High-order frequency doubling orbital angular momentum mode generating device

Abstract

The invention discloses a high-order frequency doubling orbital angular momentum mode generation device, which is characterized in that a laser direct writing technology is utilized to erase a second-order nonlinear coefficient in a lithium niobate crystal to form a three-dimensional fork-shaped grating array; each layer of fork-shaped grating is a pattern obtained by performing non-collinear interference on an OAM mode with topological charge of 10 and a Gaussian mode according to a holographic theory and then performing binarization processing; when each fork-shaped grating is processed, the central area of the fork-shaped grating is avoided; a fundamental frequency light beam meets a quasi-phase matching condition through the processed lithium niobate crystal, light field regulation and control are carried out while frequency multiplication conversion is carried out, and generation of a high-quality high-order nonlinear OAM mode is achieved.

Description

Technical Field

[0001] The present invention relates to the field of nonlinear light field control technology, and in particular to a high-quality high-order frequency-doubled orbital angular momentum mode generation technology in a three-dimensional nonlinear photonic crystal, and specifically to a high-order frequency-doubled orbital angular momentum mode generation device. Background Art

[0002] Photon orbital angular momentum (OAM) has the characteristics of spirally distributed phase, infinite dimension and orthogonality. The light spot is like a donut with a circular dark spot in the middle. Since it was verified by Allen et al. in 1992, it has been widely used in light manipulation, optical communication, quantum, imaging, detection and optical encryption. High-order OAM modes can significantly improve application performance. The important application of OAM modes in the field of optics has promoted the research on its generation methods. In addition to the commonly used spiral phase plates, q-plates, computational holography, π / 2 mode converters, etc., there are also various micro-integrated OAM generating devices. If you want to generate OAM modes at new frequencies, the traditional method requires two devices: first, frequency conversion is performed through a nonlinear medium, and then OAM is loaded through a linear device. In order to save cost and space, a direct method was born to achieve beam shaping in one step while performing nonlinear conversion. Three-dimensional nonlinear photonic crystals not only design the second-order nonlinear coefficient χ in the vertical propagation direction, but also have a linear device to load the OAM. (2) The light field can be distributed and regulated, and the fundamental wave can be quasi-phase matched by considering the periodic structure in the propagation direction. Compared with metal metasurfaces and two-dimensional nonlinear photonic crystals, the nonlinear efficiency of three-dimensional nonlinear photonic crystals has been greatly improved.

[0003] Femtosecond laser direct writing technology provides a flexible way to prepare three-dimensional nonlinear photonic crystals. It can not only erase the second-order nonlinear coefficient at any position inside the crystal, but also change the refractive index, with the advantages of low thermal effect and high resolution. Using femtosecond laser to erase nonlinear coefficients in lithium niobate crystal to form a three-dimensional fork grating array can be used to generate efficient frequency-doubling OAM modes. However, processing the center position of the fork grating is a challenge. Due to the limitations of laser processing resolution and thermal effects, the center position is often deformed, which reduces the quality of the spot, especially for the fork grating structure used to generate high-order OAM modes. Summary of the invention

[0004] In view of this, in order to solve the above problems in the prior art, the present invention proposes a high-order frequency-doubled orbital angular momentum mode generating device, which performs light field regulation while performing frequency doubling conversion to achieve the generation of high-quality high-order nonlinear OAM modes.

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

[0006] In a first aspect, the present invention provides a high-order frequency-doubled orbital angular momentum mode generating device, wherein the device uses laser direct writing technology to erase the second-order nonlinear coefficient in a lithium niobate crystal to form a three-dimensional fork-shaped grating array;

[0007] The fork grating of each layer is a pattern obtained by non-collinear interference between the OAM mode with topological charge of l0 and the Gaussian mode according to the holographic theory, and then binarization processing;

[0008] When processing each fork-shaped grating, avoid its central area;

[0009] The fundamental frequency beam passes through the processed lithium niobate crystal to meet the quasi-phase matching conditions, and the light field is controlled while the frequency doubling conversion is performed to achieve the generation of high-quality high-order nonlinear OAM modes.

[0010] Preferably, the processed lithium niobate crystal is expressed by the formula:

[0011]

[0012] Among them, x, y, and z are the three orientation values ​​of the rectangular coordinate system. is the azimuth angle in the xz plane, l0 is the topological charge of the OAM mode, and a is the radius of the center area avoided by processing; in the yz plane, the spatial frequency G y =G z =2π / Λ, where Λ is the grating period; T is the binarization function, for any value Z, it is expressed as:

[0013]

[0014] Change of the second-order nonlinear coefficient of the crystal caused by laser processing Δχ (2) , the nonlinear fork grating array is expressed as:

[0015]

[0016] in Represents the original nonlinear coefficient of lithium niobate crystal.

[0017] Preferably, when the fundamental wave is incident on the crystal along the y direction and polarized along the x direction, the change of the second-order nonlinear coefficient directly performs frequency doubling conversion on the fundamental wave, satisfying the quasi-phase matching condition:

[0018]

[0019] in are the wave vectors of fundamental frequency and double frequency light, The reciprocal lattice vector required for the quasi-phase matching condition is expressed as:

[0020]

[0021] Where m and n are arbitrary integers. are the unit vectors in the y and z directions respectively; the quasi-phase matching of the positive and negative first-order frequency-doubled OAM modes requires the reciprocal lattice vector and Quasi-phase matching of positive and negative double-frequency OAM modes requires a reciprocal lattice vector and

[0022] Preferably, the laser direct writing technology is a femtosecond laser direct writing technology.

[0023] In a second aspect, the present invention provides a system for preparing a high-order frequency-doubled orbital angular momentum mode generating device, which is used to prepare the high-order frequency-doubled orbital angular momentum mode generating device, including a first laser, a first half-wave plate, a first polarization beam splitter, a second half-wave plate, a first lens, a second lens, a pinhole, a gradient attenuation plate, an objective lens, a CCD, a computer, a three-dimensional nano-translation stage, and a magnesium-doped lithium niobate crystal;

[0024] The first laser emits laser along the y direction, and the power of the laser is controlled by the first half-wave plate and the first polarization beam splitter. The polarization of the laser is changed to be along the z direction by the second half-wave plate. The light beam is shaped by the first lens, the pinhole and the second lens. The energy of the laser is controlled by the gradient attenuation plate to compensate the energy in depth of the laser processing. After the laser passes through the objective lens, it is focused onto the magnesium-doped lithium niobate crystal. The reflected light beam is collected by the CCD to observe the processing status in real time. The lithium niobate crystal is placed on a three-dimensional nano-translation stage, and the stroke of the three-dimensional nano-translation stage is controlled by a computer to make the laser scribe along the x direction to form a fork-shaped grating. The same grating is processed in multiple layers along the y direction to form a fork-shaped grating array while avoiding the central area.

[0025] Preferably, the preparation system of the high-order frequency-doubled orbital angular momentum mode generating device further comprises a shutter, which is arranged between the first laser and the first half-wave plate and is used to control the opening and closing of the laser.

[0026] Preferably, the preparation system of the high-order frequency-doubled orbital angular momentum mode generating 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.

[0027] 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 y direction.

[0028] Preferably, the focal length of the first lens and the second lens is 75 mm.

[0029] In a third aspect, the present invention provides a high-order frequency-doubled orbital angular momentum mode generating system, comprising the high-order frequency-doubled orbital angular momentum mode generating device, and also comprising a second laser, a third half-wave plate, a second polarization beam splitter, a fourth half-wave plate, a third lens, a filter and a receiving screen;

[0030] The second laser emits laser along the y direction, and the energy of the laser 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 fundamental wave is polarized along the x direction. The laser is then focused by the third lens onto the high-order frequency-doubled orbital angular momentum mode generating device. After the fundamental wave is filtered out by a filter, the frequency-doubled signal is observed on the receiving screen.

[0031] Compared with the prior art, the beneficial effects of the present invention include at least:

[0032] The present invention is a high-quality high-order frequency-doubled orbital angular momentum mode generation technology, which uses femtosecond laser direct writing technology to erase the second-order nonlinear coefficient in the lithium niobate crystal to form a three-dimensional fork grating array. When processing each fork grating, avoid its central area. The fundamental frequency beam passes through the structure to meet the quasi-phase matching conditions, and the light field is regulated while the frequency doubling conversion is performed. Compared with the fork grating array that is not processed without avoiding the central area, the quality of the high-order frequency-doubled OAM mode spot generated by the invention is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] 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.

[0034] Figure 1 The invention is the design principle of the high-order frequency-doubled orbital angular momentum mode generating device (three-dimensional nonlinear photonic crystal), wherein (a) a fork-shaped grating array in a lithium niobate crystal; (b) a fork-shaped grating; (c) a quasi-phase matching principle in reciprocal space;

[0035] Figure 2 Schematic diagram of the preparation system (three-dimensional nonlinear photonic crystal processing) of the high-order frequency-doubled orbital angular momentum mode generating device of the present invention, wherein (a) the processing optical path; (bd) the fork-shaped grating processing structure, the upper figure is the processing structure with a center, and the lower figure is the processing structure without a center;

[0036] Figure 3It is a structural diagram of the high-order frequency-doubled orbital angular momentum mode generation system of the present invention, wherein (a) the generation optical path of the frequency-doubled high-order OAM mode, (bd) are the light spot diagrams of the centered and non-centered three-dimensional nonlinear photonic crystals, (b) the frequency-doubled light spot diagram with l0=2; (c) the frequency-doubled light spot diagram with l0=3; and (d) the frequency-doubled light spot diagram with l0=4. DETAILED DESCRIPTION

[0037] In order to make the above-mentioned purposes, features and advantages of the present invention more obvious and easy to understand, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be pointed out that the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0038] The present invention may also be implemented or applied through other different specific examples, and the details in this specification may also be modified and changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention.

[0039] It should be noted that if the embodiments of the present invention involve directional indications (such as up, down, left, right, front, back, etc.), the directional indications are only used to explain the relative position relationship, movement status, etc. between the components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

[0040] In addition, if there are descriptions involving "first", "second", etc. in the embodiments of the present invention, the descriptions of "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or suggesting their relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. Secondly, the technical solutions between the various embodiments can be combined with each other, but they must be based on the ability of ordinary technicians in the field to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be deemed that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0041] Example 1

[0042] The present invention provides a high-order frequency-doubled orbital angular momentum mode generating device, wherein the device uses a femtosecond laser direct writing technique to erase the second-order nonlinear coefficient in a lithium niobate crystal to form a three-dimensional fork-shaped grating array;

[0043] The fork grating of each layer is a pattern obtained by non-collinear interference between the OAM mode with topological charge of l0 and the Gaussian mode according to the holographic theory, and then binarization processing;

[0044] When processing each fork-shaped grating, avoid its central area;

[0045] The fundamental frequency beam passes through this structure to meet the quasi-phase matching condition, and the light field is controlled while the frequency doubling conversion is performed, thus achieving the generation of high-quality high-order nonlinear OAM modes.

[0046] This embodiment provides a high-order frequency-doubled orbital angular momentum mode generating device, in which a fork-shaped grating array processed by femtosecond laser in a lithium niobate crystal is formed. Figure 1 As shown in (a), the fork grating of each layer is a pattern obtained by non-collinear interference between the OAM mode with topological charge of l0 and the Gaussian mode according to the holographic theory, and then binarization processing. Figure 1 (b) The figure above is a theoretical diagram including the central area calculated according to the holographic principle. It can be seen that the central area of ​​the hologram becomes more and more dense as l0 increases, and the processing difficulty becomes greater. The present invention proposes a processing scheme to avoid the central area of ​​the fork grating, and improve the spot quality of the high-order frequency-doubled OAM mode by sacrificing a small amount of frequency-doubled conversion efficiency. This processing scheme is as follows Figure 1 (b) As shown in the figure below, it can be expressed by the formula:

[0047]

[0048] Among them, x, y, and z are the three orientation values ​​of the rectangular coordinate system. is the azimuth angle of the xz plane, l0 is the topological charge of the OAM mode, is the azimuth angle of the xz plane, l0 is the topological charge of the OAM mode, and a is the radius of the center area avoided by processing. On the yz plane, the spatial frequency G y =G z =2π / Λ, where Λ is the grating period. T is a binary function, which can be expressed as:

[0049]

[0050] Femtosecond laser processing can cause a change in the second-order nonlinear coefficient of the crystal Δχ (2) , the nonlinear fork grating array can be expressed as:

[0051]

[0052] in Represents the original nonlinear coefficient of lithium niobate crystal.

[0053] When the fundamental wave is incident on the crystal along the y direction and polarized along the x direction, the change of the second-order nonlinear coefficient directly doubles the fundamental wave, satisfying the quasi-phase matching condition:

[0054]

[0055] in are the wave vectors of fundamental frequency and double frequency light, The reciprocal lattice vector required for the quasi-phase matching condition is expressed as:

[0056]

[0057] Where m and n are arbitrary integers. are unit vectors in the y and z directions respectively. Figure 1 (c) is the principle diagram of quasi-phase matching of the structure inverse space on the yoz plane, where the quasi-phase matching of the positive and negative first-order frequency-doubled OAM modes requires the inverse lattice vector and Quasi-phase matching of positive and negative double-frequency OAM modes requires a reciprocal lattice vector and

[0058] Example 2

[0059] like Figure 2 As shown, the present invention provides a preparation system for a high-order frequency-doubling orbital angular momentum mode generating device, which is used to prepare the high-order frequency-doubling orbital angular momentum mode generating device, including a first laser, a shutter, a first half-wave plate, a first polarization beam splitter, a second half-wave plate, a first lens, a second lens, a pinhole, a gradient attenuation plate, an objective lens, a CCD, a computer, a three-dimensional nano-translation stage and a magnesium-doped lithium niobate crystal;

[0060] A fork-shaped grating array was fabricated in a 1.5 mm (x) × 2.5 mm (y) × 1 mm (z) lithium niobate crystal, such as Figure 2As shown in (a), the first laser emits a laser with a wavelength of 800nm, a pulse width of 34fs, and a repetition frequency of 1kHz along the y direction. The laser switch is controlled by a shutter, the laser power is controlled by a first half-wave plate and a first polarization beam splitter, the laser polarization is changed to be along the z direction by a second half-wave plate, the beam is shaped by a first lens with a focal length of 75mm, a pinhole, and a second lens with a focal length of 75mm, the laser energy is controlled by a gradient attenuation plate, and the laser processing is compensated for the energy in depth to make the lines of the structure more uniform. After the laser passes through an objective lens with a numerical aperture of 0.75, it is focused onto a 5% magnesium-doped lithium niobate crystal, and the reflected beam is collected by a CCD to observe the processing status in real time. A lithium niobate crystal was placed on a three-dimensional nano-translation stage, and a computer (Labview software) was used to control the travel of the three-dimensional nano-translation stage, so that the laser was engraved along the x-direction. The energy at 120 μm to 20 μm below the crystal surface was continuously reduced from 240 nJ to 200 nJ. The length of the line was 100 μm, forming a forked grating. The same grating was processed along the y-direction for 15 layers to form a forked grating array. The period of the y-axis and z-axis was Λ=3.15 μm, and the radius of the avoided central area was a=1 μm. Figure 2 (b)-(d) are fork-shaped gratings with l0=2, 3, and 4 respectively. The upper figure is the processing structure diagram without removing the center position, and the lower figure is the processing structure diagram without removing the center position.

[0061] Example 3

[0062] like Figure 3 As shown, the present invention provides a high-order frequency-doubled orbital angular momentum mode generating system, comprising the high-order frequency-doubled orbital angular momentum mode generating device, and also comprising a second laser, a third half-wave plate, a second polarization beam splitter, a fourth half-wave plate, a third lens, a filter and a receiving screen;

[0063] Figure 3 (a) is the optical path for generating the frequency-doubled OAM mode. The second laser emits a laser with a wavelength of 800nm, a pulse width of 140fs, and a repetition frequency of 80MHz along the y direction. The energy of the laser is regulated by the third half-wave plate and the second polarization beam splitter. The polarization direction of the fundamental wave is changed by the fourth half-wave plate so that the fundamental wave is polarized along the x direction. Then, the third lens with a focal length of 75mm is used to focus the laser onto the high-order frequency-doubled orbital angular momentum mode generating device. After filtering out the fundamental wave through the filter, the frequency-doubled signal is observed on the receiving screen. Figure 3(b)-(d) are the frequency doubling spot images of the l0=2, 3, and 4 fork-shaped grating arrays in lithium niobate crystals, respectively. The upper image shows the frequency doubling spot without removing the center position, and the lower image shows the frequency doubling spot without removing the center position. It can be clearly seen that after removing the center position of the fork-shaped grating, the central dark spot of the generated frequency doubling OAM mode is more round and of higher quality. For the decentered l0=2, 3, and 4 fork-shaped grating arrays, the normalized frequency doubling efficiency of the first-order OAM mode is 2×10 -5 W -1 , 1.6×10 -5 W -1 , 1.3×10 -5 W -1 .

[0064] The present invention is a high-quality high-order frequency-doubled orbital angular momentum mode generation technology, which uses femtosecond laser direct writing technology to erase the second-order nonlinear coefficient in the lithium niobate crystal to form a three-dimensional fork grating array. When processing each fork grating, avoid its central area. The fundamental frequency beam passes through the structure to meet the quasi-phase matching conditions, and the light field is regulated while the frequency doubling conversion is performed. Compared with the fork grating array that is not processed without avoiding the central area, the quality of the high-order frequency-doubled OAM mode spot generated by the invention is improved.

[0065] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A high-order frequency-multiplied orbital angular momentum mode generating device, characterized in that: The device uses laser direct writing technology to erase the second-order nonlinear coefficient in the lithium niobate crystal to form a three-dimensional fork-shaped grating array; The fork grating of each layer is a pattern obtained by non-collinear interference between the OAM mode with topological charge of l0 and the Gaussian mode according to the holographic theory, and then binarization processing; When processing each fork-shaped grating, avoid its central area; The fundamental frequency beam passes through the processed lithium niobate crystal to meet the quasi-phase matching conditions, and the light field is controlled while the frequency doubling conversion is performed to achieve the generation of high-quality high-order nonlinear OAM modes.

2. The high-order frequency-multiplied orbital angular momentum mode generating device according to claim 1, characterized in that: The processed lithium niobate crystal is expressed by the formula: Among them, x, y, and z are the three orientation values ​​of the rectangular coordinate system. is the azimuth angle in the xz plane, l0 is the topological charge of the OAM mode, and a is the radius of the center area avoided by processing; in the yz plane, the spatial frequency G y =G z =2π / Λ, where Λ is the grating period; T is the binarization function, for any value Z, it is expressed as: Change of the second-order nonlinear coefficient of the crystal caused by laser processing Δχ (2) , the nonlinear fork grating array is expressed as: in Represents the original nonlinear coefficient of lithium niobate crystal.

3. The high-order frequency-doubled orbital angular momentum mode generating device according to claim 2, characterized in that: When the fundamental wave is incident on the crystal along the y direction and polarized along the x direction, the change of the second-order nonlinear coefficient directly doubles the fundamental wave, satisfying the quasi-phase matching condition: in are the wave vectors of fundamental frequency and double frequency light respectively, The reciprocal lattice vector required for quasi-phase matching conditions is expressed as: Where m and n are arbitrary integers. are the unit vectors in the y and z directions respectively; the quasi-phase matching of the positive and negative first-order frequency-doubled OAM modes requires the reciprocal lattice vector and Quasi-phase matching of positive and negative double-frequency OAM modes requires a reciprocal lattice vector and 4. The high-order frequency-multiplied orbital angular momentum mode generating device according to claim 1, characterized in that: The laser direct writing technology is a femtosecond laser direct writing technology.

5. A system for preparing a high-order frequency-multiplied orbital angular momentum mode generating device, used for preparing a high-order frequency-multiplied orbital angular momentum mode generating device as claimed in any one of claims 1 to 4, characterized in that: It includes a first laser, a first half-wave plate, a first polarization beam splitter, a second half-wave plate, a first lens, a second lens, a pinhole, a gradient attenuation plate, an objective lens, a CCD, a computer, a three-dimensional nano-translation stage and a magnesium-doped lithium niobate crystal; The first laser emits laser along the y direction, and the power of the laser is controlled by the first half-wave plate and the first polarization beam splitter. The polarization of the laser is changed to be along the z direction by the second half-wave plate. The light beam is shaped by the first lens, the pinhole and the second lens. The energy of the laser is controlled by the gradient attenuation plate to compensate the energy in depth of the laser processing. After the laser passes through the objective lens, it is focused onto the magnesium-doped lithium niobate crystal. The reflected light beam is collected by the CCD to observe the processing status in real time. The lithium niobate crystal is placed on a three-dimensional nano-translation stage, and the stroke of the three-dimensional nano-translation stage is controlled by a computer to make the laser scribe along the x direction to form a fork-shaped grating. The same grating is processed in multiple layers along the y direction to form a fork-shaped grating array while avoiding the central area.

6. The system for preparing a high-order frequency-doubled orbital angular momentum mode generating device according to claim 5, characterized in that: The preparation system of the high-order frequency-doubled orbital angular momentum mode generating 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 high-order frequency-doubled orbital angular momentum mode generating device according to claim 5, characterized in that: The preparation system of the high-order frequency-doubled orbital angular momentum mode generating 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 high-order frequency-doubled orbital angular momentum mode generating device according to claim 5, characterized in that: The first laser emits a laser with a wavelength of 800 nm, a pulse width of 34 fs and a repetition frequency of 1 kHz along the y direction.

9. The system for preparing a high-order frequency-doubled orbital angular momentum mode generating device according to claim 5, characterized in that: The focal length of the first lens and the second lens is 75 mm.

10. A high-order frequency-multiplied orbital angular momentum mode generation system, comprising the high-order frequency-multiplied orbital angular momentum mode generation device as claimed in 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 filter and a receiving screen; The second laser emits laser along the y direction, and the energy of the laser 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 fundamental wave is polarized along the x direction. The laser is then focused by the third lens onto the high-order frequency-doubled orbital angular momentum mode generating device. After the fundamental wave is filtered out by a filter, the frequency-doubled signal is observed on the receiving screen.

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