Multiplexing Orbital Angular Momentum Mode Exchange System and Method Based on Controllable Unitary Transformation

Abstract

The present invention discloses a system and method for exchanging multiplexed orbital angular momentum modes based on controllable unitary transformation, comprising: a multiplexed pattern generation module for generating multiple coaxially transmitted multiplexed orbital angular momentum modes; a pattern exchange module for performing mode conversion between the multiple coaxially transmitted multiplexed orbital angular momentum modes; a pattern detection module for detecting the exchanged orbital angular momentum modes and outputting corresponding detection results; the pattern exchange module is arranged after the multiplexed pattern generation module and on the same optical axis as the multiplexed pattern generation module; and the pattern detection module is arranged after the pattern exchange module and on the same optical axis as the mode exchange module. The present invention can realize coaxial independent exchange of multiple input orbital angular momentum modes, and the exchanged modes have high mode purity. In addition, the mode coaxial exchange system has the characteristics of a large number of controllable modes, flexible functions, and independent modulation, and is suitable for all-optical mode information exchange in high-dimensional multiplexed communications.

Description

Technical Field

[0001] The present invention relates to the field of deep learning and information optics technology, and in particular to a multiplexed orbital angular momentum mode exchange system and method based on controllable unitary transformation. Background Art

[0002] As a complete set of orthogonal bases, orbital angular momentum modes can form an infinite-dimensional Hilbert space, providing a new multiplexing dimension for data communications, significantly improving communication capacity and transmission rates. Current research focuses on mode multiplexing and demultiplexing. Due to the lack of effective passive mode modulation devices, coaxial conversion of multiple orbital angular momentum modes has been less studied. This is key to achieving all-optical information processing in orbital angular momentum mode-multiplexed channels.

[0003] The essence of achieving multi-mode coaxial conversion is to control the angular quantum number carried by the target beam. Currently, there are two main known methods for achieving multi-mode coaxial conversion: one is spiral transformation, which uses the spiral phase to quantitatively control the wavefront phase delay of the target beam, which can achieve arbitrary addition and subtraction of the angular quantum number, but the number of convertible modes and the conversion function are limited; the other is spiral gradient phase modulation, which can use grating diffraction to achieve one-to-many mode conversion. However, the achievable conversion relationship is strictly limited by the spiral phase and the number of diffraction orders, and there is a certain amount of resource waste due to the spectral characteristics of the grating. In addition, these methods usually only perform indistinguishable spiral modulation on all input beams, which makes it difficult to independently modulate multiple orbital angular momentum modes. Therefore, how to coaxially and independently modulate multiple input orbital angular momentum modes with greater degrees of freedom is an important problem that needs to be solved in fields such as optical communications.

[0004] Therefore, the existing technology needs to be improved. Summary of the Invention

[0005] The technical problem to be solved by the present invention is that, in response to the defects of the existing technology, the present invention provides a multiplexed orbital angular momentum mode exchange system and method based on controllable unitary transformation to solve the technical problem of low degree of freedom of coaxial independent modulation of existing multi-input orbital angular momentum modes.

[0006] The technical solutions adopted by the present invention to solve the technical problems are as follows:

[0007] In a first aspect, the present invention provides a multiplexed orbital angular momentum mode exchange system based on controllable unitary transformation, comprising:

[0008] A multiplexing pattern generation module, used for generating multiplexed orbital angular momentum patterns for coaxial transmission;

[0009] A mode exchange module, configured to perform mode conversion between the plurality of coaxial transmission multiplexed orbital angular momentum modes;

[0010] and a mode detection module for detecting the orbital angular momentum mode after the exchange and outputting the corresponding detection result;

[0011] The mode exchange module is arranged behind the multiplexing mode generation module and is located on the same optical axis as the multiplexing mode generation module; the mode detection module is arranged behind the mode exchange module and is located on the same optical axis as the mode exchange module.

[0012] In one implementation, the multiplexing mode generation module includes:

[0013] A light source, a beam splitting device, a first single-mode vortex light generating device, a second single-mode vortex light generating device, a first reflecting device, a second reflecting device, a first beam combining device, and a second beam combining device;

[0014] The beam splitting device is arranged behind the light source and is located on the same optical axis as the light source; the first single-mode vortex light generating device and the first reflecting device form a vortex light path; the second single-mode vortex light generating device, the first beam combining device and the second beam combining device form a vortex composite light path; the second reflecting device and the second beam combining device form a composite light path.

[0015] In one implementation, the light source is a fundamental mode laser with a wavelength of 1550 nm; and the beam splitting device is a 1×3 optical coupler with a splitting ratio of 1:1:1.

[0016] In one implementation, the first single-mode vortex light generating device and the second single-mode vortex light generating device are both spiral phase masks, the first reflecting device and the second reflecting device are both plane mirrors, and the first beam combining device and the second beam combining device are both non-polarization combiners.

[0017] In one implementation, the mode switching module includes:

[0018] a first transition phase mask, a second transition phase mask, and a third transition phase mask;

[0019] The first conversion phase mask is provided after the multiplexing mode generation module, and the first conversion phase mask, the second conversion phase mask and the third conversion phase mask are provided in sequence;

[0020] The first transformed phase mask, the second transformed phase mask, and the third transformed phase mask are used to perform a controllable unitary transformation.

[0021] In one implementation, the mode detection module includes:

[0022] A light intensity detector and a computer; the light intensity detector is arranged after the mode exchange module and is used to detect the optical signal after the controllable unitary transformation; the computer is connected to the light intensity detector.

[0023] In a second aspect, the present invention further provides a method for exchanging multiplexed orbital angular momentum modes based on controllable unitary transformation, which is applied to the multiplexed orbital angular momentum mode exchange system based on controllable unitary transformation as described in the first aspect, comprising:

[0024] The Gaussian beam emitted by the light source passes through the beam splitting device to output a first Gaussian sub-beam, a second Gaussian sub-beam and a third Gaussian sub-beam;

[0025] Passing the first Gaussian sub-beam through a first single-mode vortex light generating device to obtain a first vortex beam, and passing the second Gaussian sub-beam through a second single-mode vortex light generating device to obtain a second vortex beam; wherein the first vortex beam and the second vortex beam are vortex beams carrying a single orbital angular momentum mode;

[0026] After the first vortex beam passes through a first reflecting device, the first vortex beam is combined with the second vortex beam in a first beam combining device to form a first multiplexed vortex beam;

[0027] After the third Gaussian sub-beam passes through a second reflecting device, the third Gaussian sub-beam is combined with the first multiplexed vortex beam in a second beam combining device to form a second multiplexed vortex beam;

[0028] After the second multiplexed vortex beam passes through the first conversion phase mask, the second conversion phase mask, and the third conversion phase mask to perform mode conversion, the orbital angular momentum mode required by the target is output;

[0029] The orbital angular momentum pattern required by the target is recorded, detected and analyzed by the pattern detection device, and the detection and analysis results are output.

[0030] In one implementation, the light field distributions of the first vortex beam and the second vortex beam are both:

[0031]

[0032] Where r is the radial component, φ is the angular component, w0 is the beam waist radius, w(z) is the beam waist size at z, is the associated Laguerre polynomial, l is the topological charge that characterizes the orbital angular momentum mode, p is the radial parameter, z r is the Rayleigh distance, z is the beam transmission distance, k is the wave vector, the phase factor exp(-ilφ) indicates that the beam has a spiral structure, and i is the imaginary unit.

[0033] In one implementation, the interconnection between layers in the first conversion phase mask, the second conversion phase mask, and the third conversion phase mask is expressed as:

[0034]

[0035] Among them, F and F -1 They are fast Fourier transform and inverse fast Fourier transform;

[0036]

[0037] Among them, (f x ,f y ) is the spatial frequency of (x, y), λ is the operating wavelength, k=2π / λ, Δz is the interlayer distance, is the output complex-valued distribution.

[0038] In one implementation, the method further includes:

[0039] Multi-level phase modulation based on deep learning algorithms to achieve high mode purity of the converted light field:

[0040]

[0041]

[0042] Among them, T out (x,y) is the true field distribution, E out (x, y) is the output complex-valued distribution, α=1 is the weight parameter, and Relu and Tanh are both activation functions.

[0043] The present invention adopts the above technical solution to achieve the following effects:

[0044] The controllable unitary transformation strategy of the system of the present invention enables coaxial independent exchange of multiple input orbital angular momentum modes, resulting in high mode purity. Compared to traditional mode exchange schemes, the coaxial mode exchange system of the present invention offers a large number of controllable modes, flexible functionality, and independent modulation, making it suitable for all-optical mode information exchange in high-dimensional multiplexing communications. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. 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 the structures shown in these drawings without paying any creative work.

[0046] Figure 1 This is a structural schematic diagram (part one) of a multiplexed orbital angular momentum mode exchange system based on controllable unitary transformation in one implementation of the present invention.

[0047] Figure 2 This is a structural schematic diagram (part 2) of a multiplexed orbital angular momentum mode exchange system based on controllable unitary transformation in one implementation of the present invention.

[0048] Figure 3 It is a flow chart of a multiplexed orbital angular momentum mode exchange method based on controllable unitary transformation in one implementation of the present invention.

[0049] Figure 4 This is a modulation diagram of a multi-level phase modulation screen designed based on deep learning assistance in one implementation of the present invention.

[0050] In the figure: 100, multiplexing mode generation module; 200, mode switching module; 300, mode detection module; 1, light source; 2, beam splitting device; 3, first single-mode vortex light generating device; 4, second single-mode vortex light generating device; 5, first reflecting device; 6, first beam combining device; 7, second beam combining device; 8, second reflecting device; 9, first conversion phase mask; 10, second conversion phase mask; 11, third conversion phase mask; 12, light intensity detector; 13, computer.

[0051] The purpose, features and advantages of the present invention will be further described with reference to the accompanying drawings and in conjunction with the embodiments. DETAILED DESCRIPTION

[0052] In order to make the purpose, technical solutions and advantages of the present invention more clear and distinct, the present invention is further described in detail below with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0053] Exemplary Systems

[0054] Currently, there are two main known methods for achieving multimode coaxial conversion: one is spiral transformation, which uses the spiral phase to quantitatively control the wavefront phase delay of the target beam, allowing arbitrary addition and subtraction of the angular quantum number, but the number of convertible modes and the conversion function are limited; the other is spiral gradient phase modulation, which can achieve one-to-many mode conversion using grating diffraction. However, the achievable conversion relationship is strictly limited by the spiral phase and the number of diffraction orders, and there is a certain amount of resource waste due to the spectral characteristics of the grating. In addition, these methods usually only perform indistinguishable spiral modulation on all input beams, which makes it difficult to independently modulate multiple orbital angular momentum modes.

[0055] To address the above technical issues, this embodiment provides a multiplexed orbital angular momentum mode exchange system based on controllable unitary transformation. The controllable unitary transformation strategy of this system enables coaxial independent exchange of multiple input orbital angular momentum modes, resulting in high mode purity. Compared to traditional mode exchange schemes, the coaxial mode exchange system of this embodiment offers advantages such as a large number of controllable modes, flexible functionality, and independent modulation, making it suitable for all-optical mode information exchange in high-dimensional multiplexed communications.

[0056] like Figure 1 As shown, this embodiment provides a multiplexed orbital angular momentum mode exchange system based on controllable unitary transformation, including:

[0057] A multiplexing pattern generating module 100 is used to generate multiplexed orbital angular momentum patterns for coaxial transmission;

[0058] A mode exchange module 200 is configured to perform mode conversion between the plurality of coaxial transmission multiplexed orbital angular momentum modes;

[0059] and a mode detection module 300 for detecting the orbital angular momentum mode after the exchange and outputting the corresponding detection result;

[0060] The mode exchange module 200 is disposed behind the multiplexing mode generation module 100 and is located on the same optical axis as the multiplexing mode generation module 100 ; the mode detection module 300 is disposed behind the mode exchange module 200 and is located on the same optical axis as the mode exchange module 200 .

[0061] In this embodiment, the multiplexed pattern generation module 100 is a multiplexed orbital angular momentum pattern generation module, the pattern exchange module 200 is an orbital angular momentum pattern exchange module 200 , and the pattern detection module 300 is an orbital angular momentum pattern detection module 300 .

[0062] In the system of this embodiment, the multiplexed pattern generation module 100 generates a plurality of coaxially transmitted vortex beams. After the plurality of coaxially transmitted vortex beams undergo multiple mode conversions in the mode exchange module 200, the exchanged modes are detected and analyzed by the mode detection module 300 to obtain the target orbital angular momentum mode.

[0063] like Figure 2 As shown, in one implementation of this embodiment, the multiplexing mode generation module 100 includes:

[0064] Light source 1, beam splitting device 2, first single-mode vortex light generating device 3, second single-mode vortex light generating device 4, first reflecting device 5, second reflecting device 8, first beam combining device 6 and second beam combining device 7;

[0065] The beam splitting device 2 is arranged behind the light source 1 and is located on the same optical axis as the light source 1; the first single-mode vortex light generating device 3 and the first reflecting device 5 form a vortex light path; the second single-mode vortex light generating device 4, the first beam combining device 6 and the second beam combining device 7 form a vortex composite light path; the second reflecting device 8 and the second beam combining device 7 form a composite light path.

[0066] In this embodiment, the light source 1 is a fundamental mode laser with a wavelength of 1550 nm; the beam splitter 2 is a 1×3 optical coupler with a splitting ratio of 1:1:1. The first single-mode vortex light generating device 3 and the second single-mode vortex light generating device 4 are both spiral phase masks; the first reflector 5 and the second reflector 8 are both plane mirrors; and the first beam combiner 6 and the second beam combiner 7 are both non-polarization combiners. The multiplexed pattern generation module 100 in this embodiment is not limited to these devices and can be replaced by devices with corresponding functions.

[0067] The optical path principle of the multiplexing pattern generation module 100 in this embodiment is:

[0068] A Gaussian beam is generated by a light source 1. After passing through a beam splitter 2, the Gaussian beam forms three sub-Gaussian beams with balanced energy. Two of the three sub-Gaussian beams pass through a first single-mode vortex light generating device 3 and a second single-mode vortex light generating device 4, respectively, to generate corresponding vortex beams carrying a single orbital angular momentum mode. Among the two vortex beams, one of the vortex beams passes through a first reflecting device 5 and then merges with the other vortex beam in a first beam combining device 6 to form two multiplexed vortex beams. After being reflected by a second reflecting device 8, the third sub-Gaussian beam merges with the two multiplexed vortex beams in a second beam combining device 7 to form three multiplexed vortex beams. The Gaussian beam of the multiplexing pattern generating module 100 in this embodiment can be regarded as a vortex beam carrying a zero orbital angular momentum mode.

[0069] like Figure 2 As shown, in one implementation of this embodiment, the mode switching module 200 includes:

[0070] a first conversion phase mask 9, a second conversion phase mask 10, and a third conversion phase mask 11;

[0071] The first conversion phase mask 9 is provided after the multiplexing pattern generation module 100, and the first conversion phase mask 9, the second conversion phase mask 10 and the third conversion phase mask 11 are provided in sequence;

[0072] The first conversion phase mask 9, the second conversion phase mask 10 and the third conversion phase mask 11 are used to perform controllable unitary transformation and can be implemented by loading multiple phase-type spatial light modulators, but are not limited to these devices and can be replaced by devices with corresponding functions.

[0073] like Figure 2 As shown, in one implementation of this embodiment, the mode detection module 300 includes:

[0074] A light intensity detector 12 and a computer 13; the light intensity detector 12 is arranged after the mode switching module 200 and is used to detect the optical signal after the controllable unitary transformation; the computer 13 is connected to the light intensity detector 12; the light intensity detector 12 in this embodiment is a 1550nm light intensity detector, but is not limited to these devices and can be replaced by devices with corresponding functions.

[0075] The principles of the mode exchange module 200 and the mode detection module 300 in this embodiment are as follows:

[0076] After the three multiplexed vortex light beams undergo mode conversion through the first conversion phase mask 9, the second conversion phase mask 10, and the third conversion phase mask 11, the orbital angular momentum pattern required by the target is output; the orbital angular momentum pattern required by the target is detected by the light intensity detector 12, and is recorded by the computer 13 and subjected to pattern detection and analysis.

[0077] The multiplexed orbital angular momentum mode exchange system based on controllable unitary transformation in this embodiment can implement a controllable unitary transformation strategy in the mode exchange module 200, thereby achieving coaxial independent exchange of multiple input orbital angular momentum modes and obtaining a target mode with high mode purity.

[0078] This embodiment achieves the following technical effects through the above technical solution:

[0079] The controllable unitary transformation strategy of this embodiment enables coaxial independent exchange of multiple input orbital angular momentum modes, resulting in high mode purity. Compared to traditional mode exchange schemes, this embodiment's coaxial mode exchange system offers a large number of controllable modes, flexible functionality, and independent modulation, making it suitable for all-optical mode information exchange in high-dimensional multiplexing communications.

[0080] Exemplary Methods

[0081] like Figure 3 As shown, an embodiment of the present invention provides a multiplexed orbital angular momentum mode exchange method based on controllable unitary transformation, which is applied to the multiplexed orbital angular momentum mode exchange system based on controllable unitary transformation as described in the above embodiment, including the following steps:

[0082] Step S100, a Gaussian beam emitted by a light source is passed through a beam splitter to output a first Gaussian sub-beam, a second Gaussian sub-beam, and a third Gaussian sub-beam;

[0083] Step S200: passing the first Gaussian sub-beam through a first single-mode vortex light generating device to obtain a first vortex beam, and passing the second Gaussian sub-beam through a second single-mode vortex light generating device to obtain a second vortex beam; wherein the first vortex beam and the second vortex beam are both vortex beams carrying a single orbital angular momentum mode;

[0084] Step S300, after the first vortex beam passes through a first reflecting device, the first vortex beam is combined with the second vortex beam in a first beam combining device to form a first multiplexed vortex beam;

[0085] Step S400, after the third Gaussian sub-beam passes through a second reflecting device, the third Gaussian sub-beam is combined with the first multiplexed vortex beam in a second beam combining device to form a second multiplexed vortex beam;

[0086] Step S500, after the second multiplexed vortex beam passes through a first conversion phase mask, a second conversion phase mask, and a third conversion phase mask to perform mode conversion, outputting an orbital angular momentum mode required by the target;

[0087] Step S600: Record, detect and analyze the orbital angular momentum pattern required by the target through a pattern detection device, and output the detection and analysis results.

[0088] like Figure 2 As shown, in this embodiment, fundamental mode Gaussian light is emitted from a 1550nm laser (i.e., light source 1), and after passing through a 1×3 optical coupler with a splitting ratio of 1:1:1 (i.e., beam splitting device 2), three sub-Gaussian beams with balanced energy are generated. Two of the three sub-Gaussian beams are respectively passed through two spiral phase masks (i.e., first single-mode vortex light generating device 3 and second single-mode vortex light generating device 4), respectively generating vortex beams carrying a single orbital angular momentum mode. At this time, the light field distribution of the two vortex beams (i.e., the first vortex beam and the second vortex beam) can be approximated by the Laguerre-Gaussian beam expression:

[0089]

[0090] Where r is the radial component, φ is the angular component, w0 is the beam waist radius, w(z) is the beam waist size at z, is the associated Laguerre polynomial, l is the topological charge that characterizes the orbital angular momentum mode, p is the radial parameter, z r is the Rayleigh distance, z is the beam transmission distance, k is the wave vector, the phase factor exp(-ilφ) indicates that the beam has a spiral structure, and i is the imaginary unit.

[0091] Among the two vortex light beams generated, one of them passes through a plane reflector (i.e., the first reflecting device 5) and is combined with the other vortex light beam by a non-polarizing beam splitter (i.e., the first beam combining device 6) to form two multiplexed vortex light beams, and its light field distribution is the superposition of the light fields of the two vortex light beams.

[0092] After the third sub-Gaussian beam passes through the beam plane mirror (i.e., the second reflecting device 8), it and the two multiplexed vortex beams are combined by a non-polarization beam splitter (i.e., the second beam combining device 7) to form three multiplexed vortex beams. The light field distribution is the superposition of the light fields of the three vortex beams, among which the Gaussian beam can be regarded as a vortex beam carrying a zero orbital angular momentum mode.

[0093] The three multiplexed vortex beams undergo mode conversion through a multi-stage phase modulation structure (i.e., the first conversion phase mask 9, the second conversion phase mask 10, and the third conversion phase mask 11) to output the orbital angular momentum mode required by the target.

[0094] Assuming the three input patterns are [-1, 0, 1] and the target output pattern is [0, 1, -1], its coaxial independent conversion mechanism can be expressed by unitary transformation as follows:

[0095]

[0096] In order to achieve this unitary transformation, a three-layer multi-level phase modulation structure is constructed in this embodiment, and deep learning technology is introduced to assist in solving the entire unitary matrix. Through the alternating modulation of the phase screen and light propagation, the unitary matrix can be obtained:

[0097] X1=H·f3·H·f2·H·f1·H,

[0098] Where H is the optical transmission matrix with a transmission distance d in free space, f i Indicates L i The modulation phase of the layer.

[0099] For the same input pattern, the unitary matrix can not only trigger independent transformations corresponding to different modes, but also develop other excitation modulation functions by adjusting its own structural parameters. This adjustment includes spatial rotation, position shift, etc., which provides a new operational dimension for the unitary transformation of the mode. Assuming that a spatial rotation modulation mechanism is introduced in the third layer phase screen of the unitary matrix, this adjustment can transform the previous unitary transformation into:

[0100] X2=H·f3'·H·f2·H·f1·H.

[0101] The phase modulation f3' is defined by the rotation matrix:

[0102]

[0103] Where a is the rotation angle.

[0104] Multi-level modulation achieves the conversion of multi-mode [-1, 0, 1] to [0, 1, -1] when rotated 0°. When the third phase screen is rotated 90° counterclockwise, this adjustment can trigger another mode conversion function, that is, the conversion of multi-mode [-1, 0, 1] to [1, -1, 0]. The unitary transformation at this time can be expressed as:

[0105]

[0106] As mentioned above, for the known unitary transformation based on multi-level phase modulation, its modulation of the target orbital angular momentum mode can be simply expressed as:

[0107]

[0108] The above formula can be simply converted to:

[0109]

[0110] Then, the above formula is modulated and multiplied on the left by a new unitary matrix, which is obtained by transposing the first and third modulation phase screens of the original unitary matrix. In this case, the following conversion relationship can be obtained:

[0111]

[0112] Therefore, the new unitary transformation matrix is expressed as:

[0113] X3=H·f1·H·f2·H·f3·H

[0114] The above derivation shows that in the constructed orbital angular momentum mode unitary transformation system, the conversion of the conjugate mode can be achieved by replacing the first and third phase screens in space. This conversion is an approximate inverse modulation of the original unitary transformation. For example:

[0115]

[0116] In order to distinguish these two transformations, the original transformation function is called the forward transformation, and the modulation function after spatial position transformation is called the reverse transformation.

[0117] After the orbital angular momentum pattern required by the target is controllably unitary transformed by the multi-level phase modulation structure, it is recorded, detected and analyzed by the light intensity detector 12 and the electronic computer 13.

[0118] like Figure 4 As shown, Figure 4 This is a schematic diagram of the principle of deep learning-assisted design of a multi-level phase modulation screen in the present invention.

[0119] The diffraction unit in the multi-level phase modulation structure is a learnable parameter that can be automatically adjusted in an iterative process through a computer-based deep learning algorithm. Based on the traditional fully connected network structure, this embodiment introduces the wave propagation and modulation physical mechanism into the neural network and constructs a multi-level phase modulation structure in the TensorFlow framework. Each complex-valued input field is combined with the diffraction unit as a characteristic parameter, and the calculated output is a complex-valued field with an additional phase delay. The principle of solving the phase distribution in the diffraction layer of the multi-level phase modulation structure is as follows: Figure 4 As shown, the following steps are included:

[0120] Step S11, inputting a complex-valued light field: complex amplitude distributions of a given input plurality of orbital angular momentum modes, including amplitude and phase distributions;

[0121] Step S12, free space propagation: multiple orbital angular momentum modes are transmitted by near-field Fresnel diffraction in free space;

[0122] Step S13, multi-layer progressive modulation: The above-mentioned multi-level phase modulation structure can be regarded as an all-optical light field manipulation system consisting of an input plane, multiple phase or amplitude diffraction layers, and an output plane. After the incident light wave propagates forward in free space, it undergoes progressive wavefront modulation through multiple diffraction layers. Each diffraction layer contains M*M programmable diffraction units for modulating the wavefront. According to the Huygens scalar wave analysis principle, each diffraction unit can be regarded as a secondary wave source, and its output wave is determined by the interference superposition of the diffraction unit and the front light field. Under the paraxial approximation, this optical response can be further simplified in the frequency domain using Fresnel diffraction theory:

[0123]

[0124] Where (fx, fy) is the spatial frequency of (x, y), λ is the operating wavelength, k = 2π / λ, Δz is the interlayer distance, The light field distribution of the secondary wave is determined by the product of the transmission coefficient and the input wave, and both are complex-valued functions:

[0125]

[0126] In the formula and is the complex value distribution before and after the Lth layer, t L (x,y) is the transmission coefficient of the Lth layer, which can be expressed as:

[0127] t L (x,y)=A L (x,y)·exp(iθ L (x,y)),

[0128] Where A L (x,y) and θ L (x,y) is the relative amplitude and additional phase delay added by the diffraction unit to the secondary wave, constrained to [0,1] and [0,2π] respectively. After modulation by the diffraction unit, the interconnection between layers can be expressed as:

[0129]

[0130] Where F and F-1 are fast Fourier transform and inverse fast Fourier transform respectively.

[0131] Step S14, free space propagation: the light modulated by the multi-layer phase structure is again transmitted by Fresnel diffraction in free space;

[0132] Step S15, outputting a complex-valued light field: outputting a plurality of orbital angular momentum modes after free-space transmission to a receiving screen and collecting their complex-valued light field distributions;

[0133] Step S16, evaluating the loss function: During the training process, the calculated output complex-valued light field distribution and the target complex-valued light field distribution are used to calculate the loss function, and the phase weight parameters are updated through error evaluation and backpropagation. Since the optimization target is a complex-valued function, a loss function is designed to achieve high mode purity of the converted light field, which can be expressed as:

[0134]

[0135]

[0136] Where T out (x, y) is a real complex-valued distribution, α=1 is a weight parameter, and Relu and Tanh are two common activation functions in neural networks:

[0137] relu(x)=max{0,x},

[0138]

[0139] The first term in L(x, y) considers the complex-valued difference between the output and target distributions. The Tanh function, with its pronounced gradient, can amplify this difference, accelerating convergence and accuracy. The second term focuses on the intensity difference between the output and target distributions. The ReLU function can discard outliers and further modify the intensity details of the output field distribution. It is important to note that considering both the complex value and intensity of the loss function may be superior to traditional optical neural networks that rely solely on the L1 or L2 norm, hoping to leverage their powerful linear representation capabilities. A stochastic gradient descent algorithm is employed, with Adam chosen as the optimizer to backpropagate the error and update the diffraction layer to minimize the loss function.

[0140] The optimal multi-layer diffraction structure after optimal iteration can obtain the best multi-layer phase mask and output the final required complex-valued light field distribution.

[0141] After step S15 and before step S16, the following steps are further included:

[0142] Step S15a, determining whether the output complex-valued light field is less than 3000; if yes, executing step S16; if no, executing step S15b;

[0143] Step S15b: output the optimal phase mask.

[0144] Accordingly, before step S16, the following steps are also included:

[0145] Step S16a: input the target complex-valued light field.

[0146] In this embodiment, a multiplexed orbital angular momentum mode exchange system based on controllable unitary transformation (CUT) generates multiple coaxially transmitted vortex beams using a CUT mode generation module. After the CUT mode exchange module converts these multiple modes, the CUT mode detection module detects and analyzes the exchanged modes. This CUT-based CUT multiplexed orbital angular momentum mode exchange method implements a CUT strategy, enabling coaxial, independent exchange of multiple input orbital angular momentum modes. The exchanged modes exhibit high mode purity.

[0147] This embodiment achieves the following technical effects through the above technical solution:

[0148] The controllable unitary transformation strategy of this embodiment enables coaxial independent exchange of multiple input orbital angular momentum modes, resulting in high mode purity. Compared to traditional mode exchange schemes, this embodiment's coaxial mode exchange system offers a large number of controllable modes, flexible functionality, and independent modulation, making it suitable for all-optical mode information exchange in high-dimensional multiplexing communications.

[0149] Those skilled in the art will appreciate that all or part of the processes in the above-described method embodiments can be implemented by instructing related hardware through a computer program. The computer program can be stored in a non-volatile storage medium. When executed, the computer program can include the processes in the above-described method embodiments. Any reference to memory, storage, database, or other media used in the various embodiments provided herein may include non-volatile and / or volatile memory.

[0150] In summary, the present invention provides a multiplexed orbital angular momentum mode exchange system and method based on controllable unitary transformation, comprising: a multiplexed mode generation module for generating multiple coaxially transmitted multiplexed orbital angular momentum modes; a mode exchange module for performing mode conversion between multiple coaxially transmitted multiplexed orbital angular momentum modes; a mode detection module for detecting the exchanged orbital angular momentum modes and outputting corresponding detection results; the mode exchange module is arranged after the multiplexed mode generation module and is located on the same optical axis as the multiplexed mode generation module; the mode detection module is arranged after the mode exchange module and is located on the same optical axis as the mode exchange module. The present invention can realize coaxial independent exchange of multiple input orbital angular momentum modes, and the exchanged modes have high mode purity. In addition, the mode coaxial exchange system has the characteristics of a large number of controllable modes, flexible functions, and independent modulation, and is suitable for all-optical mode information exchange in high-dimensional multiplexed communications.

[0151] It should be understood that the application of the present invention is not limited to the above examples. For those skilled in the art, improvements or changes can be made based on the above description. All these improvements and changes should fall within the scope of protection of the claims attached to the present invention.

Claims

1. A multiplexed orbital angular momentum mode exchange system based on controllable unitary transformation, characterized in that: include: A multiplexing pattern generation module, used for generating multiplexed orbital angular momentum patterns for coaxial transmission; A mode exchange module, configured to perform mode conversion between the plurality of coaxial transmission multiplexed orbital angular momentum modes; and a mode detection module for detecting the orbital angular momentum mode after the exchange and outputting the corresponding detection result; The mode switching module is arranged after the multiplexing mode generating module and is located on the same optical axis as the multiplexing mode generating module; The mode detection module is arranged behind the mode exchange module and is located on the same optical axis as the mode exchange module; The multiplexing mode generation module includes: A light source, a beam splitting device, a first single-mode vortex light generating device, a second single-mode vortex light generating device, a first reflecting device, a second reflecting device, a first beam combining device, and a second beam combining device; The beam splitting device is arranged behind the light source and is located on the same optical axis as the light source; the first single-mode vortex light generating device and the first reflecting device form a vortex light path; the second single-mode vortex light generating device, the first beam combining device and the second beam combining device form a vortex composite light path; the second reflecting device and the second beam combining device form a composite light path; The mode switching module includes: a first transition phase mask, a second transition phase mask, and a third transition phase mask; The first conversion phase mask is provided after the multiplexing mode generation module, and the first conversion phase mask, the second conversion phase mask and the third conversion phase mask are provided in sequence; The first transformed phase mask, the second transformed phase mask, and the third transformed phase mask are used to perform a controllable unitary transformation.

2. The multiplexed orbital angular momentum mode exchange system based on controllable unitary transformation according to claim 1, characterized in that: The light source is a fundamental mode laser with a wavelength of 1550 nm; the beam splitting device is a 1×3 optical coupler with a splitting ratio of 1:1:

1.

3. The multiplexed orbital angular momentum mode exchange system based on controllable unitary transformation according to claim 1, characterized in that: The first single-mode vortex light generating device and the second single-mode vortex light generating device are both spiral phase masks, the first reflecting device and the second reflecting device are both plane mirrors, and the first beam combining device and the second beam combining device are both non-polarization beam combiners.

4. The multiplexed orbital angular momentum mode exchange system based on controllable unitary transformation according to claim 1, characterized in that: The mode detection module includes: A light intensity detector and a computer; the light intensity detector is arranged after the mode exchange module and is used to detect the optical signal after the controllable unitary transformation; the computer is connected to the light intensity detector.

5. A method for multiplexing orbital angular momentum mode exchange based on controllable unitary transformation, applied to the multiplexing orbital angular momentum mode exchange system based on controllable unitary transformation according to any one of claims 1 to 4, characterized in that: include: The Gaussian beam emitted by the light source passes through the beam splitting device to output a first Gaussian sub-beam, a second Gaussian sub-beam and a third Gaussian sub-beam; Passing the first Gaussian sub-beam through a first single-mode vortex light generating device to obtain a first vortex beam, and passing the second Gaussian sub-beam through a second single-mode vortex light generating device to obtain a second vortex beam; wherein the first vortex beam and the second vortex beam are vortex beams carrying a single orbital angular momentum mode; After the first vortex beam passes through a first reflecting device, the first vortex beam is combined with the second vortex beam in a first beam combining device to form a first multiplexed vortex beam; After the third Gaussian sub-beam passes through a second reflecting device, the third Gaussian sub-beam is combined with the first multiplexed vortex beam in a second beam combining device to form a second multiplexed vortex beam; After the second multiplexed vortex beam passes through the first conversion phase mask, the second conversion phase mask, and the third conversion phase mask to perform mode conversion, the orbital angular momentum mode required by the target is output; The pattern detection module records, detects and analyzes the orbital angular momentum pattern required by the target and outputs the detection and analysis results.

6. The method for multiplexing orbital angular momentum mode exchange based on controllable unitary transformation according to claim 5, characterized in that: The light field distributions of the first vortex beam and the second vortex beam are both: in is the radial component, is the angular component, is the waist radius, for The waist size, is the associated Laguerre polynomial, To characterize the topological charge of the orbital angular momentum mode, is the radial parameter, is the Rayleigh distance, is the beam transmission distance, is the wave vector, the phase factor It means that the light beam has a helical structure. Is an imaginary unit.

7. The method for multiplexing orbital angular momentum mode exchange based on controllable unitary transformation according to claim 5, characterized in that: In the first conversion phase mask, the second conversion phase mask, and the third conversion phase mask, the interconnection between layers is expressed as: in, and They are fast Fourier transform and inverse fast Fourier transform; in, for The spatial frequency, is the working wavelength, , is the inter-layer distance, is an imaginary unit; In the formula and is the complex-valued distribution before and after the Lth layer, is the transmission coefficient of the Lth layer.

8. The method for multiplexing orbital angular momentum mode exchange based on controllable unitary transformation according to claim 5, characterized in that: The method further comprises: Multi-level phase modulation based on deep learning algorithms to achieve high mode purity of the converted light field: in, is a real complex-valued distribution, is the output complex-valued distribution, is the weight parameter, relu and tanh are both activation functions.

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