Infrared orbital angular momentum pattern recognition method based on nonlinear optics and corner lenses

Abstract

This paper presents an infrared orbital angular momentum pattern recognition method based on nonlinear optics and a corner lens, belonging to the field of optical technology. A corner lens images a potassium titanate oxyphosphate (KTP) crystal, and infrared vortex light carrying the orbital angular momentum to be identified is incident on the KTP crystal. Simultaneously, second harmonics are generated, and the corner lens focuses different orbital angular momentum patterns to different angular positions. In other words, nonlinear optics and a corner lens are used to convert infrared vortices carrying different orbital angular momentum into visible light focused at different azimuth coordinates. Experiments demonstrate the feasibility of this infrared orbital angular momentum pattern recognition method, showcasing its novel technology and simple, compact device. By converting the infrared light to be detected into visible light through nonlinear frequency upconversion before CCD detection, the drawbacks of low efficiency and high cost associated with infrared light detection are avoided. This method holds promise for applications in specialized molecular spectroscopy, biological imaging, and infrared monitoring.

Description

Technical Field

[0001] This invention relates to the field of optical technology, and in particular to an infrared orbital angular momentum pattern recognition method based on nonlinear optics and corner lenses. Background Technology

[0002] The complex amplitude expression has The spiral phase term of the vortex light, on average each photon carries The orbital angular momentum, where The azimuth angle is represented by l, which is an eigenvalue of the orbital angular momentum and can be any integer (Allen L, Beijersbergen MW, Spreeuw RJC, et al. Orbital angular momentum of light and the transformation of Laguerre-Gaussian laser modes[J]. Physical review A, 1992, 45(11): 8185.). Due to the uncertainty of the phase at the center of the spiral phase wavefront of the vortex light, its optical field exhibits a ring-shaped distribution with zero intensity at the center.The spiral phase, hollow optical field, and infinite bandwidth of vortex beams make them suitable for applications in optical remote sensing (Courtial J, Dholakia K, Robertson DA, et al. Measurement of the rotational frequency shift imparted to a rotating light beam possessing orbital angular momentum[J]. Physical Review Letters, 1998, 80(15):3217.), particle manipulation (Grier D GA revolution in optical manipulation[J]. Nature, 2003, 424(6950):810-816.), optical imaging (Jesacher A, Fürhapter S, Bernet S, et al. Shadow effects inspiral phase contrast microscopy[J]. Physical Review Letters, 2005, 94(23):233902.), and optical communication (Wang J, Yang JY, Fazal IM, et al. Terabit free-space data transmission employing orbital angular momentum). Multiplexing[J].Naturephotonics,2012,6(7):488-496.), quantum information (Mair A,Vaziri A,Weihs G,etal.Entanglement of the orbital angular momentum states of photons[J].Nature,2001,412(6844):313-316.), etc. have important applications. With the widespread application of vortex light fields in various fields, how to effectively prepare and detect the orbital angular momentum of vortex light fields has become a key issue. Compared with directly generating vortex light using components such as spiral phase plates, spatial light modulators, and metasurfaces, the effective identification of orbital angular momentum modes is challenging because traditional detectors acquire intensity information but lack phase information.

[0003] Common methods for detecting orbital angular momentum include interferometry, diffraction, and logarithmic polar transformation. In 2018, inspired by the ability of lenses to achieve Fourier transformations of momentum and position spaces, Rishabh Sah et al. proposed a method for detecting the spiral phase. Phase e of the plane wave ikz By analogy, a mathematical expression for an angular lens was proposed, and an orbital angular momentum pattern recognition experiment was conducted using a spatial light modulator to simulate the angular lens (Sahu R, Chaudhary S, Khare K, et al. Angular lens[J]. Optics Express, 2018, 26(7): 8709-8718.). In 2022, a new hybrid lens composed of an angular lens and a radial lens was proposed, which solved the main drawback of traditional angular lenses being more suitable for recognizing orbital angular momentum patterns of a specific radius, and significantly improved the recognition performance of orbital angular momentum patterns (Zhou J, Pu H, Wang Q. Orbital angular momentum mode sorting based on a hybrid radial-angular hybrid lens[J]. Optics Express, 2022, 30(6): 9703-9713.). In addition, a single-phase multi-ring azimuthal-quadratic phase optical element based on a single-ring azimuthal-quadratic phase optical element has been proposed to solve the problem that traditional angle lenses cannot simultaneously satisfy the separation of adjacent orbital angular momentum modes and the detection range of orbital angular momentum modes as large as possible (Lv Y, Shang Z, Fu S, et al. Sorting orbital angular momentum of photons through a multi-ring azimuthal-quadratic phase[J]. Optics Letters,2022,47(19):5032-5035.).

[0004] The aforementioned method for identifying orbital angular momentum using a corner lens is only applicable to visible light within a linear optical framework. The infrared band, with its strong penetrating power and low photon energy, has wide applications in two-dimensional infrared molecular spectrometers, biological imaging, and infrared remote sensing; therefore, identifying infrared orbital angular momentum is equally important. However, infrared optical detectors require special photoelectric response materials and processes, making their manufacturing cost higher than that of visible light detectors. Furthermore, their lower detection efficiency and larger dark current limit the application of infrared vortex light (Hadfield RH. Single-photon detectors for optical quantum information applications[J]. Nature photonics,2009,3(12):696-705.). Therefore, ultrasensitive detection can be achieved by converting the infrared light to be detected into visible light through nonlinear frequency upconversion. A fundamental frequency beam is used to image a corner lens onto a KTP crystal, while another beam of infrared vortex light to be identified is incident. The incident light carrying different orbital angular momentum modes is converted into visible light focused at different azimuth coordinates, which can then be detected using a CCD. Summary of the Invention

[0005] The purpose of this invention is to provide an infrared orbital angular momentum pattern recognition device based on nonlinear optics and a corner lens. This device has a compact structure, simple operation, and strong practicality. When infrared vortex light is incident on a KTP crystal with corner lens imaging, the vortex light can be focused at different azimuth coordinates by the corner lens, and the second harmonic can be generated to convert the light to be detected into visible light, which can then be directly detected by a CCD.

[0006] Another objective of this invention is to provide an infrared orbital angular momentum pattern recognition method based on nonlinear optics and corner lenses. This method extends the corner lens recognition of orbital angular momentum to the nonlinear domain, solving the problems of low infrared light detection efficiency, large dark current, and high cost that make infrared vortex detection difficult. This method has considerable application prospects in fields such as special molecular spectroscopy, biological imaging, and infrared monitoring.

[0007] This invention provides an infrared orbital angular momentum pattern recognition device based on nonlinear optics and corner lenses, comprising optical components such as a laser, a telescope system, a spatial light modulator, a biconvex lens, a mirror, a beam splitter cube, an aperture, a half-wave plate, a polarization beam splitter cube, a KTP crystal, a filter, and a CCD camera.

[0008] The telescope system and the spatial light modulator are coaxially and sequentially placed in the output optical path of the laser beam. The reflected light from the spatial light modulator, after passing through a biconvex lens, is split into two fundamental frequency beams by a beam splitter: one is infrared light carrying the phase structure of a conventional corner lens, and the other is infrared vortex light to be identified. One beam is sequentially equipped with an aperture and a biconvex lens. The biconvex lens, together with the biconvex lens before the beam splitter, forms a 4f system. The aperture is placed in the Fourier plane of this 4f system to filter out the first-order diffraction light carrying the phase structure of the corner lens. Similarly... Another path is equipped with an aperture stop and a biconvex lens in sequence. The biconvex lens and the biconvex lens before the beam splitter form a 4f system. The aperture stop is placed in the Fourier plane of this 4f system to filter out the first-order diffraction light of the fork-shaped grating to obtain the infrared vortex to be identified. The image planes of the two 4f systems coincide. In the transmission path of the beam splitter, the aperture stop is used to filter the first-order diffraction light of the corner lens grating in the Fourier plane of the 4f system. In the reflection path of the beam splitter, the aperture stop is used to filter the first-order diffraction light of the fork-shaped grating in the Fourier plane of the 4f system. The reflection path of the beam splitter is equipped with a half-wave plate.

[0009] The CCD camera is located behind the filter and is used to detect the intensity distribution of the frequency-doubled light; the beam splitter is used to split the optical path into two fundamental frequency beams, one beam carrying infrared light carrying phase information of a conventional corner lens, and the other beam being detected as an infrared vortex beam; the polarization beam splitter is used to combine the two fundamental frequency beams.

[0010] In a preferred embodiment, the laser is a near-infrared laser with a wavelength of 1064nm, but other near-infrared or infrared lasers of other wavelengths can be selected as needed.

[0011] In a preferred embodiment, a half-wave plate with a fast axis at a 45° angle to the horizontal is inserted into the reflection path of the beam splitter cube using KTP. The half-wave plate is used to convert vertically polarized light into horizontally polarized light.

[0012] In a preferred embodiment, the infrared vortex light to be identified is generated by a spatial light modulator. The infrared vortex light to be identified can also be generated by a spiral phase plate, metasurface, etc. The key is to image the vortex to be identified onto a KTP crystal.

[0013] The biconvex lens, through a reasonable arrangement, makes the phase planes of the 4f system in the two fundamental frequency beams coincide.

[0014] In a preferred embodiment, the 4f system used in both fundamental frequency beams has a first lens with a focal length of 750mm and a second lens with a focal length of 100mm. Other lens combinations can be selected as needed. By reasonably arranging the lenses, the phase planes of the 4f systems in the two fundamental frequency beams can be made to coincide.

[0015] An infrared orbital angular momentum pattern recognition method based on nonlinear optics and corner lenses includes the following steps:

[0016] 1) Provide light source and collimate and expand beam: The vertically polarized infrared light emitted by the laser is used as the laser input of the system. It is collimated and expanded by the telescope system to ensure that it can cover the liquid crystal display screen of the spatial light modulator.

[0017] 2) Utilizing the characteristic that the frequency-doubled light field is proportional to the product of the two fundamental frequency light fields in a nonlinear process, an optical path is designed and constructed to extend the method of identifying orbital angular momentum using a corner lens to the nonlinear domain: After collimation and beam expansion, the light is incident on a reflective spatial light modulator divided into left and right working regions. A holographic grating for preparing the corner lens is loaded on the left side of the spatial light modulator, and a fork-shaped grating is loaded on the right side. The reflected light from the spatial light modulator passes through a biconvex lens and is then split into two fundamental frequency beams by a beam splitter: one is an infrared vortex beam, and the other uses infrared light to image the corner lens onto a KTP crystal. Specifically, a biconvex lens is added to each of the two fundamental frequency beams, forming a 4f system with the biconvex lens before the beam splitter. Then, collinear mixing is performed by the polarization beam splitter, and the optical path is adjusted so that the image planes of the two 4f systems coincide, placing the KTP crystal at the image plane. In the transmission path of the beam splitter, an aperture is used to filter the first-order diffracted light of the corner lens grating in the Fourier plane of the 4f system, resulting in a fundamental frequency beam in the KTP crystal. Where g(r) is the pupil function of the corner lens; in the reflection path of the beam-splitting cubic system, an aperture is used to filter the first-order diffracted light of the fork-shaped grating in the Fourier plane of the 4f system, and another fundamental frequency light is obtained in the KTP crystal.

[0018] 3) Frequency doubling diffraction field detection: The nonlinear process of generating second harmonics in the KTP crystal results in a frequency doubling light field that is proportional to the two fundamental frequency light fields, i.e. Where β is a constant; at this point, the infrared light has been converted into visible light, which can be directly detected by CCD after filtering, and the frequency-doubled light field form is the same as that of vortex light directly incident on the corner lens. That is, while converting infrared light into visible light using a nonlinear process, orbital angular momentum identification is achieved using a corner lens. While taking advantage of the strong penetrating power and low photon energy of infrared light, the problem of low detection efficiency and high cost of infrared light, which limits the application of infrared vortex light, is also avoided.

[0019] 4) Qualitatively verify the effectiveness of the method: Keep the positions of each component in the optical path fixed, change the parameters of the fork-shaped grating loaded on the spatial light modulator to generate vortex light carrying different orbital angular momentum; record the diffraction pattern on the CCD and observe whether the focusing peak is located at different angular positions;

[0020] 5) Quantitatively verify the effectiveness of the method: Determine the centroid position of the focusing pattern for different vortex light incidents, and calculate the angular position of the centroid of the diffraction pattern obtained by the incident beam with zero orbital angular momentum relative to the orbital angular momentum of each diffraction pattern centroid.

[0021] Compared with the prior art, the main advantages of the present invention are:

[0022] (1) The present invention is novel in technology and has a simple and compact device. When infrared vortex light carrying orbital angular momentum is incident on a KTP crystal with a corner lens imaging, it can be converted into visible light focused at different azimuth coordinates, and can be directly detected by CCD to realize infrared orbital angular momentum identification.

[0023] (2) This invention is highly practical. It converts the infrared light to be detected into visible light through nonlinear frequency upconversion and then uses CCD for detection, thus avoiding the disadvantages of low efficiency and high cost of infrared light detection.

[0024] (3) This invention is forward-looking and has considerable application prospects in fields such as special molecular spectroscopy, biological imaging and infrared monitoring. Attached Figure Description

[0025] Figure 1 This is a structural diagram of an infrared orbital angular momentum pattern recognition device based on nonlinear optics and corner lenses. The components in the diagram are labeled as follows: 1-Laser, 2-Telescope system, 3-Reflective spatial light modulator, 4-First biconvex lens, 5-First reflector, 6-Beam splitter cube, 7-First aperture, 8-Second reflector, 9-Second biconvex lens, 10-Third reflector, 11-Second aperture, 12-Half-wave plate, 13-Third biconvex lens, 14-Polarization beam splitter cube, 15-KTP crystal, 16-Filter, 17-CCD camera, 18-Laptop.

[0026] Figure 2 This is a grating pattern loaded on the spatial light modulator in an embodiment of the present invention. The left side is the grating that generates the corner lens, and the right side is the fork-shaped grating.

[0027] Figure 3 The image shows the diffraction results obtained on a CCD as the infrared orbital angular momentum l of path one in this embodiment of the invention changes from -4 to 4, with the focused peak marked by a circle.

[0028] Figure 4 These are nine diffraction patterns obtained by varying the path-orbit angular momentum l from -4 to 4 in an embodiment of the present invention (i.e., Figure 3 The light intensity centroid of the sample is fitted to a circle, and then translated until the center of the fitted circle is located at (0,0). (See figure.) The solid line represents the position of the centroid of the light intensity in the diffraction patterns obtained from different orbital angular momentum modes, and the dot represents the center of the fitted circle.

[0029] Figure 5This is an experimental result of the relative orientation coordinates of the centroid of the diffraction pattern (relative to the orientation coordinates of the centroid of the diffraction pattern when l=0) in an embodiment of the present invention. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of this invention clearer, the following embodiments will be used to further illustrate this invention in conjunction with the accompanying drawings.

[0031] This invention provides an infrared orbital angular momentum pattern recognition device based on nonlinear optics and corner lenses, comprising a laser, a telescope system, a spatial light modulator, a biconvex lens, a mirror, a beam splitter cube, an aperture, a half-wave plate, a polarizing beam splitter cube, a filter, a KTP crystal, and a CCD camera.

[0032] The telescope system and the spatial light modulator are coaxially and sequentially placed in the output optical path of the laser beam. The reflected light from the spatial light modulator passes through a biconvex lens and is then split into two fundamental frequency beams by a beam splitter. One beam is the infrared vortex light to be detected, and the other is used to prepare the phase structure of a conventional corner lens. A biconvex lens is added to each of the two fundamental frequency beams, forming a 4f system with the biconvex lens before the beam splitter. The 4f imaging system of the two fundamental frequency beams consists of a biconvex lens, a beam splitter, and a polarization beam splitter. The cubes are placed coaxially; and the image planes of the two 4f systems coincide. In the transmission path of the beam-splitting cube, an aperture is used to filter the first-order diffraction light of the corner lens grating in the Fourier plane of the 4f system. In the reflection path of the beam-splitting cube, an aperture is used to filter the first-order diffraction light of the fork grating in the Fourier plane of the 4f system. A half-wave plate with the fast axis at a 45° angle to the horizontal is inserted in the reflection path of the beam-splitting cube to convert vertically linearly polarized light into horizontally linearly polarized light. The CCD camera is located behind the filter to detect the intensity distribution of the frequency-doubled light.

[0033] Furthermore, the KTP crystal is a type II KTP crystal.

[0034] This invention provides an infrared orbital angular momentum pattern recognition method based on nonlinear optics and corner lenses, comprising the following steps:

[0035] 1) Provide light source and collimate and expand beam: The vertically polarized infrared light emitted by the laser is used as the laser input of the system. It is collimated and expanded by the telescope system to ensure that it can cover the liquid crystal display screen of the spatial light modulator.

[0036] 2) Utilizing the characteristic that the frequency-doubled light field is proportional to the product of the two fundamental frequency light fields in a nonlinear process, an optical path is designed and constructed to extend the method of identifying orbital angular momentum using a corner lens to the nonlinear domain: After collimation and beam expansion, the light is incident on a reflective spatial light modulator divided into left and right working regions. A holographic grating for preparing the corner lens is loaded on the left side of the spatial light modulator, and a fork-shaped grating is loaded on the right side. The reflected light from the spatial light modulator passes through a biconvex lens and is then split into two fundamental frequency beams by a beam splitter: one is an infrared vortex beam, and the other uses infrared light to image the corner lens onto a KTP crystal. Specifically, a biconvex lens is added to each of the two fundamental frequency beams, forming a 4f system with the biconvex lens before the beam splitter. Then, collinear mixing is performed by the polarization beam splitter, and the optical path is adjusted so that the image planes of the two 4f systems coincide, placing the KTP crystal at the image plane. In the transmission path of the beam splitter, an aperture is used to filter the first-order diffracted light of the corner lens grating in the Fourier plane of the 4f system, resulting in a fundamental frequency beam in the KTP crystal. Where g(r) is the pupil function of the corner lens; in the reflection path of the beam-splitting cubic system, an aperture is used to filter the first-order diffracted light of the fork-shaped grating in the Fourier plane of the 4f system, and another fundamental frequency light is obtained in the KTP crystal.

[0037] 3) Frequency doubling diffraction field detection: The nonlinear process of generating second harmonics in the KTP crystal results in a frequency doubling light field that is proportional to the two fundamental frequency light fields, i.e. Where β is a constant; at this point, the infrared light has been converted into visible light, which can be directly detected by CCD after filtering, and the frequency-doubled light field form is the same as that of vortex light directly incident on the corner lens. That is, while converting infrared light into visible light using a nonlinear process, orbital angular momentum identification is achieved using a corner lens. While taking advantage of the strong penetrating power and low photon energy of infrared light, the problem of low detection efficiency and high cost of infrared light, which limits the application of infrared vortex light, is also avoided.

[0038] 4) Qualitatively verify the effectiveness of the method: Keep the positions of each component in the optical path fixed, change the parameters of the fork-shaped grating loaded on the spatial light modulator to generate vortex light carrying different orbital angular momentum; record the diffraction pattern on the CCD and observe whether the focusing peak is located at different angular positions;

[0039] 5) Quantitatively verify the effectiveness of the method: Determine the centroid position of the focusing pattern for different vortex light incidents, and calculate the angular position of the centroid of the diffraction pattern obtained by the incident beam with zero orbital angular momentum relative to the orbital angular momentum of each diffraction pattern centroid.

[0040] Working principle of the invention:

[0041] Angular momentum and angular position can be related via a Fourier relationship, just like linear momentum and linear position (Yao E, Franke-Arnold S, Courtial J, et al. Fourier relationship between angular position and optical orbital angular momentum[J]. Optics Express, 2006, 14(20): 9071-9076.); the phase distribution of a conventional radial lens is: f is the focal length of the lens. Let λ be the incident wavenumber and λ be the incident wavelength. Incident light with different wave vector modes, after passing through a radial lens, is focused at different spatial positions on the focal plane. Analogous to a radial lens, the phase distribution and azimuth angle of a corner lens can be derived. Having a quadratic relationship, written as Where α is the azimuth quadratic coefficient; the transmission function of the corner lens can be written as:

[0042]

[0043] Where g(r) is the pupil function of the angular lens, r i It is the inner radius of the corner lens, r o It is the outer radius of the corner lens:

[0044]

[0045] If the corner lens is imaged onto the KTP crystal, then... At incidence, under the paraxial approximation and the slowly varying amplitude approximation, the coupled wave equation of the second harmonic process is:

[0046]

[0047] Where ω3 is the frequency of the generated frequency-doubled light field, and d eff is the effective nonlinear coefficient of the crystal, k3 is the wavenumber of the frequency-doubled optical field, and c is the speed of light; in the experiment, the nonlinear conversion efficiency of the crystal is relatively low, so a small-signal approximation is used. If the crystal length is L, let The frequency-doubled light field output by KTP for:

[0048]

[0049] The frequency-doubled light field is proportional to the product of the two fundamental frequencies, and mathematically and formally equivalent to the transmitted light field obtained by directly incident vortex light onto a corner lens. This allows for the generation of second harmonics while simultaneously identifying orbital angular momentum using a corner lens. The frequency-doubled light emitted from the KTP propagates at a distance z in free space, and its field is described by Fresnel diffraction as follows:

[0050]

[0051] This invention leverages the characteristic that frequency-doubled light in nonlinear optics is proportional to the product of two fundamental frequencies under the small-signal approximation and the slowly varying amplitude approximation. By using a fundamental frequency beam to image a corner lens onto a KTP crystal, and then incident another vortex beam onto the KTP crystal, the resulting frequency-doubled light field has the same form as the transmitted light field of the vortex beam directly passing through the corner lens. This allows for the conversion of infrared vortex light carrying different orbital angular momentum into visible light focused at different angular positions.

[0052] In the following embodiments, a 1064nm laser is used as the light source. Before the second harmonic is generated, all components are 1064nm (such as a half-wave plate) or corresponding broadband coated devices containing 1064nm (such as a mirror, a biconvex lens, a beam splitter cube, or a polarization beam splitter cube). The second harmonic phase matching range of the KTP crystal contains 1064nm, and the center wavelength of the filter is 532nm.

[0053] like Figure 1 An infrared orbital angular momentum pattern recognition device based on nonlinear optics and corner lenses. This device uses a 1064nm wavelength laser as the infrared light source. The vertically polarized beam emitted by laser 1 is first collimated and expanded by telescope system 2 before being incident on a computer-controlled reflective spatial light modulator 3. For example... Figure 2As shown, the pixel array of the reflective spatial light modulator 3 is divided into left and right working areas, which are respectively loaded with a fork-shaped grating and a holographic grating for generating a corner lens. The reflected light from the reflective spatial light modulator 3 passes through the first biconvex lens 4 and is reflected by the first reflecting mirror 5 into the beam splitter cube 6, splitting into two fundamental frequency beams. The transmission path of the beam splitter cube is used to prepare infrared light with a conventional corner lens phase structure, denoted as path one; the reflection path of the beam splitter cube is used to generate the infrared vortex to be identified, denoted as path two. In path one, the second biconvex lens 9 and the first biconvex lens 4 before the beam splitter cube 6 form a 4f system, and the first diffracted light of the corner lens grating is filtered by the first aperture 7 and then reflected by the polarization beam splitter cube 14. In path two, the third biconvex lens 13 and the first biconvex lens 4 before the beam splitter cube 6 form a 4f system, and the first diffracted light of the fork-shaped grating is filtered by the second aperture 11. During this process, the incident light is converted from vertically polarized to horizontally polarized by a half-wave plate 12 with its fast axis at a 45° angle to the horizontal, and then transmitted through a polarization beam splitter 14. At this point, the vertically and horizontally polarized light, carrying different phase structures, are combined, and the image planes of the two 4f systems in path one and path two are made to coincide by adjusting the directions of the second and third reflecting mirrors 8 and 10. The combined light is then placed in a KTP crystal 15 on the image plane of the 4f system, generating a second harmonic light field proportional to the product of the two fundamental frequencies. This light field is filtered by a narrowband filter 16 with a center wavelength of 532 nm, and the diffracted light field is recorded by a CCD 17 and displayed on a laptop computer 18.

[0054] The laser 1 can be a Cobolt or Rumba.

[0055] The reflective spatial light modulator 3 can be Hamamatsu, X10468-07.

[0056] The focal length of the first biconvex lens 4 is f1 = 750 mm.

[0057] The focal length of the second biconvex lens 9 is f2 = 100 mm.

[0058] The focal length of the third biconvex lens 13 is f3 = 100mm.

[0059] The fast axis of the half-wave plate 12 forms a +45° angle with the horizontal.

[0060] The KTP crystal 15 is a type II KTP crystal.

[0061] The CCD camera 17 is a Thorlabs DCU224C.

[0062] The focused peaks in the diffraction pattern recorded by the CCD are marked with circles, such as... Figure 3As shown. In the experiment, the azimuth quadratic coefficient α of the corner lens generated by the holographic grating loaded on the spatial light modulator 3 is 4, and the inner radius r of the pupil function is... i = 1.3mm, outer radius r o =1.7mm. With the CCD position fixed, the parameters of the fork-shaped grating in the spatial light modulator were changed, causing the orbital angular momentum of the incident light to increase from -4 to 4 in steps of 1. It was observed that the position of the light intensity focusing peak rotated counterclockwise, qualitatively demonstrating that an angle lens can effectively identify infrared orbital angular momentum modes within a nonlinear optical framework. For quantitative verification, such as... Figure 4 As shown, the centroid coordinates of the light intensity of 9 diffraction patterns obtained from orbital angular momentum from -4 to 4 were calculated, and the least squares method was used to perform circle fitting, and the image was translated to the fitted circle with center coordinates (0,0). Figure 5 This paper demonstrates the orientation coordinates of the intensity centroids of diffraction patterns obtained by incident infrared vortex light carrying different orbital angular momentum, relative to the orientation coordinates of the intensity centroids of diffraction patterns obtained by incident infrared vortex light at l=0. It shows that the effect is similar to that of a direct angle lens in identifying orbital angular momentum; the detected orbital angular momentum mode is proportional to the relative orientation coordinates of the intensity centroids of the diffraction pattern. However, during detection, nonlinear optics converts the infrared light into visible light for direct detection using a CCD. This solves the limitations of infrared vortex applications due to low efficiency, high dark current, and high cost of infrared detectors, and holds promise for applications in specialized molecular spectroscopy, biological imaging, and infrared monitoring.

[0063] This invention images a corner lens onto a KTP crystal, and then directs infrared vortex light carrying the orbital angular momentum to be identified into the KTP crystal. Simultaneously with the generation of second harmonics, the corner lens focuses different orbital angular momentum patterns to different angular positions. In other words, nonlinear optics and the corner lens are used to convert infrared vortices carrying different orbital angular momentum into visible light focused at different azimuth coordinates. Experiments demonstrate the feasibility of this infrared orbital angular momentum pattern recognition method.

[0064] The above embodiments are merely preferred embodiments of the present invention and should not be considered as limiting the scope of the present invention. All equivalent variations and improvements made within the scope of the present invention should still fall within the patent coverage of the present invention.

Claims

1. An infrared orbital angular momentum mode recognition device based on nonlinear optics and angular lens, characterized in that It is equipped with a laser, a telescope system, a spatial light modulator, a biconvex lens, a mirror, a beam splitter cube, an aperture, a half-wave plate, a polarizing beam splitter cube, a KTP crystal, a filter, and a CCD camera. The telescope system and the spatial light modulator are coaxially and sequentially placed in the output optical path of the laser beam. The reflected light from the spatial light modulator, after passing through a biconvex lens, is split into two fundamental frequency beams by a beam splitter: one is infrared light carrying the phase structure of a conventional corner lens, and the other is infrared vortex light to be identified. One beam is sequentially equipped with an aperture and a biconvex lens. The biconvex lens, together with the biconvex lens before the beam splitter, forms a 4f system. The aperture is placed in the Fourier plane of this 4f system to filter out the first-order diffraction light carrying the phase structure of the corner lens. Similarly... Another path is equipped with an aperture stop and a biconvex lens in sequence. The biconvex lens and the biconvex lens before the beam splitter form a 4f system. The aperture stop is placed in the Fourier plane of this 4f system to filter out the first-order diffraction light of the fork-shaped grating to obtain the infrared vortex to be identified. The image planes of the two 4f systems coincide. In the transmission path of the beam splitter, the aperture stop is used to filter the first-order diffraction light of the corner lens grating in the Fourier plane of the 4f system. In the reflection path of the beam splitter, the aperture stop is used to filter the first-order diffraction light of the fork-shaped grating in the Fourier plane of the 4f system. The reflection path of the beam splitter is equipped with a half-wave plate. The CCD camera is located behind the filter and is used to detect the intensity distribution of the frequency-doubled light; the beam splitter is used to split the optical path into two fundamental frequency beams, one beam carrying infrared light carrying phase information of a conventional corner lens, and the other beam being detected as an infrared vortex beam; the polarization beam splitter is used to combine the two fundamental frequency beams.

2. The infrared orbital angular momentum pattern recognition device based on nonlinear optics and corner lenses as described in claim 1, characterized in that... A half-wave plate with a fast axis at a 45° angle to the horizontal is inserted into the reflection path of the beam splitter cube using KTP. The half-wave plate is used to convert vertically polarized light into horizontally polarized light.

3. The infrared orbital angular momentum pattern recognition device based on nonlinear optics and corner lenses as described in claim 1, characterized in that... The infrared vortex light to be identified is generated through a spiral phase plate and a metasurface.

4. The infrared orbital angular momentum pattern recognition device based on nonlinear optics and corner lenses as described in claim 1, characterized in that... The biconvex lens is arranged so that the phase planes of the 4f system in the two fundamental frequency beams coincide.

5. A method for infrared orbital angular momentum pattern recognition based on nonlinear optics and corner lenses, characterized in that... The method using the infrared orbital angular momentum pattern recognition device based on nonlinear optics and corner lenses as described in claim 1 includes the following steps: 1) Provide light source and collimate and expand beam: The vertically polarized infrared light emitted by the laser is used as the laser input of the system. It is collimated and expanded by the telescope system to ensure that it can cover the liquid crystal display screen of the spatial light modulator. 2) Utilizing the characteristic that the frequency-doubled light field is proportional to the product of the two fundamental frequency light fields in a nonlinear process, an optical path is designed and constructed to extend the method of identifying orbital angular momentum using a corner lens to the nonlinear domain: After collimation and beam expansion, the light is incident on a reflective spatial light modulator divided into left and right working regions. A holographic grating for preparing the corner lens is loaded on the left side of the spatial light modulator, and a fork-shaped grating is loaded on the right side. The reflected light from the spatial light modulator passes through a biconvex lens and is then split into two fundamental frequency beams by a beam splitter: one is an infrared vortex beam, and the other uses infrared light to image the corner lens onto a KTP crystal. Specifically, a biconvex lens is added to each of the two fundamental frequency beams, forming a 4f system with the biconvex lens before the beam splitter. Then, collinear mixing is performed by the polarization beam splitter, and the optical path is adjusted so that the image planes of the two 4f systems coincide, placing the KTP crystal at the image plane. In the transmission path of the beam splitter, an aperture is used to filter the first-order diffracted light of the corner lens grating in the Fourier plane of the 4f system, resulting in a fundamental frequency beam in the KTP crystal. Where g(r) is the pupil function of the corner lens; in the reflection path of the beam-splitting cubic system, an aperture is used to filter the first-order diffracted light of the fork-shaped grating in the Fourier plane of the 4f system, and another fundamental frequency light is obtained in the KTP crystal. 3) Frequency doubling diffraction field detection: The nonlinear process of generating second harmonics in the KTP crystal results in a frequency doubling light field that is proportional to the two fundamental frequency light fields, i.e. Where β is a constant; at this point, the infrared light has been converted into visible light, which is then filtered and directly detected by a CCD. Furthermore, the frequency-doubled light field is identical to that of vortex light directly incident on a corner lens. This means that while converting infrared light into visible light using a nonlinear process, orbital angular momentum identification is achieved using a corner lens. This approach leverages the advantages of infrared light's strong penetrating power and low photon energy while avoiding the limitations imposed by low detection efficiency and high cost of infrared vortex light on its applications. 4) Qualitatively verify the effectiveness of the method: Keep the positions of each component in the optical path fixed, change the parameters of the fork-shaped grating loaded on the spatial light modulator to generate vortex light carrying different orbital angular momentum; record the diffraction pattern on the CCD and observe whether the focusing peak is located at different angular positions; 5) Quantitatively verify the effectiveness of the method: Determine the centroid position of the focusing pattern for different vortex light incidents, and calculate the angular position of the centroid of the diffraction pattern obtained by the incident beam with zero orbital angular momentum relative to the orbital angular momentum of each diffraction pattern centroid.

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