Non-reciprocal high-order frequency doubling orbital angular momentum mode Bragg enhancement generation device

Abstract

According to the non-reciprocal high-order frequency doubling orbital angular momentum mode Bragg enhancement generation device, in a certain wave band range, birefringence phase matching can be achieved through lithium niobate crystals, and efficient frequency doubling conversion is achieved. A three-dimensional fork-shaped grating array is processed in a lithium niobate crystal by a femtosecond laser direct writing technology, Bragg enhancement diffraction can be realized based on the change of a refractive index, and the generation efficiency of a target diffraction order frequency doubling OAM mode is further improved. In addition, fundamental waves are incident from two opposite directions, different frequency doubling diffraction phenomena can occur, a non-reciprocal effect is achieved, and possibility is provided for different requirements.

Description

Technical Field

[0001] The present invention relates to the field of nonlinear light field control technology, in particular to the generation of high-order frequency-doubled orbital angular momentum modes in lithium niobate crystals, and specifically to a non-reciprocal high-order frequency-doubled orbital angular momentum mode Bragg enhancement generation device. Background Art

[0002] Photon orbital angular momentum (OAM) exhibits properties such as a spirally distributed phase, infinite dimensionality, and orthogonality. Since its discovery by Allen et al. in 1992, high-order OAM modes have been widely applied in optical manipulation, optical communications, quantum optics, imaging, detection, and optical encryption. The significant applications of OAM modes in optics have driven research into methods for generating these higher-order modes. In addition to commonly used methods such as spiral phase plates, q-plates, computational holography, and π / 2 mode converters, various micro-integrated OAM generating devices have also been developed. However, these methods focus on linear regimes; combining them with nonlinear effects could further expand the frequency range of OAM modes.

[0003] To efficiently generate OAM modes at new frequencies, traditional methods require two devices: first, frequency conversion through nonlinear materials, and then loading OAM through linear devices. To save cost and space, a device is needed to achieve beam shaping while performing nonlinear conversion. Two-dimensional nonlinear photonic crystals and metasurfaces can both achieve nonlinear light field control, but the efficiency is low. Three-dimensional nonlinear photonic crystals processed using femtosecond laser direct writing technology can control the light field based on quasi-phase matching. However, femtosecond laser direct writing technology can only erase about 20% of the nonlinear coefficients, and the nonlinear conversion efficiency is still low. 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 non-reciprocal high-order frequency-doubled orbital angular momentum mode Bragg enhancement generation device. Within a certain wavelength range, the lithium niobate crystal can achieve birefringence phase matching and realize efficient frequency-doubled conversion. Femtosecond laser direct writing technology processes a three-dimensional fork grating array in the lithium niobate crystal. Based on the change in refractive index, Bragg enhanced diffraction can be achieved, further improving the generation efficiency of the target diffraction-order frequency-doubled OAM mode. In addition, different frequency-doubled diffraction phenomena can occur when the fundamental wave is incident from two opposite directions, achieving a non-reciprocal effect and providing possibilities for different needs.

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

[0006] In a first aspect, the present invention provides a non-reciprocal high-order frequency-doubled orbital angular momentum mode Bragg enhancement generation device, which is processed in a lithium niobate crystal using laser processing technology, and the refractive index of the processed area will change;

[0007] The optical axis of the lithium niobate crystal is along the z direction. When the polarization direction of the light beam is along the x direction, it is ordinary light, that is, o light; when the polarization direction of the light beam is along the z direction, it is extraordinary light, that is, e light.

[0008] The processed lithium niobate crystal includes two regions, namely a first region and a second region sequentially arranged along the y direction;

[0009] The first area is a non-processing area, and the second area is a fork grating array area;

[0010] There are two directions of fundamental wave incidence: the first direction is to pass through the non-processing area first and then through the fork-shaped grating array area; the second direction is to pass through the fork-shaped grating array area first and then through the non-processing area;

[0011] When using method 1, the fundamental wave o light is incident on the surface of the lithium niobate crystal parallel to the xoy plane, polarized along the x direction, and at an angle θ with the y axis, where it is refracted. The refracted o light passes through the unprocessed area of the lithium niobate crystal, generating the doubled frequency e light, which satisfies the birefringence phase matching condition. Finally, the doubled frequency e light undergoes p-level Bragg-enhanced diffraction in the fork grating array area, generating the doubled frequency OAM mode.

[0012] When the second method is used, the fundamental wave o light is incident on the surface of the lithium niobate crystal parallel to the xoy plane, polarized along the x direction, and at an angle of π-θ with the y axis, where it is refracted. The refracted o light undergoes q-order Bragg-enhanced diffraction when passing through the fork grating array area, generating a fundamental frequency OAM mode. Finally, the fundamental frequency OAM beam passes through the non-processing area to achieve birefringence phase matching, generating a frequency-doubled OAM mode.

[0013] When the fundamental wave is incident from two opposite directions, the order of the frequency doubling conversion based on birefringence phase matching and the linear Bragg enhanced diffraction will be different, resulting in different frequency doubling diffraction phenomena and achieving a non-reciprocal effect.

[0014] Preferably, the processed lithium niobate crystal includes a non-processed area and a fork grating array area, which is expressed as:

[0015]

[0016] 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, which can be expressed as follows for any value of Z:

[0017]

[0018] Preferably, when the first method is adopted, when the fundamental wave is incident in a direction parallel to the xoy plane, polarized along the x direction, and incident at an angle θ with the y axis on the lithium niobate crystal, refraction occurs:

[0019] sinθ=n o (λ)sinθ'(3)

[0020] Among them, θ' is the refraction angle, the fundamental wave is o light, n o (λ) is the refractive index of the fundamental frequency o light, and λ is the wavelength of the fundamental wave. When passing through the non-processed area of the lithium niobate crystal, the doubled frequency e light is generated, and the birefringence phase matching condition is satisfied:

[0021] k e (λ / 2,θ')=k o (λ)+k o (λ)(4)

[0022] in, are the wave vectors of the doubled frequency e light and the fundamental frequency o light, n e (λ / 2,θ') is the refractive index of the frequency-doubled e-light. Then, the frequency-doubled e-light undergoes Bragg p-order enhanced diffraction in the fork grating array area:

[0023]

[0024] Where p is the diffraction order, which is an integer. At the p-order, the frequency-doubled e-light OAM beam is output, and the topological charge is pl o .

[0025] Preferably, when the second method is adopted, when the fundamental wave is incident in a direction parallel to the xoy plane, polarized along the x direction, and incident at an angle of π-θ with the y axis on the lithium niobate crystal, refraction occurs:

[0026] sin(π-θ)=sin(θ)=n o (λ)sin(π-θ')=n o (λ)sin(θ')(6)

[0027] Where π-θ' is the refraction angle, and the fundamental frequency is o-light. After passing through the fork grating array area, the fundamental frequency o-light undergoes q-order Bragg-enhanced diffraction:

[0028]

[0029] Where q is the diffraction order, which is an integer. At this time, the fundamental frequency o light OAM mode is output at the q level, and the topological charge is ql o Finally, birefringence phase matching is achieved through the non-processed area:

[0030] k e (λ / 2,π-θ')=k o (λ)+k o (λ) (8)

[0031] in, The frequency doubling process doubles the topological charge, and the topological charge of the frequency-doubled e-light OAM mode output at the q level is 2ql o .

[0032] In a second aspect, the present invention provides a system for preparing a non-reciprocal high-order frequency-doubling orbital angular momentum mode Bragg enhancement generating device, which is used to prepare the non-reciprocal high-order frequency-doubling orbital angular momentum mode Bragg enhancement generating 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 pinhole, a gradient attenuation plate, an objective lens, a CCD, a three-dimensional nano-translation stage and a computer;

[0033] The first laser emits laser light 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 laser polarization to the z-direction. The first lens, the pinhole, and the second lens shape the beam. The laser power is continuously varied by a gradient attenuation plate, achieving energy compensation in depth during laser processing. After passing through the objective lens, the laser is focused onto a magnesium-doped lithium niobate crystal. The reflected beam is collected by a CCD for real-time observation of the processing status. 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. The laser is directed to scribe lines along the x-direction, forming a fork-shaped grating. Multiple layers of the same grating are processed along the y-direction, forming a fork-shaped grating array.

[0034] Preferably, the preparation system of the non-reciprocal high-order frequency-doubled orbital angular momentum mode Bragg enhancement 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 opening and closing of the laser.

[0035] Preferably, the preparation system of the non-reciprocal high-order frequency-doubled orbital angular momentum mode Bragg enhancement generation device also includes a dichroic mirror, which is arranged between the gradient attenuation plate and the objective lens, and is used to reflect the laser emitted by the gradient attenuation plate to the objective lens, and at the same time transmit the light beam reflected by the lithium niobate crystal to the CCD for collection.

[0036] 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; and the focal lengths of the first lens and the second lens are 75 mm.

[0037] In a third aspect, the present invention provides a non-reciprocal high-order frequency-doubled orbital angular momentum mode Bragg enhancement generation system, comprising the non-reciprocal high-order frequency-doubled orbital angular momentum mode Bragg enhancement generation 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, a rotation translation stage and a receiving screen;

[0038] The second laser emits laser light along the y direction, and the energy of the laser light is regulated by the third half-wave plate and the second polarization beam splitter. The polarization direction of the fundamental wave is changed by the fourth half-wave plate so that the polarization is along the x direction, and then the laser light is focused by the third lens onto the non-reciprocal high-order frequency doubling orbital angular momentum mode Bragg enhancement generating device. The non-reciprocal high-order frequency doubling orbital angular momentum mode Bragg enhancement generating device is placed on a rotating translation stage, and the frequency doubling signal is observed on the receiving screen by changing the incident angle.

[0039] Preferably, the second laser emits laser light with a wavelength, a pulse width of 140 fs and a repetition frequency of 80 MHz along the y direction; and the focal length of the third lens is 75 mm.

[0040] Compared with the prior art, the beneficial effects of the present invention include at least:

[0041] This invention proposes a nonreciprocal high-order frequency-doubled orbital angular momentum (OAM) Bragg-enhanced generator. Within a specific wavelength range, lithium niobate crystals can achieve birefringence phase matching, enabling efficient frequency-doubled conversion. Using femtosecond laser direct writing technology, a three-dimensional fork-shaped grating array is fabricated in the lithium niobate crystal. Based on the change in refractive index, Bragg-enhanced diffraction can be achieved, further improving the generation efficiency of the target diffraction-order OAM mode. Furthermore, by injecting the fundamental wave from two opposing directions, different frequency-doubled diffraction phenomena can occur, achieving a nonreciprocal effect and providing the possibility to meet different needs. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] 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.

[0043] Figure 1Schematic diagram of the Bragg enhancement device for generating a non-reciprocal high-order frequency-doubled orbital angular momentum mode according to the present invention; wherein (a) the Bragg enhancement device for generating a non-reciprocal high-order frequency-doubled orbital angular momentum mode; (b) the reciprocal space principle diagram of mode 1; (c) the light spot distribution diagram of mode 1; (d) the reciprocal space principle diagram of mode 2; (e) the light spot distribution diagram of mode 2;

[0044] Figure 2 Schematic diagram of the system for preparing the non-reciprocal high-order frequency-doubled orbital angular momentum mode Bragg enhancement generation device of the present invention; (a) device processing optical path diagram; (b) device topography in the xoz and yoz planes;

[0045] Figure 3 It is the characterization optical path of the Bragg enhancement generation system of the non-reciprocal high-order frequency-doubled orbital angular momentum mode of the present invention;

[0046] Figure 4 The incident angle and fundamental wavelength used for the Bragg enhancement generation of the frequency-doubled OAM mode at different positions of the present invention;

[0047] Figure 5 These are the spot diagrams of mode 1 and mode 2 when l0=3, θ=9°, and λ=1048nm. DETAILED DESCRIPTION

[0048] To make the above-mentioned objectives, features, and advantages of the present invention more clearly understood, the technical solutions of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that the embodiments described are only some of the embodiments of the present invention, not all of them. All other embodiments obtained by persons of ordinary skill in the art based on the embodiments of the present invention without creative effort are also within the scope of protection of the present invention.

[0049] Example 1

[0050] The present invention proposes a non-reciprocal high-order frequency-doubled orbital angular momentum mode Bragg enhancement generation device, which is processed in a lithium niobate crystal using laser processing technology, and the refractive index of the processed area will change;

[0051] The optical axis of the lithium niobate crystal is along the z direction. When the polarization direction of the light beam is along the x direction, it is ordinary light (abbreviated as o light); when the polarization direction of the light beam is along the z direction, it is extraordinary light (abbreviated as e light).

[0052] The processed lithium niobate crystal includes two regions, namely a first region and a second region sequentially arranged along the y direction;

[0053] The first area is a non-processing area, and the second area is a fork grating array area. Figure 1 (a)

[0054] There are two directions of fundamental wave incidence: the first direction is to pass through the non-processing area first and then through the fork-shaped grating array area; the second direction is to pass through the fork-shaped grating array area first and then through the non-processing area;

[0055] When using method 1, the fundamental wave o light is incident on the surface of the lithium niobate crystal parallel to the xoy plane, polarized along the x direction, and at an angle θ with the y axis, where it is refracted. The refracted o light passes through the unprocessed area of the lithium niobate crystal, generating the doubled frequency e light, which satisfies the birefringence phase matching condition. Finally, the doubled frequency e light undergoes p-level Bragg-enhanced diffraction in the fork grating array area, generating the doubled frequency OAM mode.

[0056] When the second method is used, the fundamental wave o light is incident on the surface of the lithium niobate crystal parallel to the xoy plane, polarized along the x direction, and at an angle of π-θ with the y axis, where it is refracted. The refracted o light undergoes q-order Bragg-enhanced diffraction when passing through the fork grating array area, generating a fundamental frequency OAM mode. Finally, the fundamental frequency OAM beam passes through the non-processing area to achieve birefringence phase matching, generating a frequency-doubled OAM mode.

[0057] When the fundamental wave is incident from two opposite directions, the order of the frequency doubling conversion based on birefringence phase matching and the linear Bragg enhanced diffraction will be different, resulting in different frequency doubling diffraction phenomena and achieving a non-reciprocal effect.

[0058] Specifically, the processed lithium niobate crystal includes a non-processed area and a fork-shaped grating array area, such as Figure 1 (a) is shown as:

[0059]

[0060] 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, which can be expressed as follows for any value of Z:

[0061]

[0062] Figure 1 (b) Demonstrating the first method in reciprocal space, when the fundamental wave is parallel to the xoy plane, polarized along the x direction, and incident on the lithium niobate crystal at an angle θ to the y axis, refraction occurs:

[0063] sinθ=n o (λ)sinθ'(3)

[0064] Among them, θ' is the refraction angle, the fundamental wave is o light, n o(λ) is the refractive index of the fundamental frequency o light, and λ is the wavelength of the fundamental wave. After passing through the non-processed area of the lithium niobate crystal, the doubled frequency e light is generated, and the birefringence phase matching condition is satisfied:

[0065] k e (λ / 2,θ')=k o (λ)+k o (λ) (4)

[0066] in, are the wave vectors of the doubled frequency e light and the fundamental frequency o light, n e (λ / 2,θ') is the refractive index of the frequency-doubled e-light. Then, the frequency-doubled e-light undergoes Bragg p-order enhanced diffraction in the fork grating array area:

[0067]

[0068] Where p is the diffraction order, which is an integer. The frequency-doubled e-light OAM beam is output at the p-order, and the topological charge of the OAM beam is pl o When p=1, 2, 3, 4, the frequency-doubled light spots are distributed at positions 1-4, respectively, as shown in the following figure: Figure 1 (c) shows that the corresponding topological charges are l o 、2l o 、3l o 、4l o .

[0069] Figure 1 (d) Demonstrating the second method in reciprocal space, when the fundamental wave is parallel to the xoy plane, polarized along the x direction, and incident on the lithium niobate crystal at an angle of π-θ with the y axis, refraction occurs:

[0070] sin(π-θ)=sin(θ)=n o (λ)sin(π-θ')=n o (λ)sin(θ')(6)

[0071] Where π-θ' is the refraction angle; the fundamental frequency is o-light. Then, after passing through the fork grating array area, the fundamental frequency o-light undergoes q-order Bragg enhanced diffraction:

[0072]

[0073] Where q is the diffraction order, which is an integer. At this time, the fundamental frequency o light OAM mode is output at the q level, and the topological charge is ql o Finally, birefringence phase matching is achieved through the non-processed area:

[0074] k e (λ / 2,π-θ')=k o (λ)+k o (λ) (8)

[0075] in, The frequency doubling process doubles the topological charge, and the topological charge of the frequency-doubled e-light OAM mode output at the q level is 2ql o When q = 1, 2, the frequency-doubled light spot is only distributed at position 2 and position 4, such as Figure 1 As shown in (e), the topological charges are 2l o and 4l o Therefore, the spot distributions of method 1 and method 2 are different and have non-reciprocal properties.

[0076] Example 2

[0077] like Figure 2 As shown, the present invention provides a preparation system for a non-reciprocal high-order frequency-doubling orbital angular momentum mode Bragg enhancement generating device, which is used to prepare the non-reciprocal high-order frequency-doubling orbital angular momentum mode Bragg enhancement 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, a dichroic mirror, an objective lens, a CCD, a three-dimensional nano-translation stage and a computer.

[0078] The non-reciprocal high-order frequency-doubled orbital angular momentum mode Bragg enhancement generation device is fabricated in a 1.5 mm (x) × 2.5 mm (y) × 1 mm (z) lithium niobate crystal, such as Figure 2 As shown in Figure (a), the first laser emits 800 nm of laser light along the x-direction with a pulse width of 34 fs and a repetition rate of 1 kHz. A shutter controls the laser's on / off, while a first half-wave plate and a first polarization beam splitter control the laser's power. A second half-wave plate polarizes the laser light along the z-direction. A first lens with a focal length of 75 mm, a pinhole, and a second lens with a focal length of 75 mm shape the beam. A gradient attenuation plate continuously varies the laser power, achieving energy compensation for laser processing depth. After passing through an objective lens with a numerical aperture of 0.75, the laser light is focused onto a 5% magnesium-doped lithium niobate crystal. The reflected beam is collected by a CCD for real-time monitoring of the processing status. A dichroic mirror is placed between the gradient attenuation plate and the objective lens, reflecting the laser light emitted by the gradient attenuation plate to the objective lens and transmitting the beam reflected by the lithium niobate crystal to the CCD for collection. A lithium niobate crystal was placed on a three-dimensional nano-translation stage. The travel of the three-dimensional nano-translation stage was controlled by a computer (Labview software) to make the laser scribe lines along the x-direction. The laser processing energy changed from 240nJ to 20nJ between 120nm and 20nm below the crystal surface. The line length was 100μm, forming a fork-shaped grating. Fifteen layers of the same grating were processed along the y-direction to form a fork-shaped grating array. The period of the y-axis and z-axis was Λ = 3.35μm. The prepared structure is shown in FIG. Figure 2 (b).

[0079] Example 3

[0080] like Figure 3 As shown, the present invention provides a non-reciprocal high-order frequency-doubled orbital angular momentum mode Bragg enhancement generation system, including the non-reciprocal high-order frequency-doubled orbital angular momentum mode Bragg enhancement 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, a rotation translation stage and a receiving screen.

[0081] The second laser emits laser light along the y-direction with a pulse width of 140 fs and a repetition rate of 80 MHz. The laser's energy is controlled by a third half-wave plate and a second polarization beam splitter, while a fourth half-wave plate changes the polarization of the fundamental wave along the y-direction. The laser is then focused by a third lens with a focal length of 75 mm onto the device, which is placed on a rotary stage. By varying the angle of incidence, the frequency-doubled signal is observed on a receiving screen.

[0082] The experimental results are as follows Figure 4 As shown, the black, red, and blue curves are theoretically the relationship between wavelength and incident angle for birefringence phase matching, Bragg diffraction mode 1, and Bragg diffraction mode 2, respectively. The intersection of the red and black curves is the wavelength and incident angle that simultaneously satisfies formulas (4)-(6) in mode 1, and the intersection of the blue and black curves is the wavelength and incident angle that simultaneously satisfies formulas (7)-(11) in mode 2. When p = 1-4, the theoretical intersection points are marked in black, and are (4.41°, 1031nm), (8.87°, 1033nm), (13.41°, 1036nm), and (18.11°, 1041nm), respectively. The experimentally measured values are marked in red, and are (4°, 1026nm), (9.67°, 1028nm), (16.67°, 1032nm), and (20.83°, 1036nm), respectively. When q=1 and 2, the theoretical intersection points are (8.87°, 1033nm) and (18.11°, 1041nm). The experimentally measured values are marked in blue, which are (8°, 1028nm) and (20.16°, 1032nm), respectively. The theory and experiment are basically consistent.

[0083] Use l o = 1, the fundamental power is 230mW. When the incident angle is 20.83°, as the fundamental wavelength changes, the frequency-doubled power at each position is as follows: Figure 5 As shown in (a), it can be seen that when the fundamental wavelength is 1036nm, the frequency-doubled power at position 4 is the highest, and the frequency-doubled powers at positions 3, 1, and 2 decrease accordingly, indicating that the birefringence frequency-doubled conversion of Bragg enhanced diffraction with p=4 is achieved. When the fundamental wavelength is 1036nm, the emitted frequency-doubled light spot is as follows: Figure 5 (b) As shown. A cylindrical lens is used to convert the OAM mode at position 4. The conversion spot is as follows Figure 5 As shown in (c), it can be seen that the number of dark fringes is 4, indicating that the topological charge is 4. When the second incident mode is adopted and the incident angle is 20.16°, as the fundamental wavelength changes, the doubled power at each position is as follows: Figure 5 As shown in (d), when the fundamental wavelength is 1032nm, the frequency-doubled power at position 4 is the highest, and the frequency-doubled power at other positions is extremely low, indicating that the birefringence frequency-doubled conversion of Bragg enhanced diffraction with q=2 is achieved. Figure 5 (d) A cylindrical lens is used to convert the OAM mode at position 4. The conversion spot is shown as Figure 5 As shown in (e), the number of dark fringes is 4, indicating a topological charge of 4. A comparison of the two methods reveals that while the frequency-doubled power at position 4 is the strongest in Methods 1 and 2, the distribution of the frequency-doubled power at other positions is different. Method 1 maintains the same power level at all positions, enabling simultaneous four-channel output. Method 2, however, only has a single-channel output at position 4, but the frequency-doubled efficiency is 2.5 times that of Method 1. These two methods exhibit non-reciprocal properties.

[0084] This invention proposes a nonreciprocal high-order frequency-doubled orbital angular momentum (OAM) Bragg-enhanced generator. Within a specific wavelength range, lithium niobate crystals can achieve birefringence phase matching, enabling efficient frequency-doubled conversion. Using femtosecond laser direct writing technology, a three-dimensional fork-shaped grating array is fabricated in the lithium niobate crystal. Based on the change in refractive index, Bragg-enhanced diffraction can be achieved, further improving the generation efficiency of the target diffraction-order OAM mode. Furthermore, by injecting the fundamental wave from two opposing directions, different frequency-doubled diffraction phenomena can occur, achieving a nonreciprocal effect and providing the possibility to meet different needs.

[0085] The above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be determined by the appended claims.

Claims

1. A non-reciprocal high-order frequency-doubled orbital angular momentum mode Bragg enhancement generation device, characterized in that: The device is processed in a lithium niobate crystal using laser processing technology, and the refractive index of the processed area changes; The optical axis of the lithium niobate crystal is along the z direction. When the polarization direction of the light beam is along the x direction, it is ordinary light, that is, o light; when the polarization direction of the light beam is along the z direction, it is extraordinary light, that is, e light. The processed lithium niobate crystal includes two regions, namely a first region and a second region sequentially arranged along the y direction; The first area is a non-processing area, and the second area is a fork grating array area; There are two directions of fundamental wave incidence: the first direction is to pass through the non-processing area first and then through the fork-shaped grating array area; the second direction is to pass through the fork-shaped grating array area first and then through the non-processing area; When using method 1, the fundamental wave o light is incident on the surface of the lithium niobate crystal parallel to the xoy plane, polarized along the x direction, and at an angle θ with the y axis, where it is refracted. The refracted o light passes through the unprocessed area of the lithium niobate crystal, generating the doubled frequency e light, which satisfies the birefringence phase matching condition. Finally, the doubled frequency e light undergoes p-level Bragg-enhanced diffraction in the fork grating array area, generating the doubled frequency OAM mode. When the second method is used, the fundamental wave o light is incident on the surface of the lithium niobate crystal parallel to the xoy plane, polarized along the x direction, and at an angle of π-θ with the y axis, where it is refracted. The refracted o light undergoes q-order Bragg-enhanced diffraction when passing through the fork grating array area, generating a fundamental frequency OAM mode. Finally, the fundamental frequency OAM beam passes through the non-processing area to achieve birefringence phase matching, generating a frequency-doubled OAM mode. When the fundamental wave is incident from two opposite directions, the order of the frequency doubling conversion based on birefringence phase matching and the linear Bragg enhanced diffraction will be different, resulting in different frequency doubling diffraction phenomena and achieving a non-reciprocal effect.

2. The non-reciprocal high-order frequency-doubling orbital angular momentum mode Bragg enhancement generating device according to claim 1, characterized in that: The processed lithium niobate crystal includes a non-processed area and a fork-shaped grating array area, which can be expressed 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, which can be expressed as follows for any value of Z:

3. The non-reciprocal high-order frequency-doubling orbital angular momentum mode Bragg enhancement generating device according to claim 2, characterized in that: When using method 1, the fundamental wave is incident in the direction parallel to the xoy plane, polarized along the x direction, and incident at an angle θ with the y axis on the lithium niobate crystal, and refraction occurs: sinθ=n o (λ)sinθ'(3) Among them, θ' is the refraction angle, the fundamental wave is o light, n o (λ) is the refractive index of the fundamental frequency o light, and λ is the wavelength of the fundamental wave. When passing through the non-processed area of the lithium niobate crystal, the doubled frequency e light is generated, and the birefringence phase matching condition is satisfied: k e (λ / 2,θ')=k o (λ)+k o (l)(4) in, are the wave vectors of the doubled frequency e light and the fundamental frequency o light, n e (λ / 2,θ') is the refractive index of the frequency-doubled e-light. Then, the frequency-doubled e-light undergoes Bragg p-order enhanced diffraction in the fork grating array area: Where p is the diffraction order, which is an integer. At the p-order, the frequency-doubled e-light OAM beam is output, and the topological charge is pl o .

4. The non-reciprocal high-order frequency-doubling orbital angular momentum mode Bragg enhancement generating device according to claim 3, characterized in that: When the second method is used, the fundamental wave is incident in the direction parallel to the xoy plane, polarized along the x direction, and incident at an angle of π-θ with the y axis on the lithium niobate crystal, and refraction occurs: sin(π-θ)=sin(θ)=n o (λ)sin(π-θ')=n o (λ)sin(θ')(6) Where π-θ' is the refraction angle, and the fundamental frequency is o-light. After passing through the fork grating array area, the fundamental frequency o-light undergoes q-order Bragg-enhanced diffraction: Where q is the diffraction order, which is an integer. At this time, the fundamental frequency o light OAM mode is output at the q level, and the topological charge is q l o Finally, birefringence phase matching is achieved through the non-processed area: k e (λ / 2,π-θ')=k o (λ)+k o (l)(8) in, The frequency doubling process doubles the topological charge, and the topological charge of the frequency-doubled e-light OAM mode output at the q level is 2ql o .

5. A system for preparing a non-reciprocal high-order frequency-doubled orbital angular momentum mode Bragg enhancement generating device, used for preparing the non-reciprocal high-order frequency-doubled orbital angular momentum mode Bragg enhancement generating device according to any one of claims 1 to 4, characterized in that: The device comprises 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 light 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 laser polarization to the z-direction. The first lens, the pinhole, and the second lens shape the beam. The laser power is continuously varied by a gradient attenuation plate, achieving energy compensation in depth during laser processing. After passing through the objective lens, the laser is focused onto a magnesium-doped lithium niobate crystal. The reflected beam is collected by a CCD for real-time observation of the processing status. 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. The laser is directed to scribe lines along the x-direction, forming a fork-shaped grating. Multiple layers of the same grating are processed along the y-direction, forming a fork-shaped grating array.

6. The system for preparing a non-reciprocal high-order frequency-doubled orbital angular momentum mode Bragg enhancement generation device according to claim 5, characterized in that: The preparation system of the non-reciprocal high-order frequency-doubled orbital angular momentum mode Bragg enhancement generation device also 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.

7. The system for preparing a non-reciprocal high-order frequency-doubled orbital angular momentum mode Bragg enhancement generation device according to claim 5, characterized in that: The preparation system of the non-reciprocal high-order frequency-doubled orbital angular momentum mode Bragg enhancement generation device also includes a dichroic mirror, which is arranged between the gradient attenuation plate and the objective lens, and is used to reflect the laser emitted by the gradient attenuation plate 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 a non-reciprocal high-order frequency-doubled orbital angular momentum mode Bragg enhancement generation 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; the focal lengths of the first lens and the second lens are 75 mm.

9. A non-reciprocal high-order frequency-doubled orbital angular momentum mode Bragg enhancement generation system, comprising the non-reciprocal high-order frequency-doubled orbital angular momentum mode Bragg enhancement generation device according to any one of claims 1 to 4, characterized in that: It also includes a second laser, a third half-wave plate, a second polarization beam splitter, a fourth half-wave plate, a third lens, a rotation stage and a receiving screen; The second laser emits laser light along the x-direction, and the energy of the laser light 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 so that the polarization is along the x-direction, and then the laser light is focused by a third lens onto the non-reciprocal high-order frequency doubling orbital angular momentum mode Bragg enhancement generating device. The non-reciprocal high-order frequency doubling orbital angular momentum mode Bragg enhancement generating device is placed on a rotating translation stage, and the frequency doubling signal is observed on a receiving screen by changing the incident angle.

10. The non-reciprocal high-order frequency-doubled orbital angular momentum mode Bragg enhancement generation system according to claim 9, characterized in that: The second laser emits laser light with a wavelength, a pulse width of 140 fs and a repetition frequency of 80 MHz along the x direction; and the focal length of the third lens is 75 mm.

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