser source was a pulsed source with a wavelength of 1342nm, a pulse width of 50ns, and a repetition rate of 10kHz; the waist of the Gaussian beam emitted by the laser was 2mm; the second-order nonlinear coefficient parameters after modulation by the lithium tantalate crystal were... Nonlinear cylindrical lens superlattice structures, such as Figure 4 As shown in (a), the phase matching method for the frequency doubling process is as follows: Figure 4 As shown in (b).

[0063] After incident on the spatial light modulator, the resulting topological charge is: The vortex beam (i.e., LG mode), the vortex beam passes through After spatial filtering, the light is focused onto the lithium tantalate crystal. After passing through filter 10, the diffracted light field collected by the CCD camera is as follows: Figure 5 As shown, the number and orientation of the dark fringes in the diffraction field can be used to obtain the number of topological charges carried by the mid-to-far-infrared vortex field before nonlinear frequency conversion. Furthermore, compared to the chirped structure, the nonlinear cylindrical lens structure in this embodiment can more strictly satisfy the phase-matching condition, thus supporting a longer effective interaction length (device length up to 5.5 mm). Simultaneously, since the frequency doubling process tends to propagate nearly collinearly, there is a larger spatial overlap between the fundamental wave and the frequency-doubled light, significantly enhancing the nonlinear interaction. The results demonstrate that the frequency doubling conversion efficiency is approximately 0.8%, which is about two orders of magnitude higher than that of the chirped structure.

[0064] The embodiments of the present invention have been described in detail above with reference to the examples. However, the present invention is not limited to the above embodiments. For those skilled in the art, after learning the contents described in the present invention, several equivalent changes and substitutions can be made without departing from the principle of the present invention. These equivalent changes and substitutions should also be considered to fall within the protection scope of the present invention.

Claims

1. An orbital angular momentum upconversion detection device based on an optical superlattice structure, defining the x-direction as the direction perpendicular to the beam propagation direction and the y-direction as the beam propagation direction, characterized in that: Along the beam propagation direction, it includes, in sequence, a spatial light modulator, The system comprises a half-wave plate, a third lens, an optical superlattice structure, and a filter. A fork-shaped grating hologram is loaded within the spatial light modulator. Initial linearly polarized light enters the spatial light modulator, generating a vortex beam. The vortex beam passes through... The system performs spatial filtering, then passes through a half-wave plate and a third lens in sequence, and then along... The light is incident on the optical superlattice structure in a specific direction, and after the fundamental frequency beam is filtered out by a filter, it is collected by a CCD camera. The optical superlattice structure is provided with a ferroelectric domain structure with a preset two-dimensional spatial distribution. The ferroelectric domain structure is used to satisfy the quasi-phase matching condition of the nonlinear frequency conversion process and to preset the complex amplitude distribution of the optical field participating in the frequency conversion, so that the topological charge information of the incident vortex beam is directly mapped into a specific stripe pattern on the cross section of the output frequency-doubled beam during the frequency conversion process. The two-dimensional spatially distributed ferroelectric domain structure is a nonlinear cylindrical lens structure. The nonlinear cylindrical lens structure is constructed by the interference pattern of the plane nonlinear polarized wave and the spherical secondary wave. The distribution of the corresponding second-order nonlinear coefficients in the xy plane space satisfies the preset phase modulation law, so as to simultaneously realize the quasi-phase matching in the frequency doubling process and the preset modulation of the complex amplitude distribution of the optical field. The spatial distribution of the second-order nonlinear coefficients corresponding to the two-dimensional spatially distributed ferroelectric domain structure satisfies: ； in, It is a second-order nonlinear magnetic susceptibility. and These are the wave vectors of the frequency-doubled beam and the fundamental frequency beam, respectively. Let be the radius of curvature, and .

2. The orbital angular momentum upconversion detection device based on an optical superlattice structure according to claim 1, characterized in that: The The system includes, in sequence along the direction of light propagation, a first lens, a pinhole aperture, a second lens, a first reflecting mirror, and a second reflecting mirror.

3. The orbital angular momentum upconversion detection device based on an optical superlattice structure according to claim 2, characterized in that: A chirped structure replaces the nonlinear cylindrical lens structure as the two-dimensional spatially distributed ferroelectric domain structure. The chirped structure is as follows: It exhibits a periodically gradually varying distribution in the x-direction, providing equivalent secondary phase modulation to achieve preset modulation of the complex amplitude distribution of the optical field; It has a fixed periodic distribution in the y-direction, providing transverse reciprocal lattice vector components to satisfy the quasi-phase matching condition of the non-collinear frequency doubling process.

4. The orbital angular momentum upconversion detection device based on an optical superlattice structure according to claim 3, characterized in that: The spatial distribution of the second-order nonlinear coefficients corresponding to the two-dimensional spatially distributed ferroelectric domain structure satisfies: ； in, It is a second-order nonlinear magnetic susceptibility. yes direction The position is at the polarization period, It is a description The chirp of the rate of change of directional polarization period. yes Directional polarization period, It is a constant, and , Duty cycle; The positive and negative first-order reciprocal lattice vectors of the two-dimensional spatially distributed ferroelectric domain structure in the x and y directions are respectively , .

5. The orbital angular momentum upconversion detection device based on an optical superlattice structure according to claim 4, characterized in that: In optical superlattice structure The non-collinear angle between the fundamental frequency light and the second harmonic light The quasi-phase matching condition is: ； in, and These are the wave vectors of the frequency-doubled beam and the fundamental frequency beam, respectively.

6. A method for detecting orbital angular momentum upconversion based on optical superlattice structures, characterized in that: The method using the orbital angular momentum upconversion detection device based on an optical superlattice structure as described in claim 1 includes the following steps: Provide wavelength for Waist corset And a Gaussian beam with horizontal polarization; and after being incident on a spatial light modulator loaded with a fork-shaped phase grating hologram, a vortex beam is generated; The vortex beam undergoes a Fourier transform through the first lens, moving from the real space to the frequency domain. After unwanted spatial modes are filtered out on the frequency spectrum using a pinhole aperture, it undergoes another Fourier transform through the second lens, returning from the frequency domain to the real space. After passing through the first mirror, the second mirror, and the half-wave plate, it is focused by the third lens. After focusing, in the ferroelectric crystal coordinate system Position along The beam is incident in a specific direction onto the optical superlattice structure; wherein, the beam is... The inverse lattice vector of the direction will compensate for the longitudinal phase mismatch; Under the condition of quasi-phase matching, after the fundamental frequency beam is filtered out by the filter at the output port of the ferroelectric crystal, it is collected by a CCD camera to obtain the frequency-doubled beam after nonlinear diffraction. The number of OAMs carried by the fundamental frequency beam is obtained by the orientation and number of dark fringes of the cross-sectional spot of the frequency-doubled beam.

7. A method for detecting orbital angular momentum upconversion based on optical superlattice structures, characterized in that: The method employs the orbital angular momentum upconversion detection device based on an optical superlattice structure as described in any one of claims 3-5, and includes the following steps: Provide wavelength for Waist corset And a Gaussian beam with horizontal polarization; and after being incident on a spatial light modulator loaded with a fork-shaped phase grating hologram, a vortex beam is generated; The vortex beam undergoes a Fourier transform through the first lens, moving from the real space to the frequency domain. After unwanted spatial modes are filtered out on the frequency spectrum using a pinhole aperture, it undergoes another Fourier transform through the second lens, returning from the frequency domain to the real space. After passing through the first mirror, the second mirror, and the half-wave plate, it is focused by the third lens. After focusing, in the ferroelectric crystal coordinate system Position along The beam is incident in a specific direction onto the optical superlattice structure; wherein, the beam is... Nonlinear diffraction occurs in the direction; simultaneously The inverse lattice vector of the direction will compensate for the longitudinal phase mismatch; Under the condition of quasi-phase matching, after the fundamental frequency beam is filtered out by the filter at the output port of the ferroelectric crystal, it is collected by a CCD camera to obtain the frequency-doubled beam after nonlinear diffraction. The number of OAMs carried by the fundamental frequency beam is obtained by the orientation and number of dark fringes of the cross-sectional spot of the frequency-doubled beam.

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