A non-reciprocal high-order frequency multiplication orbital angular momentum mode Bragg enhanced generator device

Abstract

This invention proposes a non-reciprocal higher-order frequency-harmonic orbital angular momentum mode (OAM) Bragg enhancement generation device. Within a certain wavelength range, lithium niobate crystals can achieve birefringence phase matching, enabling efficient frequency doubling conversion. Femtosecond laser direct writing technology is used to fabricate a three-dimensional fork-shaped grating array within the lithium niobate crystal. Based on the change in refractive index, Bragg enhancement diffraction can be achieved, further improving the generation efficiency of the target diffraction-order secondary frequency-harmonic OAM mode. Furthermore, different frequency-harmonic 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.

Description

Technical Field

[0001] This invention relates to the field of nonlinear optical field manipulation technology, and more particularly to the generation of high-order frequency-harmonic orbital angular momentum modes in lithium niobate crystals, specifically to a non-reciprocal high-order frequency-harmonic orbital angular momentum mode Bragg enhancement generation device. Background Technology

[0002] Orbital angular momentum (OAM) possesses properties such as helical phase distribution, infinite dimension, and orthogonality. Since its verification by Allen et al. in 1992, higher-order OAM modes have been widely used in optical manipulation, optical communication, quantum mechanics, imaging, detection, and optical encryption. The significant applications of OAM modes in optics have driven research into methods for generating higher-order modes. Besides commonly used methods such as helical phase plates, q-plates, computational holography, and π / 2 mode converters, various miniature integrated OAM generation devices have been developed. However, these methods are primarily concentrated in the linear domain. Combining them with nonlinear effects can further expand the frequency range of OAM modes.

[0003] Traditional methods for efficiently generating OAM modes at new frequencies require two devices: first, frequency conversion via a nonlinear material, and then OAM loading via a linear device. To save cost and space, a single device is needed to simultaneously perform nonlinear conversion and beam shaping. Two-dimensional nonlinear photonic crystals and metasurfaces can achieve nonlinear optical field manipulation, but their efficiency is low. Three-dimensional nonlinear photonic crystals fabricated using femtosecond laser direct writing technology can manipulate the optical field while achieving quasi-phase matching; however, femtosecond laser direct writing can only erase about 20% of the nonlinear coefficients, and the nonlinear conversion efficiency remains low. Summary of the Invention

[0004] In view of this, to address the aforementioned problems in the prior art, this invention proposes a non-reciprocal higher-order frequency-harmonic orbital angular momentum mode (OAM) Bragg enhancement generation device. Within a certain wavelength range, lithium niobate crystals can achieve birefringence phase matching, enabling efficient frequency doubling conversion. Femtosecond laser direct writing technology is used to fabricate a three-dimensional fork-shaped grating array within the lithium niobate crystal. Based on the change in refractive index, Bragg enhancement diffraction can be achieved, further improving the generation efficiency of the target diffraction-order secondary frequency-harmonic OAM mode. Furthermore, different frequency-harmonic diffraction phenomena can occur when the fundamental wave is incident from two opposing 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 proposes a non-reciprocal higher-order frequency-harmonic orbital angular momentum mode Bragg enhancement generation device, wherein the device is fabricated in a lithium niobate crystal using laser processing technology, and the refractive index of the processing region will be changed.

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

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

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

[0010] There are two directions for the fundamental wave incident: one is to pass through the unprocessed area first, and then through the fork-shaped grating array area; the other is to pass through the fork-shaped grating array area first, and then through the unprocessed area.

[0011] When using method one, the fundamental o-beam, parallel to the xoy plane, polarized along the x-direction, and incident on the surface of the lithium niobate crystal at an angle θ with the y-axis, undergoes refraction. The refracted o-beam passes through the unprocessed region of the lithium niobate crystal, generating a frequency-doubled e-beam that satisfies the birefringence phase-matching condition. Finally, the frequency-doubled e-beam undergoes p-order Bragg enhancement diffraction in the fork-shaped grating array region, generating a frequency-doubled OAM mode.

[0012] When using method two, the fundamental o-beam, parallel to the xoy plane, polarized along the x-direction, and incident on the surface of the lithium niobate crystal at an angle of π-θ with the y-axis, undergoes refraction. The refracted o-beam undergoes q-order Bragg enhanced diffraction in the fork-shaped grating array region, generating the fundamental OAM mode. Finally, the fundamental OAM beam passes through the unprocessed area to achieve birefringence phase matching, generating the 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 an unprocessed region and a fork-shaped grating array region, as shown below:

[0015]

[0016] in, Let be the azimuth angle in the xz plane, l0 be the topology charge of the OAM mode, and x, y, and z be the three azimuth values ​​in a Cartesian coordinate system. In the yz plane, the spatial frequency G... y =G z= 2π / Λ, where Λ is the grating period; T is the binarization function, which is expressed as follows for any value Z:

[0017]

[0018] Preferably, when method one is used, when the fundamental wave incident direction is parallel to the xoy plane, polarized along the x direction, and makes an angle θ with the y axis, it is incident on the lithium niobate crystal, and refraction occurs:

[0019] sinθ=n o (λ)sinθ'(3)

[0020] Where θ' is the angle of refraction, the fundamental wave is the o-wave, and n o (λ) is the refractive index of the fundamental frequency o-ray, and λ is the wavelength of the fundamental wave; after passing through the unprocessed region of the lithium niobate crystal, the frequency-doubled e-ray is generated, satisfying the birefringence phase-matching condition:

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

[0022] in, These are the wave vectors of the frequency-doubled e-light and the fundamental frequency o-light, respectively. e (λ / 2, θ') represents the refractive index of the frequency-doubled e-ray; subsequently, the frequency-doubled e-ray undergoes Bragg p-order enhanced diffraction in the region of the fork-shaped grating array:

[0023]

[0024] Where p is the diffraction order, which is an integer; the p-order output frequency-doubled e-optical OAM beam has a topological charge of pp. o .

[0025] Preferably, when using method two, when the fundamental wave incident direction is parallel to the xoy plane, polarized along the x direction, and makes an angle of π-θ with the y axis, it is incident on the lithium niobate crystal and 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-ray; then, after passing through the forked grating array region, the fundamental frequency o-ray undergoes q-order Bragg enhanced diffraction:

[0028]

[0029] Where q is the diffraction order, which is an integer; at this point, in the q-order output fundamental frequency OAM mode, the topological charge is ql. o Finally, birefringence phase matching is achieved through the non-processed region:

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

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

[0032] Secondly, the present invention provides a fabrication system for a non-reciprocal higher-order frequency-harmonic orbital angular momentum mode Bragg enhancement generation device, used to fabricate the non-reciprocal higher-order frequency-harmonic orbital angular momentum mode Bragg enhancement generation device, comprising a first laser, a first half-wave plate, a second half-wave plate, a first polarizing beam splitter, a first lens, a second lens, a pinhole, a graded attenuator, an objective lens, a CCD, a three-dimensional nanostage, and a computer;

[0033] The first laser emits laser light along the x-direction. The first half-wave plate and the first polarizing beam splitter control the power of the laser. The second half-wave plate changes the polarization of the laser to the z-direction. The beam is shaped by the first lens, the pinhole, and the second lens. The laser power is continuously varied by a gradient attenuator to achieve 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 to monitor the processing status 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 to make the laser etch lines along the x-direction to form a fork-shaped grating. Multiple identical gratings are processed along the y-direction to form a fork-shaped grating array.

[0034] Preferably, the fabrication system for the non-reciprocal higher-order frequency-doubled orbital angular momentum mode Bragg enhancement generator further includes a shutter, which is disposed between the first laser and the first half-wave plate to control the opening and closing of the laser.

[0035] Preferably, the fabrication system of the non-reciprocal higher-order frequency-doubled orbital angular momentum mode Bragg enhancement generator further includes a dichroic mirror, which is disposed between a graded attenuator and an objective lens. The dichroic mirror is used to reflect the laser emitted from the graded attenuator to the objective lens, while simultaneously transmitting the light beam reflected by the lithium niobate crystal to the CCD for collection.

[0036] Preferably, the first laser emits a laser beam of 800 nm with a pulse width of 34 fs and a repetition frequency of 1 kHz along the x-direction; the focal length of the first lens and the second lens is 75 mm.

[0037] Thirdly, the present invention provides a non-reciprocal higher-order frequency-harmonic orbital angular momentum mode Bragg enhancement generation system, including the non-reciprocal higher-order frequency-harmonic orbital angular momentum mode Bragg enhancement generation device, and further including a second laser, a third half-wave plate, a second polarization beam splitter, a fourth half-wave plate, a third lens, a rotating displacement stage, and a receiving screen.

[0038] The second laser emits laser light along the y-direction. The energy of the laser light is controlled 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. Then, the laser light is focused by the third lens onto the non-reciprocal higher-order frequency-harmonic orbital angular momentum mode Bragg enhancement generator. The non-reciprocal higher-order frequency-harmonic orbital angular momentum mode Bragg enhancement generator is placed on a rotating displacement stage. By changing the incident angle, the frequency-harmonic signal is observed on the receiving screen.

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

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

[0041] This invention proposes a non-reciprocal higher-order frequency-harmonic orbital angular momentum mode (OAM) Bragg enhancement generation device. Within a certain wavelength range, lithium niobate crystals can achieve birefringence phase matching, enabling efficient frequency doubling conversion. Femtosecond laser direct writing technology is used to fabricate a three-dimensional fork-shaped grating array within the lithium niobate crystal. Based on the change in refractive index, Bragg enhancement diffraction can be achieved, further improving the generation efficiency of the target diffraction-order secondary frequency-harmonic OAM mode. Furthermore, different frequency-harmonic 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. Attached Figure Description

[0042] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0043] Figure 1This is a schematic diagram of the non-reciprocal higher-order harmonic orbital angular momentum mode Bragg enhancement generation device of the present invention; wherein (a) is a non-reciprocal higher-order harmonic orbital angular momentum mode Bragg enhancement generation device; (b) is a schematic diagram of the inverted space of mode one; (c) is a beam distribution diagram of mode one; (d) is a schematic diagram of the inverted space of mode two; and (e) is a beam distribution diagram of mode two.

[0044] Figure 2 This is a schematic diagram of the fabrication system of the non-reciprocal higher-order frequency-harmonic orbital angular momentum mode Bragg enhancement generation device of the present invention; wherein (a) is the optical path diagram of device fabrication; and (b) is the topographic diagram of the device in the xoz and yoz planes.

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

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

[0047] Figure 5 These are the spot patterns for Mode 1 and Mode 2 when l0 = 3, θ = 9°, and λ = 1048nm. Detailed Implementation

[0048] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, 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 described embodiments are merely some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0049] Example 1

[0050] This invention proposes a non-reciprocal higher-order frequency-harmonic orbital angular momentum mode Bragg enhancement generation device, which is fabricated in lithium niobate crystal using laser processing technology, and the refractive index of the processing region will be changed.

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

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

[0053] The first region is a non-processing region, and the second region is a fork-shaped grating array region, such as... Figure 1 As shown in (a);

[0054] There are two directions for the fundamental wave incident: one is to pass through the unprocessed area first, and then through the fork-shaped grating array area; the other is to pass through the fork-shaped grating array area first, and then through the unprocessed area.

[0055] When using method one, the fundamental o-beam, parallel to the xoy plane, polarized along the x-direction, and incident on the surface of the lithium niobate crystal at an angle θ with the y-axis, undergoes refraction. The refracted o-beam passes through the unprocessed region of the lithium niobate crystal, generating a frequency-doubled e-beam that satisfies the birefringence phase-matching condition. Finally, the frequency-doubled e-beam undergoes p-order Bragg enhancement diffraction in the fork-shaped grating array region, generating a frequency-doubled OAM mode.

[0056] When using method two, the fundamental o-beam, parallel to the xoy plane, polarized along the x-direction, and incident on the surface of the lithium niobate crystal at an angle of π-θ with the y-axis, undergoes refraction. The refracted o-beam undergoes q-order Bragg enhanced diffraction in the fork-shaped grating array region, generating the fundamental OAM mode. Finally, the fundamental OAM beam passes through the unprocessed area to achieve birefringence phase matching, generating the 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 an unprocessed region and a fork-shaped grating array region, such as... Figure 1 As shown in (a), it is represented as:

[0059]

[0060] in, Let be the azimuth angle in the xz plane, l0 be the topology charge of the OAM mode, and x, y, and z be the three azimuth values ​​in a Cartesian coordinate system. In the yz plane, the spatial frequency G... y =G z = 2π / Λ, where Λ is the grating period; T is the binarization function, which is expressed as follows for any value Z:

[0061]

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

[0063] sinθ=n o (λ)sinθ'(3)

[0064] Where θ' is the angle of refraction, the fundamental wave is the o-wave, and n o(λ) represents the refractive index of the fundamental frequency (o) light, and λ is the wavelength of the fundamental wave. After passing through the unprocessed region of a lithium niobate crystal, frequency-doubled (e) light is generated, satisfying the birefringence phase-matching condition:

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

[0066] in, These are the wave vectors of the frequency-doubled e-light and the fundamental frequency o-light, respectively. e (λ / 2, θ') represents the refractive index of the frequency-doubled e-ray; subsequently, the frequency-doubled e-ray undergoes Bragg p-order enhanced diffraction in the region of the fork-shaped grating array:

[0067]

[0068] Where p represents the diffraction order, which is an integer. In a p-order output frequency-doubled e-optical (OAM) beam, the topological charge of the OAM beam is pl. o When p = 1, 2, 3, and 4, the frequency-doubled light spots are distributed at positions 1-4, as shown in the diagram. Figure 1 As shown in (c), the corresponding topological load numbers are l o 2l o 3l o 4l o .

[0069] Figure 1 (d) As shown in reciprocal space, when the fundamental wave, parallel to the xoy plane, polarized along the x-direction, and incident on a 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 the o-wave. Then, passing through the forked grating array region, the fundamental frequency o-wave undergoes q-order Bragg-enhanced diffraction:

[0072]

[0073] Where q represents the diffraction order, which is an integer. At this point, in the q-order output fundamental frequency OAM mode, the topological charge is ql. o Finally, birefringence phase matching is achieved through the unprocessed region:

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

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

[0076] Example 2

[0077] like Figure 2 As shown, the present invention provides a fabrication system for a non-reciprocal higher-order frequency-harmonic orbital angular momentum mode Bragg enhancement generation device, which is used to fabricate the non-reciprocal higher-order frequency-harmonic orbital angular momentum mode Bragg enhancement generation device, including a first laser, a shutter, a first half-wave plate, a second half-wave plate, a first polarizing beam splitter, a first lens, a second lens, a pinhole, a graded attenuator, a dichroic mirror, an objective lens, a CCD, a three-dimensional nanostage, and a computer.

[0078] The non-reciprocal higher-order frequency-harmonic orbital angular momentum mode Bragg enhancement generation device is fabricated in a 1.5mm(x)×2.5mm(y)×1mm(z) lithium niobate crystal, such as... Figure 2 As shown in (a), the first laser emits a laser beam of 800 nm with a pulse width of 34 fs and a repetition frequency of 1 kHz along the x-direction. The laser's on / off state is controlled by a shutter, while the power is controlled by a first half-wave plate and a first polarizing beam splitter. A second half-wave plate polarizes the laser beam along the z-direction. The beam is shaped by a first lens with a focal length of 75 mm, a pinhole, and a second lens with a focal length of 75 mm. A graduated attenuator continuously varies the laser power, achieving energy compensation in depth during laser processing. After passing through a 0.75-inch objective lens, the laser beam 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 positioned between the graduated attenuator and the objective lens to reflect the laser beam emitted from the graduated attenuator to the objective lens, while simultaneously transmitting the beam reflected from the lithium niobate crystal to the CCD for collection. A lithium niobate crystal was placed on a three-dimensional nano-translation stage. The stage's travel was controlled by a computer (LabVIEW software), allowing a laser to etch lines along the x-axis. The laser processing energy decreased from 240 nJ to 20 nJ between 120 nm and 20 nm below the crystal surface. The line length was 100 μm, forming a fork-shaped grating. Fifteen identical gratings were then fabricated along the y-axis, forming a fork-shaped grating array. The periods Λ on the y and z axes were 3.35 μm. The fabricated structure is shown below. Figure 2 (b)

[0079] Example 3

[0080] like Figure 3 As shown, the present invention provides a non-reciprocal higher-order frequency harmonic orbital angular momentum mode Bragg enhancement generation system, including the non-reciprocal higher-order frequency harmonic orbital angular momentum mode Bragg enhancement generation device, and further including a second laser, a third half-wave plate, a second polarization beam splitter, a fourth half-wave plate, a third lens, a rotating displacement stage, and a receiving screen.

[0081] The second laser emits a laser beam with a wavelength of 140 fs and a repetition frequency of 80 MHz along the y-direction. The laser energy is controlled by a third half-wave plate and a second polarization beam splitter, and the polarization direction of the fundamental wave is changed by a fourth half-wave plate, so that the polarization is along the y-direction. Then, the laser beam is focused onto the device by a third lens with a focal length of 75 mm. The device is placed on a rotating displacement stage, and the frequency-doubled signal is observed on a receiving screen by changing the incident angle.

[0082] Experimental results are as follows Figure 4 As shown, the black, red, and blue curves theoretically represent the relationship between wavelength and incident angle for birefringence phase matching, Bragg diffraction in mode one, and Bragg diffraction in mode two, respectively. The intersection of the red and black curves represents the wavelength and incident angle that simultaneously satisfy formulas (4)-(6) in mode one, and the intersection of the blue and black curves represents the wavelength and incident angle that simultaneously satisfy formulas (7)-(11) in mode two. When p = 1-4, the theoretical intersections are marked in black, which are (4.41°, 1031nm), (8.87°, 1033nm), (13.41°, 1036nm), and (18.11°, 1041nm), respectively. The experimentally measured values ​​are marked in red, which 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 theoretical and experimental results are basically in agreement.

[0083] Using l o A forked grating array with a fundamental wavelength of 1 has a fundamental power of 230mW. When incident using mode one at an incident angle of 20.83°, the harmonic power at each position varies with the fundamental wavelength as follows: Figure 5 As shown in (a), it can be seen that when the fundamental wavelength is 1036 nm, the harmonic power at position 4 is the highest, and the harmonic powers at positions 3, 1, and 2 decrease accordingly, indicating that birefringent frequency doubling conversion of Bragg-enhanced diffraction with p=4 is achieved. When the fundamental wavelength is 1036 nm, the emitted harmonic light spot is as follows: Figure 5 As shown in (b). A cylindrical lens is used to perform mode conversion on the OAM mode at position 4, and the converted spot is as follows. Figure 5 As shown in (c), the number of dark fringes is 4, indicating a topological charge number of 4. When incident using mode two at an incident angle of 20.16°, the harmonic power at each position varies with the fundamental wavelength as follows: Figure 5 As shown in (d), when the fundamental wavelength is 1032 nm, the harmonic power at position 4 is the highest, while the harmonic power at other positions is extremely low, indicating that birefringent frequency doubling conversion of Bragg-enhanced diffraction with q=2 has been achieved. When the fundamental wavelength is 1032 nm, the emitted harmonic light spot is as follows... Figure 5 As shown in (d), a cylindrical lens is used to perform mode conversion on the OAM mode at position 4, and the converted spot is as follows. Figure 5 As shown in (e), the number of dark stripes is 4, indicating that the topological charge is 4. Comparing the two methods, it can be concluded that when the frequency harmonic power is strongest at position 4, the distribution of frequency harmonic power at other positions is different. In method 1, the power at each position is on the same order of magnitude, and 4-channel output can be achieved simultaneously. In method 2, only a single-channel output at position 4 is achieved, but the frequency harmonic efficiency is 2.5 times that of method 1. The two methods have non-reciprocal properties.

[0084] This invention proposes a non-reciprocal higher-order frequency-harmonic orbital angular momentum mode (OAM) Bragg enhancement generation device. Within a certain wavelength range, lithium niobate crystals can achieve birefringence phase matching, enabling efficient frequency doubling conversion. Femtosecond laser direct writing technology is used to fabricate a three-dimensional fork-shaped grating array within the lithium niobate crystal. Based on the change in refractive index, Bragg enhancement diffraction can be achieved, further improving the generation efficiency of the target diffraction-order secondary frequency-harmonic OAM mode. Furthermore, different frequency-harmonic 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.

[0085] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A non-reciprocal higher-order harmonic orbital angular momentum mode Bragg enhancement generation system, comprising a non-reciprocal higher-order harmonic orbital angular momentum mode Bragg enhancement generation device, wherein the device is fabricated in a lithium niobate crystal using laser processing technology, and the refractive index of the processed region is changed; The optical axis of the lithium niobate crystal is along the z-direction. When the polarization of the light beam is along the x-direction, it is ordinary light, also known as o-ray; when the polarization of the light beam is along the z-direction, it is extraordinary light, also known as e-ray. The processed lithium niobate crystal comprises two regions, namely a first region and a second region arranged sequentially along the y-direction; The first region is a non-processing region, and the second region is a fork-shaped grating array region; There are two directions for the fundamental wave incident: one is to pass through the unprocessed area first, and then through the fork-shaped grating array area; the other is to pass through the fork-shaped grating array area first, and then through the unprocessed area. When method one is used, the fundamental wave o-light is parallel to the xoy plane, polarized along the x-direction, and makes an angle with the y-axis of θ. When incident on the surface of a lithium niobate crystal, refraction occurs; The refracted o-ray passes through the unprocessed region of the lithium niobate crystal to generate the frequency-doubled e-ray, which satisfies the birefringence phase-matching condition. Finally, the frequency-doubled e-ray undergoes p-order Bragg enhancement diffraction in the fork-shaped grating array region, generating the frequency-doubled OAM mode. When method two is used, the fundamental wave o-light is parallel to the xoy plane, polarized along the x-direction, and makes an angle with the y-axis. When incident on the surface of the lithium niobate crystal, refraction occurs; the refracted light o-beam undergoes q-order Bragg enhanced diffraction in the fork-shaped grating array region, generating the fundamental frequency OAM mode; finally, the fundamental frequency OAM beam achieves birefringence phase matching in the unprocessed region, generating the 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. Its features include a second laser, a third half-wave plate, a second polarizing beam splitter, a fourth half-wave plate, a third lens, a rotating displacement stage, and a receiving screen; The second laser emits laser light along the x-direction. The energy of the laser light is controlled 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. Then, the laser light is focused by the third lens onto the non-reciprocal higher-order frequency-harmonic orbital angular momentum mode Bragg enhancement generator. The non-reciprocal higher-order frequency-harmonic orbital angular momentum mode Bragg enhancement generator is placed on a rotating displacement stage. By changing the incident angle, the frequency-harmonic signal is observed on the receiving screen.

2. The Bragg enhancement generation system for non-reciprocal higher-order harmonic orbital angular momentum modes according to claim 1, characterized in that, The second laser emits a laser with a wavelength of 140 fs and a repetition frequency of 80 MHz along the x-direction; the third lens has a focal length of 75 mm.

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