Optical fiber transmission image restoration method and device based on complex amplitude regulation metasurface diffraction device

Abstract

The invention discloses an optical fiber transmission image recovery method and device based on a complex amplitude regulation metasurface diffraction device, and the method comprises the steps: obtaining the complex amplitude distribution of each diffraction layer through the training of a diffraction neural network, designing a corresponding metasurface array, and preparing a cascaded metasurface diffraction device with a complex amplitude regulation function, deploying a cascade metasurface diffraction device to the far end of the multimode optical fiber and performing complex amplitude modulation on emergent circularly polarized light to realize high-resolution or high-sampling-rate image recovery; or an optical fiber transmission inverse matrix is solved by using an analytical method, and a corresponding metasurface array is designed, so that the single-layer metasurface diffraction device with a complex amplitude regulation and control function is prepared; and deploying the single-layer metasurface diffraction device to the far end of the few-mode optical fiber and carrying out complex amplitude modulation on emergent circularly polarized light to realize low-resolution or low-sampling-rate image recovery. Miniaturization of a multimode optical fiber imaging system can be realized, and the image recovery quality is effectively improved based on complex amplitude modulation of the metasurface diffraction device.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical fiber imaging, and specifically relates to an optical fiber transmission image restoration method and device based on a complex amplitude controlled metasurface diffraction device. Background Art

[0002] Multimode fiber (MMF) is capable of transmitting multiple spatial modes, which can serve as independent information channels, opening up a promising path for the transmission of ultra-high-density information and images in thin diameters. Its unique transmission characteristics not only greatly broaden the bandwidth and capacity of fiber-optic communications, but also provide strong technical support for many cutting-edge technology fields such as fiber-optic imaging.

[0003] Today, multimode optical fiber has diverse applications, encompassing fields ranging from biomedicine to information communications. In biomedicine, high-resolution three-dimensional endoscopic imaging using multimode optical fiber provides doctors with clearer and more accurate information about internal structures, significantly improving disease diagnosis and treatment outcomes. In the field of short-distance optical data interconnection, the introduction of space-division multiplexing technology has significantly increased data transmission rates and capacity, providing a strong foundation for the efficient operation of data centers. Furthermore, with the booming development of emerging fields such as quantum communications and photonic computing, multimode optical fiber, with its unique transmission characteristics, is becoming an indispensable key technology in these fields.

[0004] However, despite its numerous advantages and promising applications, multimode fiber also faces significant challenges in practical application. Modal dispersion and crosstalk are two major challenges that must be overcome during multimode fiber transmission. Because multiple spatial modes exist within a multimode fiber, these modes interfere with each other during transmission, causing the incident image and light field information to be disrupted and transformed into speckle upon exit. This speckle phenomenon not only reduces image clarity and accuracy but also severely impacts the performance and stability of multimode fiber during information transmission.

[0005] In order to fully utilize the high information capacity of multimode optical fibers, researchers have been working hard to explore effective solutions. Among them, by measuring the transmission matrix (TM) of the multimode optical fiber and using a spatial light modulator to perform dynamic wavefront shaping, a focused scanning point can be generated at the far end of the optical fiber to excite the sample signal to be detected, and the reflection or fluorescence image can be reconstructed by collecting and recording the total light intensity signal returned from each spot position. In addition, there are studies that use multi-plane optical converters (MPLCs) to achieve a single reconstruction of an image transmitted into a multimode optical fiber at a single wavelength. By precisely controlling the phase changes of the light wave on multiple planes, efficient regulation and optimization of the light wavefront are achieved, thereby improving the accuracy and efficiency of image reconstruction. However, whether using a spatial light modulator or a multi-plane optical converter, there are problems with the system being large in size and not miniaturized enough, which to a certain extent limits the application of multimode optical fibers in chip-level mode multiplexing and demultiplexing and on-chip quantum optics.

[0006] Therefore, future research needs to focus more on how to achieve the miniaturization and integration of multimode fiber transmission systems, and how to improve the accuracy and stability of image reconstruction, so as to promote the unique value of multimode fiber in more fields and make greater contributions to scientific and technological progress and social development. Summary of the Invention

[0007] In view of the above, the purpose of the present invention is to provide a fiber transmission image restoration method and device based on a complex amplitude controlled metasurface diffraction device, aiming to achieve single exposure imaging and miniaturization of multimode fiber imaging systems in specific application scenarios such as high-speed communication and endoscopy. Compared with the pure phase modulation method, complex amplitude modulation can effectively improve the image restoration quality. At the same time, in order to design a structure that can achieve the best restoration effect when the number of modulations of the metasurface diffraction device is limited, the design and arrangement of the metasurface are optimized by diffraction neural network training. For the possible situation where the number of samples between the two planes of the propagation process is different, the matrix triple product (MTP) and chirp-Z transform (chrip-Z) algorithms are used to solve the problem so that the propagation model is more in line with physical laws. For the case where the number of imaging samples is small or the resolution requirement is not high, the transmission inverse matrix can be solved by analytical methods and the corresponding metasurface arrangement structure can be designed, thereby improving the flexibility of metasurface design and application.

[0008] To achieve the above-mentioned purpose, the present invention provides the following technical solutions:

[0009] An embodiment of the present invention provides a method for restoring an optical fiber transmission image based on a complex amplitude controlled metasurface diffraction device, comprising the following steps:

[0010] The complex amplitude distribution of each diffraction layer is obtained by training a diffraction neural network and the corresponding metasurface array is designed to prepare a cascaded metasurface diffraction device with complex amplitude control function.

[0011] The cascaded metasurface diffraction device is deployed at the far end of a multimode optical fiber and the outgoing circularly polarized light is complex-amplitude modulated to achieve high-resolution or high-sampling-rate image restoration.

[0012] To achieve the same purpose of the invention, an embodiment of the present invention further provides a method for restoring an optical fiber transmission image based on a complex amplitude-controlled metasurface diffraction device, comprising the following steps:

[0013] The inverse fiber transmission matrix is solved analytically and the corresponding metasurface array is designed to fabricate a single-layer metasurface diffraction device with complex amplitude control capability.

[0014] A single-layer metasurface diffraction device is deployed at the far end of a few-mode fiber and the outgoing circularly polarized light is complex-amplitude modulated to achieve low-resolution or low-sampling-rate image restoration.

[0015] Preferably, the method of obtaining the complex amplitude distribution of each diffraction layer by training a diffraction neural network and designing a corresponding metasurface array includes:

[0016] A training set is generated using the measured actual transmission matrix of a multimode optical fiber. The training layer of the diffraction neural network includes phase and amplitude weight factors, which correspond to the normalized amplitude and phase values of the unit modulation on each layer of the metasurface device, respectively. The light field between layers is analyzed and converted through angular spectrum propagation. The modulated complex amplitude distribution of each diffraction layer is trained by the diffraction neural network. According to the trained complex amplitude distribution, metaatoms are arranged as several modulation units to obtain a metasurface array with complex amplitude control function.

[0017] Preferably, during the training process of the diffraction neural network, the matrix triple product algorithm or the chirped Z transform is used to transform the sampling number of the light field in the frequency domain to the spatial domain, so as to ensure that the sampling number of the light field diffracted by the optical fiber end face and propagated to the first layer of the metasurface diffraction device is consistent with the number of modulation units of the metasurface array corresponding to this layer.

[0018] Preferably, the method of solving the fiber transmission inverse matrix using an analytical method and designing a corresponding metasurface array includes:

[0019] Based on the Rayleigh-Sommerfeld diffraction theory, the light field function of the optical fiber far-end facet transmitted to the metasurface and then to the detection surface is calculated and expressed in matrix form. The relationship between the metaatoms in the metasurface array and the sampling number of the image generated by the detection surface and the optical fiber far-end facet is analyzed through the matrix expression, which is expressed as:

[0020] M I×1 =B I×J ×A J×1

[0021]

[0022] Among them, A J×1 represents the output field distribution at the far end of the optical fiber, B I×J It represents the transmission matrix between the output field at the far end of the optical fiber and the detection surface, M I×1 represents the field distribution of the detection surface, I represents the number of image plane samples at the far end of the optical fiber, J represents the number of object plane samples at the near end of the optical fiber, and V (I*J)×K The matrix represents the transfer matrix B I×J The decomposition matrix, K represents the number of modulation units in the metasurface array, B (i,j) Denotes the transmission matrix B I×J The element in row i and column j in , where i and j are the indices of I and J respectively, represents the modulation function of the kth metaatom, the superscript 2 represents the plane of the metasurface diffraction device, and the superscript T represents the transpose. When K=I*J, the transmission matrix can be freely modulated, and then the complex amplitude distribution of the metasurface diffraction device unit is solved based on the transmission matrix, so as to design the metaatom arrangement corresponding to the complex amplitude distribution to obtain the metasurface array.

[0023] Preferably, a geometric phase-type metasurface is used, that is, each metaatom in the metasurface array adopts an X-shaped structure or a single rectangular nanopillar structure, and complex amplitude control is achieved by modulating the angle (amplitude) and / or rotation angle (phase) of the two rectangular nanopillars that make up the X-shaped structure, or by modulating the height (amplitude) and / or rotation angle (phase) of a single rectangular nanopillar structure. Further preferably, the use of an X-shaped structure can achieve a uniform height of the metasurface array, simplify the process and design, and achieve more flexible and efficient complex amplitude control.

[0024] Preferably, the required metasurface structure is printed on the end face of the optical fiber using two-photon laser direct writing technology.

[0025] To achieve the above-mentioned purpose of the invention, an embodiment of the present invention further provides an optical fiber transmission image restoration device based on a complex amplitude controlled metasurface diffraction device, which is implemented using the above-mentioned optical fiber transmission image restoration method based on a complex amplitude controlled metasurface diffraction device, and includes: a light source module, a spatial light modulation module, a polarization modulation module, a multimode optical fiber projection device module, and a matrix measurement and imaging conversion module;

[0026] The light source module includes a laser, a first collimator and a second collimator. The laser is used to emit object light and reference light. The object light is collimated by the first collimator and then incident on the spatial light modulation module. The reference light is collimated by the second collimator and then incident on the matrix measurement and imaging conversion module.

[0027] The spatial light modulation module includes a spatial light modulator, which is used to perform light field modulation on the received object light and emit the modulated light beam to the polarization modulation module;

[0028] The polarization modulation module includes a first polarizer, a first quarter-wave plate and a first reflector. The modulated light beam is polarized and modulated into circularly polarized light by the first polarizer and the first quarter-wave plate, and then the reflected light is incident on the multimode fiber projection device module by the first reflector.

[0029] The multimode fiber projection device module includes a first microscope objective lens, an optical fiber, a metasurface diffraction device, and a second microscope objective lens, wherein the optical fiber includes a multimode optical fiber or a few-mode optical fiber, and the metasurface diffraction device includes a cascaded metasurface diffraction device or a single-layer metasurface diffraction device. The first microscope objective lens focuses on the proximal end of the optical fiber to generate a projection pattern, which is then transmitted to the distal end of the optical fiber via the optical fiber. The metasurface diffraction device is processed on the end face of the distal end of the optical fiber for complex amplitude control, and the emitted light is transmitted to the matrix measurement and imaging conversion module via the second microscope objective lens.

[0030] The matrix measurement and imaging conversion module includes a second quarter glass, a lens, a beam splitter, a second polarizer, a photodetector, a second reflector, a third polarizer, and a server. Light emitted from the far end of the optical fiber passes through the second quarter glass, the lens, and the beam splitter in sequence. At the same time, the reference light collimated by the second collimator is incident on the third polarizer and then reflected by the second reflector into the beam splitter. The light emitted from the far end of the optical fiber and the reference light interfere with each other based on the beam splitter, and the interference pattern is collected by the photodetector after passing through the second polarizer. Finally, the interference pattern is transmitted to the server for processing to obtain a restored image.

[0031] Preferably, the matrix measurement function in the matrix measurement and imaging conversion module includes:

[0032] When the metasurface diffraction device is not processed on the far end of the optical fiber, the circularly polarized light emitted by the second microscope objective is converted into linearly polarized light through the second quarter glass. The rear lens interferes with the reference light on the beam splitter and then passes through the second polarizer. The interference pattern is collected by the photodetector and transmitted to the server to calculate the transmission matrix.

[0033] Preferably, the imaging function in the matrix measurement and imaging conversion module includes:

[0034] After processing the metasurface diffraction device on the far end of the optical fiber, the second polarizer in front of the photodetector set in the matrix measurement function is rotated 90° to realize the imaging function conversion, and finally the restored image is processed in the server.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] (1) The present invention processes a metasurface diffraction device on the far end face of the optical fiber, which can replace the spatial light modulator to realize the light field control function in certain application scenarios, and uses complex amplitude control to restore the image or light field information with higher precision.

[0037] (2) The present invention determines the arrangement distribution of the metasurface by using diffraction neural network training to optimize the complex amplitude modulation parameters of each diffraction layer, which is suitable for high-resolution or high-sampling rate image restoration. At the same time, the matrix triple product or chirped Z transform is introduced in the training process, which optimizes the physical model while ensuring the training speed, which helps to accurately design and prepare cascaded metasurface diffraction devices.

[0038] (3) The present invention solves the arrangement distribution of the metasurface through analytical methods, which is suitable for image restoration with low resolution or low sampling rate, and improves the flexibility and comprehensiveness of metasurface design and application fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] 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 these drawings without creative work.

[0040] Figure 1 1 is a flow chart of a method for restoring an optical fiber transmission image based on a complex amplitude controlled metasurface diffraction device provided by an embodiment of the present invention;

[0041] Figure 2 2 is a flow chart of a method for restoring an image transmitted via an optical fiber based on a complex amplitude controlled metasurface diffraction device according to another embodiment of the present invention;

[0042] Figure 3 This is a schematic diagram of the principle and effect of image restoration technology using a diffraction neural network to design a cascaded metasurface diffraction device provided by an embodiment of the present invention;

[0043] Figure 4 It is a schematic diagram of the three-dimensional structure of the optical fiber end face processing metasurface diffraction device and a schematic diagram of the X-type complex amplitude modulation metasurface diffraction device structure and parameter scanning results provided by an embodiment of the present invention;

[0044] Figure 5 This is a schematic diagram of the principle and effect of image restoration technology for designing a single-layer surface diffraction device using an analytical method provided by an embodiment of the present invention;

[0045] Figure 6 It is a structural schematic diagram of an optical fiber transmission image restoration device based on a complex amplitude controlled metasurface diffraction device provided in an embodiment of the present invention.

[0046] The specific symbols in the accompanying drawings are as follows:

[0047] 1. Laser; 2. First collimator; 3. Spatial light modulator; 4. First polarizer; 5. First quarter-wave plate; 6. First reflector; 7. First microscope objective; 8. Optical fiber; 9. Metasurface diffraction device; 10. Second microscope objective; 11. Second quarter-wave plate; 12. Lens; 13. Beam splitter; 14. Second polarizer; 15. Photodetector; 16. Second reflector; 17. Third polarizer; 18. Second collimator; 19. Server. DETAILED DESCRIPTION

[0048] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and do not limit the scope of protection of the present invention.

[0049] The inventive concept of the present invention is: in response to the problems in the prior art that multimode fiber imaging systems are large in size, not miniaturized enough, and have insufficient accuracy and stability in image reconstruction, the embodiments of the present invention provide a fiber transmission image restoration method and device based on a complex amplitude controlled metasurface diffraction device. The stability of the chirality of circularly polarized light when propagating in a multimode optical fiber is utilized, and an improved diffraction neural network is trained to obtain the complex amplitude distribution of each diffraction layer unit and arrange the metaatoms accordingly. An X-shaped isogeometric phase metasurface diffraction device is designed and processed on the end face of the optical fiber to perform complex amplitude modulation on the input circularly polarized light, so that the restored image has both high structural similarity and signal-to-noise ratio.

[0050] Based on the above invention concept, Figure 1 As shown, the embodiment provides a fiber transmission image restoration method based on complex amplitude controlled metasurface diffraction devices, which uses a diffraction neural network to design a cascaded metasurface diffraction device, and uses two-photon laser direct writing to process a tiny metasurface diffraction device with the same light field modulation capability on the fiber end face, which can achieve effective recovery of single-wavelength multimode fiber light field information, and the metasurface with complex amplitude modulation function plays a more effective role in the restoration of images that are also affected by the complex amplitude linear operator (multimode fiber transmission matrix).

[0051] In an embodiment, a diffraction neural network is trained to obtain the complex amplitude distribution of each diffraction layer and a corresponding metasurface array is designed to prepare a cascaded metasurface diffraction device with complex amplitude control function, and the cascaded metasurface diffraction device is deployed to the far end of a multimode optical fiber and the outgoing circularly polarized light is complex amplitude modulated to achieve high-resolution or high-sampling rate image restoration.

[0052] Due to the influence of the fiber mode number, the number of pattern samples at the fiber output end differs from the number of modulations of the metasurface diffraction device. Using FFT to calculate the mathematical model of diffraction propagation will reduce the optimization quality, while using the Rayleigh-Sommerfeld spatial domain diffraction equation will significantly reduce the calculation speed. The application of diffraction neural networks (D2NNs) can optimize the design of metasurface structures that can achieve the best recovery effect when the number of modulations of the metasurface diffraction device is limited. Specifically, a training set is generated using the measured actual transmission matrix of a multimode fiber. The diffraction neural network is then used for training within the TensorFlow or PyTorch learning framework. The training layers contain phase and amplitude weight factors, corresponding to the normalized amplitude (0 to 1) and phase (0 to 2π) values of the unit modulation on each layer of the metasurface device. The light field between layers is converted through angular spectrum propagation analysis. Therefore, the diffraction neural network is an iterative training method based on an optical physics model. The modulated complex amplitude distribution of each diffraction layer is trained through a diffraction neural network, and the metaatoms are arranged accordingly as several modulation units according to the trained complex amplitude distribution to obtain a metasurface array with complex amplitude control function.

[0053] In the angular spectrum propagation calculation method, the fast Fourier transform (FFT) requires that the number of sampling points in the spatial domain and the frequency domain be consistent, such as Figure 3 As shown, the sampling points at the output end of the multimode fiber are 64×64. Generally, the number of orthogonal sampling points must be less than or equal to the number of propagation modes of the target multimode fiber at a specific wavelength (mutually orthogonal), and the number of unit arrays of each layer of the metasurface can be set according to the processing accuracy. Within the diffraction area with effective diffraction efficiency, the more modulation units there are, the better the modulation effect. The number of metasurface array arrangements is set to 120×120 using two-photon laser direct writing technology. The sampling points of the fiber output image are different from the modulation sampling number of the metasurface array. The FFT calculation process needs to be zero-filled, which affects the final training results. Therefore, in the angular spectrum propagation calculation of the diffraction neural network training in the embodiment, the matrix triple product (MTP) algorithm or chirp-Z transform is used to achieve independent sampling of the incident field and the transfer function when calculating the diffraction field distribution of the next layer. By transforming the sampling number in the frequency domain of the light field to the spatial domain, the sampling number of the light field diffracted by the fiber end face to the first layer of the metasurface diffraction device is consistent with the number of modulation units of the metasurface array corresponding to that layer. After the spatial domain of the multimode fiber output pattern is transformed into the frequency domain through the MTP algorithm or chrip-Z, the number of samples can be smoothly increased, matching the transfer function and the number of samples propagated to the end face of the diffraction device. Figure 3As shown, the restored image has high clarity, a significant improvement over the FFT method and a significant speed advantage over methods that directly utilize spatial propagation calculations. The training results and images collected on the detection surface are optimized using the SSIM and MSE loss functions to determine the phase and amplitude weights of each unit in the layer.

[0054] Taking advantage of the fact that the chirality of circularly polarized light propagating in multimode optical fiber remains almost unchanged, a geometric phase-type metasurface array based on the Pancharatnam-Berry phase principle is used for light field manipulation. Each metaatom is designed with an X-shaped structure (composed of two rectangular nanopillars with adjustable angles and rotation angles) or a single rectangular nanopillar structure with adjustable height / rotation angles, thereby achieving complex amplitude manipulation of phase and amplitude while changing the chirality of the output light field.

[0055] like Figure 4 The figure shows a complex amplitude control metasurface based on an X-shaped structure. Complex amplitude control is performed using X-shaped metaatoms printed using photoresist IP-DIP (refractive index approximately 1.52). The structure is numerically simulated using finite-difference time-domain (FDTD) and finite element method (FEM), with parameter sweeps performed to select structural parameters within an appropriate modulation range. The X-shaped structure resembles the letter "X," and its modulation function is equivalent to the combined control effect of two rectangular metaatoms. Based on the principle of geometric phase modulation, the theoretical modulation effect can be calculated as follows:

[0056]

[0057] Among them, t cross represents the transmission control result of a single X-type metaatom (chirality inversion), θ (or α2-α1) represents the angle between the two rectangular nanopillars, α2+α1 represents the sum of the two rotation angles α relative to the set coordinate axis, j represents the imaginary number sign, σ represents the chirality of circular polarization (±1), the amplitude term cos(θ) is related to the angle between the rectangular nanopillars, and the phase It is related to the sum of the deflection angles. By scanning the parameters, a meta-atomic structure with high polarization conversion efficiency is selected. The target structural unit period is 1.5 microns, and the rectangular nanocolumns are 0.82 microns long, 0.4 microns wide and 3.14 microns high. The phase modulation range of 2π and the polarization conversion efficiency of 0 to 0.8 are obtained by scanning the meta-unit.

[0058] Based on the same inventive concept, Figure 2 As shown, in another embodiment, an analytical method is used to solve the inverse matrix of optical fiber transmission and design the corresponding metasurface array to prepare a single-layer metasurface diffraction device with complex amplitude control function, and the single-layer metasurface diffraction device is deployed to the far end of the few-mode optical fiber and the outgoing circularly polarized light is complex amplitude modulated to achieve low-resolution or low sampling rate image restoration.

[0059] When the number of samples of the object plane and the image plane is reduced to a certain range, such as signal transmission of few-mode optical fiber, the inverse transmission matrix required to restore the image can be solved by analytical methods, and a single-layer metasurface diffraction device can be designed for control. Figure 5 As shown, the physical process of the light at the output end of the few-mode fiber passing through the single-plane diffraction device and then propagating to the detection surface is the same as that of the multi-plane diffraction device, and can be expressed by a linear operator. In multi-mode and few-mode fibers, the light field incident on the near-end face of the fiber can be divided into n×n sampling points. After reaching the far end of the fiber, the light field of all orthogonal sampling points will have a weight distribution (light field coupling) at all sampling points at the far end, and the weights of all orthogonal points can be arranged into a transmission matrix. The number of orthogonal sampling points is generally set to match the number of modes allowed to propagate in the fiber. The effect of the optical fiber output light and the metasurface diffraction device is also a similar linear operator. Assuming that the output distribution of the j-th sampling point at the far end of the few-mode fiber is for:

[0060]

[0061] Wherein, j represents the light field sampling point at the output end of the optical fiber, and the superscript 1 represents the light field plane at the output end (the first plane). represents the amplitude term, Represents the phase term. Assuming that the sampling point of the metasurface diffraction device unit is k, the complex amplitude of the light field incident on the kth sampling point of the metasurface layer by the jth sampling point at the output end is:

[0062]

[0063] in, represents the complex amplitude of the light field at the jth sampling point on the output end incident on the kth sampling point on the metasurface layer, The coordinates of the propagation from the jth sampling point on the first plane to the upper surface (second plane) of the metasurface diffraction device are: The transfer function of the kth sampling point is, The coordinates of the kth sampling point of the metasurface diffraction device incident on the detection surface (third plane) are: The complex amplitude of the light field at the i-th sampling point is, represents the light field at the i-th sampling point on the detection surface, and the complex amplitude of the light field on the lower surface of the metasurface diffraction device after being modulated by the meta-atoms is for:

[0064]

[0065] in, The coordinates are The super-atomic modulation function of represents the total light field of all light fields on the first plane incident on the kth sampling point on the second plane, and J represents the total number of samples on the first plane. Then, the complex amplitude of the light field corresponding to the kth sampling point of the metasurface diffraction device incident on the i-th sampling point on the detection surface is for:

[0066]

[0067] in, Indicates the propagation from the second plane to a unit point on the detection surface (third plane) The transfer function of the detection surface can be obtained by analogy. The total light field of a sampling point i on the detection surface is The summation process can be converted into matrix calculation, and the light field distribution on the detection surface can be expressed as:

[0068]

[0069] in, is the light field of each sampling point on the detection surface, 1≤i≤I, I is the total number of sampling points on the detection surface, the first matrix on the right side of the equation is the transmission matrix of the light after the optical fiber is emitted from the far end, the size of this matrix is I×J, K represents the total number of samples on the second plane, Indicates the propagation from the second plane to a unit point on the detection surface (third plane) The transfer function (that is, the transfer function in the previous ), It means that the coordinates propagated from the first plane to the upper surface (second plane) of the metasurface diffraction device are The transfer function of the unit point (that is, the previous ), Represents the output distribution of each sampling point at the far end of the few-mode fiber, 1≤j≤J, J is the total number of sampling points at the far end of the few-mode fiber. Each element in the matrix is represented by B (i,j) , further decomposing the matrix yields:

[0070]

[0071] Among them, B I×J The matrix size is (I*J)×K. Since the goal of the metasurface diffraction device is to construct an inverse fiber matrix, the matrix B I×J It should be the inverse matrix of the optical fiber transmission matrix TM. Given the measured optical fiber transmission matrix TM, calculate its inverse matrix, then the matrix B I×J The values of each element in can be determined (one-to-one correspondence with the inverse matrix), and the equation can be expressed as:

[0072]

[0073] According to the matrix calculation principle, only when the matrix V (I*J)×K With the matrix (V(I*J)×K ,(B I×J ) T ) has a solution when the ranks are the same. Therefore, the matrix needs to be I*J=K to solve. This means that the number of samples at the output multiplied by the number of samples at the detection surface equals the number of metasurface modulations. Due to machining precision limitations, this can only be achieved in few-mode fibers. The complex amplitude distribution of the metasurface diffraction device unit obtained from this analytical solution is then used to design a metastructured atomic arrangement corresponding to this complex amplitude distribution to form a metasurface array. Complex amplitude modulation can almost perfectly restore the image with fewer samples.

[0074] Based on the same inventive concept, Figure 6 As shown, an embodiment of the present invention also provides an optical fiber transmission image restoration device based on a complex amplitude controlled metasurface diffraction device, including: a light source module, a spatial light modulation module, a polarization modulation module, a multimode optical fiber projection device module, and a matrix measurement and imaging conversion module.

[0075] The light source module includes a laser 1, a first collimator 2 and a second collimator 18. The laser 1 is used to emit object light and reference light. The object light is collimated by the first collimator 2 and then incident on the spatial light modulation module. The reference light is collimated by the second collimator 18 and then incident on the matrix measurement and imaging conversion module.

[0076] The spatial light modulation module includes a spatial light modulator 3, which modulates the received object light (matrix measurement and pattern projection onto the near end of the optical fiber) and outputs the modulated beam to the polarization modulation module. Spatial light modulator 3 can be used to control the phase and amplitude of the incident light. It can use a digital micromirror array, liquid crystal spatial light modulator, or deformable mirror to measure the multimode fiber transmission matrix and generate the desired projection pattern.

[0077] Considering that the use of a DMD spatial light modulator (SLM) results in binomial attenuation in both S and P polarizations due to metal reflection from the micromirrors, as well as a small phase difference, leading to the generation of elliptically polarized light, a polarization modulation module is constructed to convert elliptically polarized light into circularly polarized light. The polarization modulation module comprises a first polarizer 4, a first quarter-wave plate 5, and a first reflector 6. The modulated light beam is polarized into circularly polarized light by the first polarizer 4 and the first quarter-wave plate 5. The first reflector 6 then directs the reflected light into the multimode fiber projection device module.

[0078] The multimode fiber projection device module includes a first microscope objective 7, an optical fiber 8, a metasurface diffraction device 9 and a second microscope objective 10, wherein the optical fiber 8 includes a multimode optical fiber or a few-mode optical fiber, and the metasurface diffraction device 9 includes a cascaded metasurface diffraction device or a single-layer metasurface diffraction device. The first microscope objective 7 focuses on the proximal end of the optical fiber 8 to generate a projection pattern, which is then transmitted to the far end of the optical fiber through the optical fiber 8. The metasurface diffraction device 9 is processed on the end face of the far end of the optical fiber for complex amplitude control and the output light is transmitted to the matrix measurement and imaging conversion module through the second microscope objective 10.

[0079] The matrix measurement and imaging conversion module includes a second quarter glass 11, a lens 12, a beam splitter 13, a second polarizer 14, a photodetector 15, a second reflector 16, a third polarizer 17, and a server 19. Light emitted from the far end of the optical fiber passes through the second quarter glass 11, lens 12, and beam splitter 13 in sequence. The reference light, collimated by the second collimator 18, is incident on the third polarizer 17 and then reflected by the second reflector 16 into the beam splitter 13. Interference between the light emitted from the far end of the optical fiber and the reference light occurs based on the beam splitter 13. After passing through the second polarizer 14, the interference pattern is collected by the photodetector 15 and transmitted to the server 19 for processing to recover the image. The server 19 is also used to calculate the axial propagation circularly polarized light transmission matrix and generate a D2NN training set for designing the metasurface diffraction device 9.

[0080] In the embodiment, in the matrix measurement stage, the metasurface diffraction device 9 is not processed on the far end face of the optical fiber, and the right (left) circularly polarized light emitted by the second microscope objective 10 is converted into x (y) linearly polarized light through the second quarter glass slide 11. The light then passes through the lens 12 (a 4f system composed of the second microscope objective 10 and the lens 12 is used to image the image emitted from the far end of the optical fiber) and interferes with the x (y) linear polarized light generated by the reference light path on the beam splitter 13. After that, the interference pattern is collected by the photodetector 15 through the second polarizer 14 in the x (y) direction and transmitted to the server 19 to calculate the transmission matrix.

[0081] In the embodiment, during the imaging stage, a metasurface diffraction device 9 is processed on the far end face of the optical fiber. Since the chirality of the modulated circularly polarized light changes, the polarization direction of the target light field after passing through the second quarter glass slide 11 is orthogonal to that during the matrix measurement, that is, it is converted into linearly polarized light y(x) orthogonal to the previous direction. It is only necessary to rotate the second polarizer 14 in front of the photodetector 15 set in the matrix measurement by 90° to adjust it to y(x) polarized light, so as to collect the information of the restored image and filter out the unmodulated x(y) polarization direction stray light and reference light, thereby realizing functional conversion, and finally processing in the server 19 to obtain the restored image.

[0082] The specific implementation methods described above provide a detailed description of the technical solutions and beneficial effects of the present invention. It should be understood that the above is only the most preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, supplements and equivalent substitutions made within the scope of the principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for restoring an optical fiber transmission image based on a complex amplitude controlled metasurface diffraction device, characterized in that: The following steps are involved: The complex amplitude distribution of each diffraction layer is obtained by training a diffraction neural network and the corresponding metasurface array is designed to prepare a cascaded metasurface diffraction device with complex amplitude control function. The cascaded metasurface diffraction device is deployed at the far end of a multimode optical fiber and the outgoing circularly polarized light is complex-amplitude modulated to achieve high-resolution or high-sampling-rate image restoration.

2. A method for restoring an optical fiber transmission image based on a complex amplitude-controlled metasurface diffraction device, characterized in that: The following steps are involved: The inverse fiber transmission matrix is solved analytically and the corresponding metasurface array is designed to fabricate a single-layer metasurface diffraction device with complex amplitude control capability. A single-layer metasurface diffraction device is deployed at the far end of a few-mode fiber and the outgoing circularly polarized light is complex-amplitude modulated to achieve low-resolution or low-sampling-rate image restoration.

3. The optical fiber transmission image restoration method based on complex amplitude controlled metasurface diffraction device according to claim 1 is characterized in that: The method of obtaining the complex amplitude distribution of each diffraction layer by using a diffraction neural network training and designing a corresponding metasurface array includes: A training set is generated using the measured actual transmission matrix of a multimode optical fiber. The training layer of the diffraction neural network includes phase and amplitude weight factors, which correspond to the normalized amplitude and phase values of the unit modulation on each layer of the metasurface device, respectively. The light field between layers is analyzed and converted through angular spectrum propagation. The modulated complex amplitude distribution of each diffraction layer is trained by the diffraction neural network. According to the trained complex amplitude distribution, metaatoms are arranged as several modulation units to obtain a metasurface array with complex amplitude control function.

4. The optical fiber transmission image restoration method based on complex amplitude controlled metasurface diffraction device according to claim 3 is characterized in that: During the training process of the diffraction neural network, the matrix triple product algorithm or chirped Z transform is used to transform the sampling number of the light field in the frequency domain to the spatial domain, so that the sampling number of the light field diffracted by the optical fiber end face and propagated to the first layer of the metasurface diffraction device is consistent with the number of modulation units of the metasurface array corresponding to this layer.

5. The optical fiber transmission image restoration method based on complex amplitude controlled metasurface diffraction device according to claim 2, characterized in that: The method of using an analytical method to solve the inverse optical fiber transmission matrix and designing a corresponding metasurface array includes: Based on the Rayleigh-Sommerfeld diffraction theory, the light field function of the optical fiber far-end facet transmitted to the metasurface and then to the detection surface is calculated and expressed in matrix form. The relationship between the metaatoms in the metasurface array and the sampling number of the image generated by the detection surface and the optical fiber far-end facet is analyzed through the matrix expression, which is expressed as: M I×1 =B I×J ×A J×1 Among them, A J×1 represents the output field distribution at the far end of the optical fiber, B I×J It represents the transmission matrix between the output field at the far end of the optical fiber and the detection surface, M I×1 represents the field distribution of the detection surface, I represents the number of image plane samples at the far end of the optical fiber, J represents the number of object plane samples at the near end of the optical fiber, and V (I*J)×K The matrix represents the transfer matrix B I×J The decomposition matrix, K represents the number of modulation units in the metasurface array, B (i,j) Denotes the transmission matrix B I×J The element in row i and column j in , where i and j are the indices of I and J respectively, represents the modulation function of the kth metaatom, the superscript 2 represents the plane of the metasurface diffraction device, and the superscript T represents the transpose. When K=I*J, the transmission matrix can be freely modulated, and then the complex amplitude distribution of the metasurface diffraction device unit is solved based on the transmission matrix, so as to design the metaatom arrangement corresponding to the complex amplitude distribution to obtain the metasurface array.

6. The optical fiber transmission image restoration method based on complex amplitude controlled metasurface diffraction device according to claim 1 or 2, characterized in that: A geometric phase-type metasurface is used, that is, each metaatom in the metasurface array adopts an X-shaped structure or a single rectangular nanopillar structure. Complex amplitude control is achieved by modulating the angle and / or rotation angle of the two rectangular nanopillars that constitute the X-shaped structure, or by modulating the height and / or rotation angle of a single rectangular nanopillar structure.

7. The optical fiber transmission image restoration method based on complex amplitude controlled metasurface diffraction device according to claim 6, characterized in that: The metasurface structure required for printing processing is achieved by using two-photon laser direct writing technology on the end face of the optical fiber.

8. A fiber transmission image restoration device based on a complex amplitude controlled metasurface diffraction device, implemented using the fiber transmission image restoration method based on a complex amplitude controlled metasurface diffraction device according to any one of claims 1 to 7, characterized in that: include: Light source module, spatial light modulation module, polarization modulation module, multimode fiber projection device module, and matrix measurement and imaging conversion module; The light source module includes a laser, a first collimator and a second collimator. The laser is used to emit object light and reference light. The object light is collimated by the first collimator and then incident on the spatial light modulation module. The reference light is collimated by the second collimator and then incident on the matrix measurement and imaging conversion module. The spatial light modulation module includes a spatial light modulator, which is used to perform light field modulation on the received object light and emit the modulated light beam to the polarization modulation module; The polarization modulation module includes a first polarizer, a first quarter-wave plate and a first reflector. The modulated light beam is polarized and modulated into circularly polarized light by the first polarizer and the first quarter-wave plate, and then the reflected light is incident on the multimode fiber projection device module by the first reflector. The multimode fiber projection device module includes a first microscope objective lens, an optical fiber, a metasurface diffraction device, and a second microscope objective lens, wherein the optical fiber includes a multimode optical fiber or a few-mode optical fiber, and the metasurface diffraction device includes a cascaded metasurface diffraction device or a single-layer metasurface diffraction device. The first microscope objective lens focuses on the proximal end of the optical fiber to generate a projection pattern, which is then transmitted to the distal end of the optical fiber via the optical fiber. The metasurface diffraction device is processed on the end face of the distal end of the optical fiber for complex amplitude control, and the emitted light is transmitted to the matrix measurement and imaging conversion module via the second microscope objective lens. The matrix measurement and imaging conversion module includes a second quarter glass, a lens, a beam splitter, a second polarizer, a photodetector, a second reflector, a third polarizer, and a server. Light emitted from the far end of the optical fiber passes through the second quarter glass, the lens, and the beam splitter in sequence. At the same time, the reference light collimated by the second collimator is incident on the third polarizer and then reflected by the second reflector into the beam splitter. The light emitted from the far end of the optical fiber and the reference light interfere with each other based on the beam splitter, and the interference pattern is collected by the photodetector after passing through the second polarizer. Finally, the interference pattern is transmitted to the server for processing to obtain a restored image.

9. The optical fiber transmission image restoration device based on the complex amplitude controlled metasurface diffraction device according to claim 8, characterized in that: The matrix measurement functions in the matrix measurement and imaging conversion module include: When the metasurface diffraction device is not processed on the far end of the optical fiber, the circularly polarized light emitted by the second microscope objective is converted into linearly polarized light through the second quarter glass. The light then passes through the lens and interferes with the reference light on the beam splitter. The interference pattern is then collected by the photodetector through the second polarizer and transmitted to the server to calculate the transmission matrix.

10. The optical fiber transmission image restoration device based on the complex amplitude controlled metasurface diffraction device according to claim 9, characterized in that: Imaging functions in the matrix measurement and imaging conversion module include: After processing the metasurface diffraction device on the far end of the optical fiber, the second polarizer in front of the photodetector set in the matrix measurement function is rotated 90° to realize the imaging function conversion, and finally the restored image is processed in the server.

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