A system and method for separating and detecting OAM modes of a composite vortex beam

Abstract

The present application relates to vortex light orbital angular momentum multiplexing communication and optical wavefront modulation technical field, in the prior art detection technology, the signal light transmitted in the strong atmospheric turbulence environment is detected by using the coordinate transformation method, the OAM state range is small, the OAM separation and detection precision is not high when the wide range composite OAM and the medium strong turbulence environment are transmitted for a long distance, the present application provides a kind of separation and detection system and method of composite vortex light beam OAM state, based on turbulence phase compensation and beam geometric transformation, for separating and detecting OAM state multiplexing signal light under atmospheric disturbance, the emission end generates vortex light beam, one beam is used as test light, for predicting turbulence phase by GS phase recovery algorithm;Another beam is OAM state multiplexing signal light, after signal light is transmitted through atmospheric channel, it is incident on SLM loaded with turbulence compensation phase, then the OAM state carried by signal light is detected by detector again;Meanwhile, the detection device of receiving end is simplified in the present application.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical fields of optical orbital angular momentum multiplexing communication and optical wavefront modulation, and more particularly to a system and method for separating and detecting OAM states of composite vortex beams. BACKGROUND

[0002] With the emergence of emerging fields such as massive data transmission, cloud computing, artificial intelligence, high capacity and high speed are the inevitable trend of future communication industry development. Vortex beams carrying OAM appear in a new multiplexing mode, which can greatly increase the system transmission capacity and spectral efficiency without increasing the bandwidth of the communication system.

[0003] At present, from the characteristics of vortex beams, researchers have proposed many different OAM detection schemes, mainly including base mode conversion, mode conversion, interferometer interference, mode classification, aperture diffraction and other methods. These methods can only detect single or two OAM state composite beams. In recent years, with the rapid development of artificial intelligence, machine learning has been applied to many fields such as optical communication. This method has the advantages of fast recognition speed and high accuracy, but for OAM detection, it can only identify the same number of topological charge, and the absolute value of the different number of topological charge and the multi-beam random combination composite light OAM cannot be identified.

[0004] The method of geometric coordinate transformation can convert vortex beams carrying different OAM modes into different horizontal position spot distributions, and each spot position can be detected as an independent information channel. In theory, it can separate and detect light beams carrying any random complex OAM mode. In 2010, Gregorius team first proposed applying the optical geometric transformation method in optical image processing to the separation of OAM. This method can separate the OAM mode of any multiple random combination of complex light, but it has unavoidable defects. Due to the overlapping of light spots, it cannot distinguish two adjacent topological charge beams, which is determined by the mathematical transformation property. In 2013, Mohammad Mirhosseini team designed a fan-out element to copy the light beam to improve the resolution of the OAM mode, greatly improving the recognition rate and reducing the signal-to-noise ratio (SNR) in communication. In the same year, Martin team combined an optical amplifier to improve the performance of the coordinate transformation detector, expanding the effective topological charge detection range of the detector. The OAM mode and detection range can reach -27 to +27. Because the fan-out element is relatively complex to manufacture, in 2018, Yuanhui Wen team proposed a more easily manufactured Archimedes spiral transformation method to detect OAM, which essentially solves the problem of adjacent OAM mode recognition. However, in existing research, the Archimedes spiral transformation method is not as effective as the fan-out element in separating high topological charge vortex beams, and the light spot diffusion phenomenon is more serious. In 2022, Jie Cheng team integrated the above-mentioned coordinate transformation schemes and proposed a generalized spiral transformation scheme that can be flexibly switched between different geometric transformation schemes by simply changing parameters. However, these detection methods are carried out in a vacuum environment.

[0005] Free space optical communication uses the atmosphere as the transmission medium. When vortex light carrying information is transmitted in the atmospheric channel, it will inevitably be disturbed by atmospheric turbulence, causing the light intensity to fluctuate randomly, the spiral phase to distort, and the orbital angular momentum mode to interfere. It is difficult to distinguish between different OAM modes, especially when the turbulence intensity is high and the separation and detection of a large range of complex OAM modes. In the published research, the method of using coordinate transformation to detect OAM mode range is small when the signal light is transmitted in a strong atmospheric turbulence environment, with a maximum of ±5.

[0006] In summary, the existing technology has the problem of low accuracy in OAM separation and detection when a large range of complex OAM and medium-intensity turbulence environment are transmitted over a long distance. SUMMARY

[0007] The present application aims at the deficiencies of the prior art optical OAM state separation and detection technology, and provides a compact composite vortex light topological charge separation and detection system and method based on turbulence phase compensation and beam geometric transformation, which has higher OAM state recognition accuracy and range in spatial optical communication under the influence of atmospheric turbulence.

[0008] To achieve the above object, the present application provides the following technical scheme.

[0009] A composite vortex light OAM state separation and detection system based on turbulence phase compensation and beam geometric transformation, comprising a test light emitting end and a receiving end for turbulence phase compensation of signal light and separation and detection of OAM states, wherein:

[0010] The test light of the emitting end sequentially passes through a first polarization beam splitter, atmospheric turbulence transmission, a second polarization beam splitter, a reflecting mirror and a first CCD to a computer for turbulence prediction; the signal light sequentially passes through the first polarization beam splitter of the emitting end, atmospheric turbulence transmission, the second polarization beam splitter and the receiving end to the computer for image recognition.

[0011] Further, the receiving end comprises a spatial light modulator and an OAM state detector based on geometric coordinate transformation, and the OAM state detector comprises a geometric transformation phase plate, a correction phase plate, a Fourier lens and a second CCD connected in sequence.

[0012] Further, the composite phase loaded on the spatial light modulator SLM is represented as:

[0013]

[0014]

[0015] In the formula, φ (x1, y1) is the phase distribution of the spatial light modulator SLM (21), φ (x2, y2) is the phase distribution of the geometric transformation phase plate (22), φ (x3, y3) is the phase distribution of the correction phase plate (23), φ (x4, y4) is the phase distribution of the Fourier lens (24), and φ (x5, y5) is the phase distribution of the second CCD (25). is the expansion phase distribution of the expansion mirror, x1, y1 are the coordinates of the plane where the spatial light modulator SLM (21) is located, γ represents the magnification of the expansion mirror, and f is the distance between the SLM and the geometric transformation phase plate (22).

[0016] Further, the composite phase on the geometric transformation phase plate is the collimating phase of the collimating mirror plus the transformation and replication phase, and is represented as:

[0017]

[0018] wherein is the phase of the collimating mirror, x2, y2 are the coordinates of the plane in which the geometric transformation phase plate 22 is located;

[0019]

[0020] Ψ2(x2, y2) is the transformation and replication phase:

[0021]

[0022] wherein N is the replication multiple of the fan-out element, a = d / 2π is the lateral adjustment parameter, d represents the length of the rectangular light beam generated after the action of the geometric transformation phase plate (22), c m = (2m-N-1)π is the longitudinal adjustment parameter, and the rectangular light beam replicated by the parameter c m is adjusted to realize the longitudinal connection of the light beam.

[0023] A method for separating and detecting OAM states of a composite vortex light beam based on turbulence phase compensation and light beam geometric transformation, based on the above-mentioned separation and detection system of OAM states of a composite vortex light beam, at the receiving end, the turbulence phase is predicted by using the test light intensity information, and the light field distribution of the signal light is improved based on the turbulence phase, the beam quality of the signal light is improved, and the characteristics of the signal light are more obvious, and then the signal light after phase compensation is separated and detected by the geometric coordinate transformation method; The specific steps are as follows:

[0024] Step 1. Predicting atmospheric turbulence phase: generating a test light with topological charge number l = 1 at the transmitting end, after atmospheric transmission, the first CCD receives the light intensity distribution of the test light affected by turbulence disturbance, the light intensity images of the original test light and the phase distorted test light are input into the computer, and the predicted pre-compensation atmospheric turbulence phase Φ(x1, y1) is obtained based on the phase inversion algorithm of GS algorithm;

[0025] Step 2. Compensate the distortion phase of vortex light beam: take the inverse of the pre-compensation atmospheric turbulence phase Φ(x1, y1) obtained in step 1 as the compensation phase and load it into the spatial light modulator SLM, the transmitting end simultaneously generates another composite vortex light beam as signal light, which is transmitted to the receiving end through atmospheric turbulence, and the spatial light modulator SLM compensates the light field disturbance of the signal light caused by atmospheric turbulence, and improves the quality of the signal light;

[0026] Step 3. OAM state separation and detection based on geometric coordinate transformation: the phase-compensated signal light obtained in step 2 is incident on an OAM state detector, and the light beams carrying different OAM states are imaged as light spots at different spatial positions on the receiving screen of the second CCD; by detecting the light intensity in different regions of the second CCD, the OAM state carried by the signal light is determined.

[0027] Further, the transmitting end generates two vortex beams POV, one of which, POV1, has a topological charge of 1 and serves as test light, and the other, POV2, is a signal light loaded with multiplexed OAM; the two light beams are combined into one by a first polarization beam splitter PBS and transmitted in a turbulent channel; the test light reflected by the first polarization beam splitter PBS and the signal light transmitted by the first polarization beam splitter PBS have different polarization states.

[0028] Further, the test light is separated by a second polarization beam splitter after transmission in the atmospheric space and is incident on a first CCD; the first CCD receives the light intensity information of the test light disturbed by turbulence and imports it into a computer; the GS algorithm is used to calculate the turbulence compensation phase and load it on a spatial light modulator SLM.

[0029] Further, the composite phase hologram loaded on the spatial light modulator SLM is a turbulence prediction phase and a beam expander phase, which compensates for the turbulence of the signal light field distribution disturbed by turbulence and changes the divergence angle of the signal light, thereby realizing the amplification of the beam radius.

[0030] Further, the phase on the geometric transformation phase plate is the composite phase of the collimating mirror and the transformation and replication phase plate, which realizes the recovery of the transmission divergence angle of the light beam and cooperates with the phase of the beam expander to realize the γ times amplification of the light beam; at the same time, the light field distribution is subjected to logarithmic coordinate-Cartesian coordinate geometric transformation, so that the spiral wavefront phase exp(ilθ) in the vortex beam POV is converted into a wavefront phase exp(ilx / β) with a tilt gradient, wherein x is the Cartesian coordinate and β is the scaling parameter of the rectangular light.

[0031] Further, the second CCD receives the light beam spots separated by the Fourier lens and imaged at different horizontal positions on the back focal plane, and imports the light spot distribution of the light beam into a computer; according to the energy distribution detected at the corresponding position, it is determined whether the signal light contains the OAM state signal corresponding to the position.

[0032] In summary, the application has the following beneficial effects:

[0033] The OAM mode detection device with phase correction function in the application is combined by three phase plates and a Fourier lens, has simple structure and is easy to realize, simplifies the design of the detection on the receiving end by using the GS algorithm and geometric coordinate transformation, separates and detects the multi-OAM mode randomly multiplexed signal light transmitted in the atmospheric turbulence, can greatly reduce the disturbance influence of the atmospheric turbulence on the information light beam, has high detection rate and high accuracy in the OAM mode detection of the signal light, has simple device structure, can separate and detect the OAM mode in the composite signal light at the same time, effectively improves the performance of the spatial optical communication system and simplifies the device structure of the receiving end. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 is the working system diagram of the application in the vortex light spatial communication system;

[0035] Figure 2 is Figure 1 the structure schematic diagram of the OAM mode detection device in the application, the compact structure working principle diagram of the OAM separation and detection system of the phase distortion POV light beam, wherein different colors represent different OAM mode signal lights, and the ellipsis represents other arbitrary OAM mode;

[0036] Figure 3 is the light spot diagram of the imaging of the two different OAM mode POV composite signal light on the second CCD of the receiving device under the medium turbulence intensity;

[0037] Figure 4 is the light spot diagram of the detection of the multi-OAM mode multiplexed POV by the application under the medium turbulence intensity;

[0038] Figure 5 is Figure 4 the light intensity distribution diagram of the light spot data in the line position cross section;

[0039] Figure 6 is Figure 4 the energy distribution diagram of the light spot detected on the corresponding topological charge number area;

[0040] Figure 7The relative power of the carried OAM mode light beam is detected at the second CCD of the imaging element after the single OAM mode POV is transmitted 1000 meters in different turbulent environments, and the topological charge number is -15 to 15.

[0041] In the figure: 1-test light; 2-first polarization beam splitter; 3-mirror; 4-first CCD; 5-computer; 6-OAM mode separation device, 7-signal light, 8-second polarization beam splitter, 9-second CCD, 21-space light modulator SLM (compound phase of GS compensation + beam expansion); 22-geometric transformation phase plate (collimation + transformation and replication compound phase); 23-correction phase plate; 24-Fourier lens. DETAILED DESCRIPTION

[0042] The application will be further described in detail below with reference to the accompanying drawings.

[0043] It should be noted that, for the sake of convenience, the following description of the direction is consistent with the direction of the drawings itself, but does not limit the structure of the application.

[0044] As shown in Figures 1 to 7 The application discloses a composite vortex light beam OAM mode separation and detection system based on turbulent phase compensation and light beam geometric transformation, which comprises a transmitting end of test light 1 and a receiving end for turbulent phase compensation of signal light 7 and separation and detection of OAM mode, wherein:

[0045] The test light 1 of the transmitting end is transmitted to the computer for turbulent prediction through the first polarization beam splitter 2, atmospheric turbulent transmission, the second polarization beam splitter 8, the mirror 3 and the first CCD 4 in sequence; the signal light 7 is transmitted to the computer for image recognition through the first polarization beam splitter 2 of the transmitting end, atmospheric turbulent transmission, the second polarization beam splitter 8 and the receiving end in sequence.

[0046] The receiving end comprises a space light modulator 21 and an OAM mode separator based on geometric coordinate transformation, and the OAM mode separator comprises a geometric transformation phase plate 22, a correction phase plate 23, a Fourier lens 24 and a second CCD 4 connected in sequence; the space light modulator 21 performs distortion phase compensation and beam expansion on the signal light 7, collimates the divergence angle of the signal light 7 generated by the beam expansion through the geometric transformation phase plate 22 and reconstructs the light wavefront information, so that the pattern distributed in the polar coordinate system is mapped into the Cartesian coordinate system, the correction phase plate 23 corrects the slope of the light beam at each coordinate point changed by the geometric transformation phase plate 22, so that the wavefront phase of the light beam maintaining the inclined slope continues to transmit along a straight line and then parallelly propagates, and the Fourier lens 24 images the wavefront with different slope angles after reconstruction on different horizontal positions of the second CCD 9.

[0047] The application further discloses a method for separating and detecting OAM states of a composite vortex light beam based on turbulence phase compensation and light beam geometric transformation.

[0048] Step 1. Predicting the atmospheric turbulence phase: a test light with a topological charge of l = 1 is generated at the transmitting end, and after being transmitted through the atmosphere, the first CCD receives the light intensity distribution of the test light disturbed by the turbulence, and the light intensity images of the original test light and the test light with phase distortion are input into a computer, and the predicted pre-compensation atmospheric turbulence phase Φ (x1, y1) is obtained based on the phase inversion algorithm of the GS algorithm.

[0049] The transmitting end generates two vortex beams POV, wherein one of the vortex beams POV1 has a topological charge of 1 and is used as the test light, and the other vortex beam POV2 is a signal light loaded with multiplexed OAM, and the two light beams are combined into one light beam by the first polarization beam splitter PBS and transmitted in the turbulence channel, and the test light reflected by the first polarization beam splitter PBS and the signal light transmitted have different polarization states.

[0050] Step 2. Compensating the distortion phase of the vortex light beam: the pre-compensation atmospheric turbulence phase Φ (x1, y1) obtained in step 1 is taken as the compensation phase and loaded on the first polarization beam splitter, and the transmitting end simultaneously generates another composite vortex beam as the signal light, which is transmitted to the receiving end through the atmospheric turbulence, and the spatial light modulator SLM is used to compensate the light field disturbance of the signal light caused by the atmospheric turbulence, thereby improving the quality of the signal light.

[0051] The test light is separated by the second polarization beam splitter after being transmitted through the atmospheric space and is incident on the first CCD, the first CCD receives the light intensity information of the test light disturbed by the turbulence and inputs the information into the computer, the turbulence compensation phase is calculated by using the GS algorithm and is loaded on the spatial light modulator SLM.

[0052] The composite phase hologram loaded on the spatial light modulator SLM is the composite phase of the turbulence prediction phase and the expansion mirror, and the light field distribution of the signal light disturbed by the turbulence is compensated, and the diffusion angle of the signal light is changed, thereby realizing the amplification of the light beam radius.

[0053] The phase on the geometric transformation phase plate 22 is the composite phase of the collimating mirror and the transformation and replication phase plate, which realizes the recovery of the transmission divergence angle of the light beam, and cooperates with the phase of the beam expander to realize the gamma times amplification of the light beam; meanwhile, the logarithmic coordinate-Descartes coordinate geometric transformation is carried out on the light field distribution, so that the spiral wavefront phase exp(ilθ) in the vortex light beam POV is converted into the wavefront phase exp(ilx / β) with a tilted slope, wherein x is the Cartesian coordinate, and β is the scaling parameter of the rectangular light.

[0054] Step 3. OAM state separation and detection based on geometric coordinate transformation: the signal light obtained in step 2 is incident on the OAM state detector after phase compensation, and the light beams carrying different OAM states are imaged as light spots at different spatial positions on the receiving screen of the second CCD 9, and the OAM state carried by the signal light is judged by detecting the light intensity of different regions on the second CCD 9.

[0055] The second CCD 9 receives the light beam spots separated by the Fourier lens and imaged on the different horizontal positions of the back focal plane, and guides the light spot distribution of the light beam into the computer, and judges whether the OAM state signal corresponding to the position exists in the signal light according to the energy distribution detected at the corresponding position.

[0056] The OAM state separation and detection system of the composite vortex light beam of the application is based on the whole process of vortex light atmospheric space communication, and a compact composite vortex light topological charge rapid separation and detection device based on turbulence phase compensation and beam geometric transformation is designed at the receiving end, so that the phase distortion of the space communication light beam caused by atmospheric turbulence is compensated, and the composite OAM state of the light beam is rapidly separated and detected.

[0057] 1) Transmitting end

[0058] The function of the transmitting end is to generate test light to provide calculation data for the turbulence prediction algorithm. Here, the source field expression of the test light POV is given:

[0059]

[0060] In the formula, r and θ are the radial coordinate and angular coordinate of the source plane, l is the topological charge number of the light beam, l is set to 1, w g is the beam waist of the Bessel Gaussian light beam, w0 represents the ring width of the POV, R is the ring radius of the POV, and w0 and R are controllable parameters.

[0061] 2) Receiving end

[0062] The function of the receiving end is mainly to compensate the turbulence phase of the signal light, and to separate and detect the OAM state of the light beam; the structure thereof is as follows: Figure 2As shown, including spatial light modulator SLM21, geometric transformation phase plate 22, correction phase plate 23, Fourier lens 24 and second CCD 9. Will be in the same plane different functional elements bit by bit, make discrete device structure is simplified to be by three phase plate and a Fourier lens combination of compact composite vortex light OAM state separation and detection device with phase correction function.

[0063] The role of SLM is to distort the phase compensation and beam expansion of signal light, to eliminate the influence of part of atmospheric turbulence disturbance, the light field distribution of the beam is improved, and at the same time the signal light divergence angle becomes larger under the action of expansion phase, the light ring radius increases. The composite phase loaded on SLM1 is the inverse turbulence prediction phase exp[-Φ(x1,y1)] and the expansion phase of expansion mirror, which is expressed as:

[0064]

[0065] Among them The expansion phase distribution of the expansion mirror, x1, y1 is the coordinate of the plane where SLM is located.

[0066]

[0067] In the formula, γ represents the magnification of the expansion mirror, the magnification belongs to adjustable parameter, which is adjusted according to actual needs, f is the distance between SLM1 and geometric transformation phase plate 22.

[0068] The role of geometric transformation phase plate 22 is to collimate the beam divergence angle generated by expansion and reconstruct the wavefront information, so that the pattern distributed in polar coordinate system is mapped to Cartesian coordinate system. For vortex light beam carrying OAM state, the main purpose of reconstructing the wavefront is to convert the spiral wavefront phase exp(ilθ) of the vortex light beam into a wavefront phase exp(ilx / β) with inclined slope through the transformation phase plate, wherein x is the Cartesian coordinate, and β is the scaling parameter of converting the annular vortex light into rectangular light. That is, after the vortex light beam passes through the geometric transformation phase plate 22, the light intensity distribution of the vortex light beam is changed from annular to rectangular, and the composite phase on the geometric transformation phase plate 22 is the collimating phase of the collimating mirror plus the transformation and copying phase, which is expressed as:

[0069]

[0070] Among them The phase of the collimating mirror on the geometric transformation phase plate, x2, y2 is the coordinate of the plane where the geometric transformation phase plate 22 is located.

[0071]

[0072] Ψ2(x2,y2) is the transformation and copying phase:

[0073]

[0074] In the formula, N is the replication multiple of the fan-out element (transforming and replicating the part of the copied beam in the phase plate and the geometric correction phase plate), a=d / 2π is the transverse adjustment parameter, d represents the length of the rectangular beam generated after the action of the geometric transformation phase plate 22, c m =(2m-N-1)π is the longitudinal adjustment parameter, the length of the rectangular beam is adjusted by the parameter c m The length of the rectangular beam is adjusted by the parameter c

[0075] The role of the correction phase plate 23 is to correct the slope of the light beam at each coordinate point changed by the geometric transformation phase plate 22, so that the signal light energy can be transmitted in parallel again, and the phase can be represented as:

[0076]

[0077] Finally, the Fourier lens 24 is used to image the reconstructed wavefront with different tilt angles on different horizontal positions of the second CCD 9. At this time, the signal light carrying different OAM states corresponds to a specific position of the light spot, so the topological charge number of the light beam is determined by the position of the light spot on the receiving screen of the second CCD 9.

[0078] In the embodiment of the application, the atmospheric refractive index structure parameter

[0079] Figure 3 The spot diagram of the two OAM state multiplexed POV detected by the application in the medium turbulence intensity is shown in the figure, and from left to right, there are four groups of multiplexed signal light, and the OAM states are l=1, 2, l=1, 3, l=1, 4 and l=1, 5. As can be seen from the figure, the light beams of different OAM states in each signal light are incident on the corresponding positions of the second CCD in the form of a spot, and the light beams of adjacent OAM states can also be clearly distinguished.

[0080] Figure 4 The spot diagram of the multiple OAM state multiplexed POV detected by the application in the medium turbulence intensity is shown in the figure, and on the left, the topological charge number is composed of five OAM states l=[-3, 1, 5, 9, 13], and on the right, the topological charge number is composed of 21 OAM states l=[-10,..., 8, 9, 10]. As can be seen from the figure, the spot with the topological charge number of 13 is relatively weak, and the other four OAM states can be clearly presented. Figure 6The OAM state carried by the signal light can be directly determined;

[0081] According to the beam propagation theory, the topological charge *l* carried by the signal light is calculated to be linearly related to the imaging position of the light spot on the second CCD9, which can be expressed as: t = λfl / d, that is, the interval between adjacent OAM state light spots is: Δt = λf / d; and the width of each light spot is: w l =2λf / Nd, which is inversely proportional to the length d of the transformed rectangular beam. From the above calculations, it can be seen that when N=1, the width of the beam spot is greater than the interval between beam spots, which causes overlap between adjacent beam spots, making it impossible to distinguish between adjacent OAM modes. When N=2, the width of the beam spot is exactly equal to the interval between beam spots, and there is no overlap between beam spots, allowing them to be clearly distinguished. Therefore, to accurately separate adjacent OAM states, the number of replications of the fan-out element, N≥2, is required.

[0082] For beams with a large topological charge number l, the signal light can only be fully incident on the phase plate if the local tilt angle of the Poynting vector of the beam with radius R in the directional angular direction is much smaller than the angular deviation introduced by the device itself. That is, 1 / kR << R / f, which simplifies to 1 << kR. 2 Only when the beam radius is / f can the present invention effectively separate the incident beam. If the beam radius is too large, it will be incident outside the phase plate, causing detection failure. Therefore, to achieve separation and detection of a wider range of compound OAM states, it is necessary to increase the beam radius R or decrease the distance f between the phase plates. In this embodiment, the beam radius R is increased by increasing the magnification of the beam expander before the fan-out element, thereby expanding the detection range of the detector. The method for determining the beam radius R here also applies to the value of the rectangular beam length d, that is, the larger the d value, the larger the detection range. As can be seen from the previous derivation, increasing the d value will reduce the spacing and width of the imaging spot. This will not affect the recognition between adjacent spots. However, if the spacing and width of the spots are too small, it will affect the sampling accuracy of the spot intensity. The premise for determining the values ​​of the beam radius R and the rectangular beam length d is that the size of the increased beam does not exceed the size of the area of ​​other devices. In the simulation, the effect is better when the d value is set so that the rectangular length occupies 80% of the phase plate 3. The detector of the present invention can detect the topological charge range of -15 to 15 after the signal light has been transmitted for 1000 meters in moderate atmospheric turbulence.

[0083] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A system for separation and detection of OAM states of a composite vortex beam based on the combination of turbulence phase compensation and beam geometric transformation, characterized in that, The test light (1) of the transmitting end and the receiving end for turbulence phase compensation of the signal light (7) and separation and detection of OAM mode are included, wherein: The test light (1) of the transmitting end is transmitted to the computer for turbulence prediction through the first polarization beam splitter (2), atmospheric turbulence transmission, the second polarization beam splitter (8), the mirror (3) and the first CCD (4) in sequence; the signal light (7) is transmitted to the computer for image recognition through the first polarization beam splitter (2) of the transmitting end, atmospheric turbulence transmission, the second polarization beam splitter (8) and the receiving end in sequence. The receiving end includes a spatial light modulator SLM (21) and an OAM mode separation detector based on geometric coordinate transformation, the OAM mode separation detector includes a geometric transformation phase plate (22), a correction phase plate (23), a Fourier lens (24) and a second CCD (9) connected in sequence, the spatial light modulator SLM (21) performs distortion phase compensation and beam expansion on the signal light (7), the divergence angle of the signal light (7) expanded by the beam expansion is collimated through the geometric transformation phase plate (22), and the reconstructed optical wavefront information is obtained, the pattern distributed in the polar coordinate system is mapped into the Cartesian coordinate system, the slope of the light beam at each coordinate point changed by the geometric transformation phase plate (22) is corrected through the correction phase plate (23), the wavefront phase of the light beam maintaining the inclined slope continues to transmit along the straight line and is parallelly propagated again, and the wavefront with different slope angles after reconstruction is imaged on different horizontal positions of the second CCD (9) by the Fourier lens (24). The composite phase loaded on the spatial light modulator SLM (21) is represented as: wherein a beam expanding phase distribution of the beam expander, x 1, y 1 is a coordinate in a plane in which a spatial light modulator SLM (21) is located, The composite phase on the geometric transformation phase plate (22) is the collimating phase of the collimating mirror plus the transformation and replication phase, and is represented as: denotes a magnification of the beam expander, f is a distance between the SLM and a geometric transformation phase plate (22); The turbulence phase is predicted by using the test light intensity information at the receiving end, the light field distribution of the signal light is improved, the beam quality is improved, the characteristics of the signal light are more obvious, and then the phase-compensated signal light is separated and detected by the geometric coordinate transformation method; the specific steps are as follows: wherein is the phase of the collimating mirror, x 2, y 2 is the coordinate of the plane in which the geometric transformation phase plate (22) lies; To transform and copy the phase: where N is the replication factor of the fan-out element, is a lateral adjustment parameter, denotes the length of the rectangular beam generated after the action of the geometrically transformed phase plate (22), is a longitudinal adjustment parameter, by the parameter adjusting each replicated rectangular beam so that the beams achieve longitudinal continuity.

2. A method for separation and detection of OAM states of a composite vortex beam based on the combination of the turbulence phase compensation and the beam geometric transformation, based on the system for separation and detection of OAM states of a composite vortex beam according to claim 1, characterized in that, Step 3. OAM mode separation and detection based on geometric coordinate transformation: the signal light after phase compensation obtained in step 2 is incident on the OAM mode separation detector, the light beams carrying different OAM modes are imaged as light spots at different spatial positions on the receiving screen of the second CCD (9), and the OAM mode carried by the signal light is determined by detecting the light intensity of different regions on the second CCD (9). Step 1. Predicting atmospheric turbulence phase: a test light with topological charge l =1 is generated at the transmitting end, after atmospheric transmission, the first CCD receives the intensity distribution of the test light affected by turbulence disturbance, the intensity images of the original test light and the test light with phase distortion are input into the computer, and the predicted pre-compensated atmospheric turbulence phase is obtained based on the phase retrieval algorithm of GS algorithm ; Step 2. Compensate the aberrant phase of the vortex beam: load the pre-compensation atmospheric turbulence phase obtained in step 1 to the spatial light modulator SLM (21) The inverse is loaded as a compensation phase to the spatial light modulator SLM (21), and the transmitting end simultaneously generates another composite vortex beam as signal light, which is transmitted to the receiving end through atmospheric turbulence. The spatial light modulator SLM compensates the light field disturbance of the signal light caused by atmospheric turbulence, and improves the quality of the signal light. The transmitting end generates two vortex beams POV, one of which is a vortex beam POV1 with a topological charge of 1 as test light, and the other is a vortex beam POV2 with multiplexed OAM as signal light, the two light beams are combined into one beam by the first polarization beam splitter PBS and transmitted in the turbulence channel, and the test light reflected by the first polarization beam splitter PBS and the signal light transmitted have different polarization states.

3. The method of claim 2, wherein, The test light is separated by the second polarization beam splitter after atmospheric space transmission and is incident on the first CCD, the first CCD receives the light intensity information of the test light disturbed by turbulence and introduces it into the computer, and the turbulence compensation phase is calculated by using the GS algorithm and loaded on the spatial light modulator SLM (21).

4. The method of claim 2, wherein the method is based on the separation and detection of OAM states of a composite vortex beam by combining the phase compensation of turbulence and the geometric transformation of the beam. ​ 5. The method of claim 2, wherein, The complex phase hologram loaded by the spatial light modulator (SLM) (21) is turbulence prediction phase and beam expander phase, the light field distribution of the signal light disturbed by turbulence is compensated, the divergence angle of the signal light is changed, and the amplification of the beam radius is realized.

6. The method of claim 2, wherein the method is based on the separation and detection of OAM states of a composite vortex beam with compensation of the phase of turbulence and beam geometry transformation. The phase on the geometric transformation phase plate (22) is a composite phase of a collimating mirror and a transformation and replication phase plate, realizes recovery of a light beam transmission diffusion angle, and cooperates with a phase of a beam expander to realize beam expansion Gamma ; meanwhile, logarithmic coordinate-Cartesian coordinate geometric transformation is performed on the light field distribution, so that a spiral wavefront phase exp ( il theta ) in the vortex light beam POV is converted into a wavefront phase exp ( il x / β ) with a tilted slope, wherein x is a Cartesian coordinate, β is a scaling parameter for conversion into rectangular light.

7. The method of claim 2, wherein the method is based on the separation and detection of OAM states of a composite vortex beam with compensation of the phase of turbulence and beam geometry transformation. The second CCD (9) receives the light spots of the light beams separated by the Fourier lens and imaged on different horizontal positions of the back focal plane, and guides the light spot distribution of the light beams into a computer, and judges whether the OAM mode signal corresponding to the position exists in the signal light according to the energy distribution detected by the corresponding position.

Images