Few-mode fiber multi-wavelength and multi-mode mixed mode decomposition device and decomposition method based on multi-wavelength lensless coherent modulation imaging

Abstract

The invention discloses a few-mode optical fiber multi-wavelength and multi-mode mixed mode decomposition device and decomposition method based on multi-wavelength lensless coherent modulation imaging, and the method comprises the steps: exciting a few-mode optical fiber to be detected through a multi-wavelength laser source, and modulating a mixed wavelength light field outputted by the optical fiber through a binary random amplitude plate; a single-frame diffraction pattern is collected by a detector; simultaneously reconstructing complex amplitude distribution of each wavelength component in a modulation plane from a single diffraction pattern by using a low-rank mixed state reconstruction algorithm containing frequency domain constraint; then constructing and calculating a virtual 4f system, and propagating a complex amplitude value of the modulation plane to an optical fiber end face to obtain amplified optical fiber end face complex amplitude distribution; and finally, space alignment is optimized in combination with a gradient descent algorithm, and modal coefficients under all wavelengths are extracted through modal projection. According to the method, the limitation that a traditional method can only perform single-wavelength analysis or is influenced by dispersion is overcome, and rapid, accurate and calibration-free decomposition of a multi-wavelength and multi-mode optical fiber light field under single exposure is realized.

Description

Technical Field

[0001] This invention relates to the field of fiber optic mode measurement technology, specifically to a device and method for decomposing multi-wavelength and multi-mode hybrid modes of few-mode optical fibers based on multi-wavelength lensless coherent modulation imaging. Background Technology

[0002] In recent years, with the continuous development of the information industry and the constant innovation of AI-powered intelligent services, the demand for network traffic in various countries has been growing rapidly, and the capacity of traditional single-mode fiber communication systems has approached the nonlinear limit. Mode division multiplexing (MDF) technology based on few-mode fibers has attracted much attention in the field of optical communication and has been proven to be an effective method for improving the capacity of optical transmission systems. However, in practical applications, few-mode fibers can experience problems such as mode coupling and mode-related losses due to factors such as fiber bending, twisting, and manufacturing defects, resulting in differences between the input and output optical field characteristics. Therefore, in-depth research on fiber mode characteristics and the development of mode characterization techniques based on the characteristics of fiber laser mode fields are of crucial significance for the practical application of few-mode fibers.

[0003] Mode decomposition, as a key technology for effectively characterizing multimode transmission properties in optical fibers, has received considerable attention in recent years. To date, various mode decomposition techniques have been proposed, such as the ring cavity method, spatial spectroscopy method, correlation analysis method, numerical analysis method, deep learning method, and complex amplitude reconstruction method.

[0004] While each of the aforementioned mode analysis techniques offers unique advantages and methodological capabilities in analyzing the modal characteristics of few-mode fibers, they all share a fundamental limitation: they are all based on the monochromatic light assumption in their mathematical models. This means they can only resolve the mode distribution under single-wavelength excitation and cannot effectively characterize the mode field characteristics of few-mode fibers under multi-wavelength and multi-mode excitation. According to a literature review, no research has yet proposed a systematic method capable of comprehensively characterizing the mode field information of multi-wavelength and multi-mode fibers. This situation contrasts sharply with the rapid development of wavelength division multiplexing (WDM)-mode division multiplexing (MDD) hybrid multiplexing technology, which achieves orders-of-magnitude improvements in transmission capacity and spectral efficiency by integrating MDD mechanisms into traditional WDM architectures.

[0005] It is worth noting that with theoretical breakthroughs and device innovations in spatial dimension multiplexing technology, wavelength division multiplexing (WDM)-mode division multiplexing (MDD) hybrid multiplexing systems have been recognized as the core technology path to overcome the transmission limits of single-mode optical fibers, playing a crucial role in the evolution of ultra-high-speed optical communication networks. Against this backdrop, establishing a precise characterization system for multi-wavelength, multi-mode fiber mode field information has dual research value: from a fundamental theoretical perspective, this method can provide experimental evidence for the analysis of key scientific issues such as mode coupling dynamics and cross-wavelength crosstalk mechanisms in WDM-MDD hybrid multiplexing systems; from an engineering application perspective, it can not only guide the optimized design of low-crosstalk fiber waveguide structures but also provide quantitative analysis tools for system performance evaluation and nonlinear effect suppression. Summary of the Invention

[0006] To address the problems of existing technologies and fill the gap in the accurate characterization of multi-wavelength, multi-mode fiber mode field information, this invention provides a device and method for decomposing multi-wavelength and multi-mode hybrid modes in few-mode fibers based on multi-wavelength lensless coherent modulation imaging. This method innovatively uses a binary random amplitude plate instead of a traditional phase plate and frequency domain constraints instead of physical lens focusing, thereby eliminating the focal plane displacement problems caused by wavelength-related pre-calibration and dispersion. Through diffraction patterns acquired in a single exposure, this method can simultaneously reconstruct the complex amplitudes of each wavelength component in the hybrid beam, thus achieving comprehensive mode characteristic analysis of the amplitude and phase of the multi-wavelength hybrid beam. This invention effectively bridges the development needs of novel multiplexing technologies with research on fiber mode field characteristics, laying a theoretical foundation for building a next-generation optical transmission system compatible with high-order modulation formats and multi-dimensional multiplexing.

[0007] Specifically, the following technical solutions are included: In a first aspect, the present invention provides a few-mode fiber multi-wavelength and multi-mode hybrid mode decomposition device based on multi-wavelength lensless coherent modulation imaging, comprising a first laser, a first beam splitter, a second beam splitter, a coupler, a single-mode fiber, a collimating lens, a polarizer, a modulation plate, and a detector arranged sequentially along the light propagation method. A second laser is disposed on one side of the first beam splitter, and a third laser is disposed on one side of the second beam splitter. The first, second, and third lasers are used to emit three different wavelength laser beams, the emission wavelengths of which are all less than the cutoff wavelength of the single-mode fiber. The light emitted by the first and second lasers forms a first mixed beam after passing through the first beam splitter. The first mixed beam and the light emitted by the third laser form a second mixed beam after passing through the second beam splitter and are incident on the coupler. The coupler is connected to the single-mode fiber. The light-emitting end face of the single-mode fiber is placed at the front focal plane of the collimating lens. The polarizer is used to maintain the consistent polarization direction of the beam. The modulation plate is used to modulate the wavefront. The detector is used to collect the diffraction spots of the light field.

[0008] In one embodiment of the present invention, the single-mode optical fiber is configured to support the propagation of multiple transverse modes under the operating wavelengths of the first laser, the second laser, and the third laser.

[0009] In one embodiment of the invention, the polarizer is placed between the single-mode optical fiber and the detector.

[0010] In one embodiment of the present invention, the modulation plate is an amplitude-type or phase-type modulation structure.

[0011] Secondly, the present invention provides a method for decomposing multiple wavelengths and multiple modes of mixed modes in few-mode fiber based on multi-wavelength lensless coherent modulation imaging, wherein the decomposition device is used to perform mode decomposition, including the following steps: Step 1: Acquire the intensity I of the few-mode light spot using the detector. R (x,y); Step 2: Reconstruct the complex amplitude using a multi-wavelength lensless coherent modulation imaging method, obtaining the reconstructed complex amplitude corresponding to each wavelength, and using P... k (x,y) represents the reconstructed complex amplitude of the k-th wavelength; Step 3: Based on scalar diffraction theory, construct a virtual 4f system using the focal length of the collimating lens, and apply the complex amplitude distribution P obtained in Step 2. k (x, y) is virtually transmitted through this system, and the amplified complex amplitude distribution E corresponding to the end face of the single-mode fiber is finally obtained on the back focal plane of the system. k,4f (x,y); Step 4: Convert the complex amplitude E obtained in Step 3 into... k,4f (x,y) and the eigenfield E of each mode k,LPj Perform orthogonal projection operations on (x,y) to obtain the mode complex coefficients C. k,j ; Step 5: The mode complex coefficients C obtained from Step 4 k,j Synthesize the mode field and calculate the reconstructed complex amplitude intensity. I k,rec With the strength of the synthetic mode field I k,syn Relevance Corr k The pattern decomposition is now complete.

[0012] In one embodiment of the present invention, the multi-wavelength lensless coherent modulation imaging method in step 2 specifically includes: Step 2-1: Generate arbitrary initial field distribution P k,0 ( x, y ) serves as the initial estimate of the incident beam in the iterative calculation; where the subscript kThis indicates the wavelength number, and the subscript 0 indicates the initial iteration step. Step 2-2: In the first n In the next iteration, the first n The incident light field distribution in the next iteration P k,0 ( x, y ) and a predetermined modulation plate distribution t ( x , y Multiply by , and get the first . n Sub-iteration output light field distribution φ k,n ( x , y ): φ k,n ( x , y )= P n ( x , y )· t ( x, y ) Steps 2-3: [The text appears to be incomplete and contains several grammatical errors. A more accurate translation would require the full context.] n Sub-iteration output light field distribution φ k,n ( x , y The propagation reaches the detector plane, yielding the first... n Planar optical field distribution of the next iteration detector ψ k,n ( x, y ): ψ k,n ( x, y )= F { φ k,n ( x , y ), d} in, F {} indicates Fresnel propagation. d This represents the distance between the modulation plate and the detector; the total calculated intensity distribution is given by the following formula:

[0013] Steps 2-4: Retain ψ k,n ( x, y The phase of the field is determined, and its amplitude is updated using a low-rank mixed-state algorithm that enforces energy conservation between wavelengths; the modulus of each field is determined by the measured intensity recorded by the detector. I R ( x ,y The square root of ) is obtained, and the total strength is calculated. I cal ( x , y Normalization, as shown in the given formula;

[0014] Steps 2-5: Backpropagation, ... ψ' k,n ( x , y The wave propagates in the reverse direction to the modulation plate plane, and the outgoing wave is updated using the following formula: φ' k,n ( x , y )= F -1 { ψ' k,n ( x , y ), d}; in, F -1 {} indicates reverse Fresnel propagation. d This indicates the distance between the modulation board and the detector. φ' k,n ( x , y ) represents the outgoing wave function of the modulation plate plane; Steps 2-6: Incident beam update. Update the incident beam on the modulation plate plane using the following formula:

[0015] in, α To keep it as a constant, thus avoiding division by zero errors; Steps 2-7: Obtain the frequency domain distribution of the updated incident beam through two-dimensional Fourier transform: P’ k,n ( u , v )= f { P’ n (x, y),} in f {} represents a two-dimensional Fourier transform; Steps 2-8: Detector planar constraints, applying spatial constraints, the formula is:

[0016] in, S n (u , v ) represents a circular aperture function for a specific wavelength, whose radius is gradually increased during the iteration process to optimize the reconstruction effect; Steps 2-9: Propagate backward to the incident surface, and obtain the incident beam for the next iteration through reverse Fresnel propagation:

[0017] in f -1 {} represents the inverse two-dimensional Fourier transform; Step 2-10: Convergence check, calculate the intensity on the diffraction surface. I cal ( x , y (and the intensity of the diffraction spot obtained from actual recording) I R ( x , y Error calculation is performed, and the calculation expression is as follows:

[0018] When the error exponent between the two is greater than a preset value, it is considered that the iteration has converged, and the reconstructed complex amplitude spot is obtained. P k,n ( x , y ) is used for subsequent pattern decomposition processing.

[0019] In one embodiment of the present invention, the mode complex coefficient C in step 4 k,j The calculation formula is:

[0020] in, E k,LPj (x, y) represents the characteristic mode field of each LP mode at the theoretical k-th wavelength. E k,4f (x, y) represents the amplified complex amplitude distribution corresponding to the single-mode fiber end face obtained in step 3. C k,j To correspond to the mode complex coefficients of the theoretical mode field, the magnitude value represents the mode energy proportion, and the argument represents the relative phase information.

[0021] In one embodiment of the present invention, the complex amplitude intensity is reconstructed in step 5. I k,rec By analyzing the complex amplitude distribution of the 4f focal plane in step 3 E k,4f (x, y) It is obtained by taking the square of the modulus.

[0022] In one embodiment of the present invention, the synthesized mode field strength in step 5 I k,syn The method for obtaining the complex amplitude of the mode field under test is to synthesize it through modal superposition. E k,syn Specifically, based on coefficients C k,j The formula for linearly superimposing the various transmission modes is as follows:

[0023] in, E k,LPj ( x, y ) represents the first wavelength of the optical fiber propagation field corresponding to the k-th wavelength. j Each linearly polarized eigenmode C k , j This indicates the corresponding k-th wavelength. j Modal complex coefficients of the mode field, ρ k,j It is the amplitude factor of the k-th wavelength. θ k,j It is the kth wavelength. j The phase difference between each higher-order mode and the fundamental mode; all eigenmode fields must be normalized, and then the complex amplitude is adjusted. E k,syn The square of the modulus is used to obtain the combined mode field strength I. k,syn .

[0024] In one embodiment of the present invention, the correlation calculation in step 5 adopts the intensity correlation calculation:

[0025] in, I k,rec and I k,syn These are the reconstructed complex amplitude intensity and the synthetic mode field intensity, respectively. and These are their average values.

[0026] The beneficial effects of this invention are: (1) This invention enables the simultaneous resolution of mode decomposition of multi-wavelength and multi-mode light beams in a single measurement, overcoming the problem that existing technologies cannot take into account both wavelength and mode dimensions in a single measurement. This greatly improves the efficiency and completeness of complex optical field analysis, providing unprecedented real-time characterization capabilities for application scenarios such as multi-wavelength multiplexed optical communication.

[0027] (2) This invention uses a computational virtual 4f imaging system to replace traditional physical lens imaging, fundamentally avoiding the dispersion problem caused by physical lenses. This innovation not only simplifies the optical hardware structure and realizes the miniaturization and compactness of the system, but also ensures the consistency of imaging under multiple wavelengths through computational imaging, thereby improving the intrinsic accuracy of the measurement.

[0028] (3) This invention innovatively applies an automatic gradient descent algorithm for spatial alignment between experimental and theoretical modes, achieving sub-pixel-level precise alignment. This effectively overcomes the influence of unavoidable minor mechanical drift and misalignment in experimental setups, thus ensuring extremely high accuracy and reliability of the mode decomposition results. In summary, this invention provides a new, efficient, accurate, stable, and easy-to-implement method for fiber optic mode characterization, achieving substantial progress in performance, system complexity, and applicability.

[0029] (4) This invention proposes a method for mode decomposition of multi-wavelength and multi-mode mixed optical fields requiring only a single diffraction pattern. Its innovation is mainly reflected in the following four aspects: First, this invention is the first to propose a novel fiber mode decomposition method based on multi-wavelength lensless coherent amplitude modulation imaging. Second, this invention is the first to achieve mode decomposition of multi-wavelength and multi-mode mixed optical fields, filling a gap in related measurement technology. Third, this method overcomes the dependence of existing technologies on reference beams, requiring only a single diffraction image to complete the full characterization of the multimode optical field in the fiber. Finally, through a virtual 4f system propagation model, the optical system structure is significantly simplified, enabling complete mode decomposition with only the simplest configuration. These features significantly improve the feasibility of the method in practical applications, providing an effective technical solution for the promotion and application of few-mode fibers in industrial testing and engineering practice.

[0030] (5) This invention innovatively uses a binary random amplitude plate instead of a traditional phase plate and frequency domain constraints instead of physical lens focusing, thereby eliminating the focal plane displacement problem caused by wavelength-related pre-calibration and dispersion. Through diffraction patterns acquired in a single exposure, this method can simultaneously reconstruct the complex amplitudes of each wavelength component in the mixed beam, thus achieving comprehensive mode characteristic analysis of the amplitude and phase of the multi-wavelength mixed beam. This invention will effectively connect the development needs of novel multiplexing technologies with the research on fiber mode field characteristics, laying a theoretical foundation for constructing a next-generation optical transmission system compatible with high-order modulation formats and multi-dimensional multiplexing.

[0031] (6) This invention excites the few-mode fiber under test using a multi-wavelength laser source, modulates the mixed-wavelength optical field output by the fiber using a binary random amplitude plate (BRAP), and collects a single-frame diffraction pattern by a detector; using a low-rank mixed-state reconstruction algorithm with frequency domain constraints (MW-LF-CAMI), the complex amplitude distribution of each wavelength component in the modulation plane is simultaneously reconstructed from the single diffraction pattern; then, a virtual 4f system is constructed to propagate the complex amplitude value of the modulation plane to the fiber end face to obtain an amplified complex amplitude distribution of the fiber end face; finally, the spatial alignment is optimized by combining the gradient descent algorithm, and the modal coefficients at each wavelength are extracted by modal projection. This invention overcomes the limitations of traditional methods that can only perform single-wavelength analysis or are affected by dispersion, and realizes fast, accurate, and calibration-free decomposition of multi-wavelength, multi-mode fiber optical fields in a single exposure. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the structure of the few-mode fiber multi-wavelength and multi-mode hybrid mode decomposition device based on multi-wavelength lensless coherent modulation imaging of the present invention; wherein, 1, first laser; 2, second laser; 3, third laser; 4, first beam splitter; 5, second beam splitter; 6, coupler; 7, single-mode fiber; 8, collimating lens; 9, polarizer; 10, modulation plate; 11, detector.

[0033] Figure 2 This is a schematic diagram of the intensity distribution of the modulation plate and the diffraction spot acquired during measurement in the device of the present invention; wherein, Figure 2 (a) in the figure represents the intensity distribution of the modulation plate. Figure 2 (b) in the diagram represents the diffraction pattern recorded by the detector.

[0034] Figure 3 This invention utilizes Figure 2 (b) is a schematic diagram of the complex amplitude image reconstructed from the diffraction spot recorded in the diagram; where, Figure 3 In the diagram, (a), (b), and (c) represent the reconstructed intensity distribution. Figure 3 (d), (e), and (f) in the figure represent the corresponding phase distributions, with the columns from left to right corresponding to wavelengths of 450 nm, 532 nm, and 633 nm, respectively.

[0035] Figure 4 This invention will Figure 3 A schematic diagram of the amplitude and phase obtained by virtually transmitting the reconstructed complex amplitude to position 4f; where... Figure 4 In the diagram, (a), (b), and (c) represent the reconstructed intensity distribution. Figure 4 (d), (e), and (f) in the figure represent the corresponding phase distributions, with the columns from left to right corresponding to wavelengths of 450 nm, 532 nm, and 633 nm, respectively.

[0036] Figure 5 This is a schematic diagram of the modal decomposition results of the reconstructed beam of the present invention; wherein, Figure 5 In the figures (a) and (b), the modal power ratio and phase coefficient of the 450 nm beam are shown. Figure 5 In the diagram, (c) and (d) represent the modal power ratio and phase coefficient of the 532 nm beam. Figure 5 In the figure, (e) and (f) represent the mode power ratio and phase coefficient of the 633 nm beam. The horizontal axis represents the index of the linearly polarized (LP) mode, and the vertical axis represents the mode power ratio or relative phase.

[0037] Figure 6 This is a schematic diagram of the composite complex amplitude distribution of modal coefficients on the 4f focal plane based on the present invention; wherein, Figure 6 In the diagram, (a), (b), and (c) represent the intensity distribution of the synthesized material. Figure 6 (d), (e), and (f) in the figure represent the corresponding phase distributions, with the columns from left to right corresponding to wavelengths of 450 nm, 532 nm, and 633 nm, respectively. Detailed Implementation

[0038] The embodiments of the technical solution of the present invention will now be described in detail with reference to the accompanying drawings. These embodiments are merely illustrative of the technical solution of the present invention and are therefore intended to limit the scope of protection of the present invention.

[0039] Unless otherwise defined, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains; the terminology used in this invention is for the purpose of describing particular embodiments only and is not intended to limit the invention; the terms “comprising” and “having”, and any variations thereof, in the specification, claims and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0040] In the description of the embodiments of this invention, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this invention, "multiple" means two or more, unless otherwise explicitly defined.

[0041] In this invention, the reference to "embodiment" means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least some embodiments of the invention. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described in this invention can be combined with other embodiments.

[0042] Example 1: like Figure 1 As shown, this embodiment provides a few-mode fiber multi-wavelength and multi-mode hybrid mode decomposition device based on multi-wavelength lensless coherent modulation imaging. It includes a first laser 1, a first beam splitter 4, a second beam splitter 5, a coupler 6, a single-mode fiber 7, a collimating lens 8, a polarizer 9, a modulation plate 10, and a detector 11 arranged sequentially along the light propagation path. A second laser 2 is disposed on one side of the first beam splitter 4, and a third laser 3 is disposed on one side of the second beam splitter 5. The first laser 1, the second laser 2, and the third laser 3 are respectively used to emit three different wavelength laser beams. The emission wavelengths of all laser beams are smaller than the cutoff wavelength of the single-mode fiber 7. The light emitted by the first laser 1 and the second laser 2 forms a first mixed beam after passing through the first beam splitter 4. The first mixed beam and the light emitted by the third laser 3 form a second mixed beam after passing through the second beam splitter 5 and are incident on the coupler 6. The coupler 6 is connected to the single-mode fiber 7. The light-emitting end face of the single-mode fiber 7 is placed at the front focal plane of the collimating lens 8. The polarizer 9 is used to keep the beam polarization direction consistent. The modulation plate 10 is used to modulate the wavefront. The detector 11 is used to collect the light field diffraction spots.

[0043] The working principle of this embodiment is as follows: The light emitted by the first laser 1 and the second laser 2 passes through the first beam splitter 4 to form a first mixed beam. The first mixed beam and the light emitted by the third laser 3 pass through the second beam splitter 5 to form a second mixed beam. The second mixed beam is coupled into the single-mode fiber 7 using a coupler 6, and then collimated by a collimating lens 8. Subsequently, the beam polarization direction is kept consistent by a polarizer 9. The beam with a single polarization direction passes through a modulation plate 10 and is then sampled by a detector 11 to collect the diffraction spot. All of the above optical elements are perpendicular to the laser beam and their centers are kept on the optical axis.

[0044] In this embodiment, the multi-mode beam excited after the second mixed beam is coupled into the single-mode fiber 7 serves as the mode field to be measured; the collimating lens 8 has a focal length of 8.126 mm, and the modulation plate 10 is a randomly distributed amplitude plate with a known distribution, as shown in the figure. Figure 2 As shown in (a), its distribution is labeled as t(x, y), and its distance from detector 11 is 34.66 mm; detector 11 is a CMOS camera with a pixel size of 3.76 × 3.76 μm².

[0045] Example 2: This embodiment provides a method for decomposing multiple wavelengths and mixed modes of few-mode fibers based on multi-wavelength lensless coherent modulation imaging. The method utilizes the device for decomposing multiple wavelengths and mixed modes of few-mode fibers based on multi-wavelength lensless coherent modulation imaging from Embodiment 1 to perform mode decomposition on the few-mode beam. The few-mode beam is excited by lasers with wavelengths of 632 nm, 532 nm, and 450 nm coupled into an optical fiber with a core radius of 4.1 μm and a numerical aperture of 0.14, thereby generating a total of 36 linear polarization propagation modes.

[0046] The decomposition method includes the following steps: Step 1: Acquire the intensity of the few-mode light spot through detector 11, such as... Figure 2 As shown in (b), the image distribution is labeled as I R ( x, y ); Step 2: Reconstruct the complex amplitude using a multi-wavelength lensless coherent modulation imaging method, obtaining the reconstructed complex amplitude corresponding to each wavelength, such as... Figure 3 As shown, Figure 3 In the diagram, (a), (b), and (c) represent the reconstructed intensity distribution. Figure 3 In the diagram, (d), (e), and (f) represent the corresponding phase distributions. The columns from left to right correspond to wavelengths of 450 nm, 532 nm, and 633 nm, respectively. P k (x, y) Indicates the first k The reconstructed complex amplitude of each wavelength, and the multi-wavelength lensless coherent modulation imaging method specifically include: Step 2-1: Generate arbitrary initial field distribution P k,0 ( x, y ) serves as the initial estimate of the incident beam in the iterative calculation; where the subscript k This indicates the wavelength number, and the subscript "0" indicates the initial iteration step; Step 2-2: In the first n In the next iteration, the first n The incident light field distribution in the next iteration P k,0 ( x, y ) and the predetermined distribution of modulation plate 10 t ( x , y Multiply by , and get the first . n Sub-iteration output light field distribution φ k,n ( x , y ): φ k,n (x , y )= P n ( x , y )· t ( x, y ) Steps 2-3: [The text appears to be incomplete and contains several grammatical errors. A more accurate translation would require the full context.] n Sub-iteration output light field distribution φ k,n ( x , y The propagation reaches the plane of detector 11, obtaining the first... n Sub-iteration detector 11 planar light field distribution ψ k,n ( x, y ): ψ k,n ( x, y )= F { φ k,n ( x , y ), d} in, F {} indicates Fresnel propagation. d This represents the distance between modulation plate 10 and detector 11; the total calculated intensity distribution is given by the following formula:

[0047] Steps 2-4: Retain ψ k,n ( x, y The phase of the field is determined, and its amplitude is updated using a low-rank mixed-state algorithm that enforces energy conservation between wavelengths; the modulus of each field is determined by the measured intensity recorded by detector 11. I R ( x , y The square root of ) is obtained, and the total strength is calculated. I cal ( x , y Normalization, as shown in the given formula;

[0048] Steps 2-5: Backpropagation, ... ψ' k,n ( x , y The wave propagates in the reverse direction to the plane of modulation plate 10, and the outgoing wave is updated using the following formula: φ' k,n ( x, y )= F -1 { ψ' k,n ( x , y ), d}; in, F -1 {} indicates reverse Fresnel propagation. d This indicates the distance between the modulation plate 10 and the detector 11. φ' k,n ( x , y ) represents the outgoing wave function of the plane of modulation plate 10; Steps 2-6: Incident beam update. Update the incident beam on the plane of modulation plate 10 using the following formula:

[0049] in, α To keep it as a constant, thus avoiding division by zero errors; Steps 2-7: Obtain the frequency domain distribution of the updated incident beam through two-dimensional Fourier transform: P’ k,n ( u , v )= f { P’ n (x, y),} in f {} represents a two-dimensional Fourier transform; Steps 2-8: Detector planar constraints, applying spatial constraints, the formula is:

[0050] in, S n ( u , v ) represents a circular aperture function for a specific wavelength, whose radius is gradually increased during the iteration process to optimize the reconstruction effect; Steps 2-9: Propagate backward to the incident surface, and obtain the incident beam for the next iteration through reverse Fresnel propagation:

[0051] in f -1 {} represents the inverse two-dimensional Fourier transform; Step 2-10: Convergence check, calculate the intensity on the diffraction surface. I cal ( x ,y (and the intensity of the diffraction spot obtained from actual recording) I R ( x , y Error calculation is performed, and the calculation expression is as follows:

[0052] When the error exponent between the two is greater than a preset value, it is considered that the iteration has converged (the preset value can be selected between 0.8 and 1, depending on the specific situation; in this embodiment, the value is set to 0.95). At this time, the reconstructed complex amplitude light spot... P k,n ( x , y Used for subsequent pattern decomposition processing; Step 3: Based on scalar diffraction theory, construct a virtual 4f system using the focal length of collimating lens 8, and apply the complex amplitude distribution P obtained in Step 2. k (x, y) is virtually transmitted through this system, and the amplified complex amplitude distribution corresponding to the end face of the single-mode fiber 7 is finally obtained on the back focal plane of the system, such as Figure 4 As shown, Figure 4 In the diagram, (a), (b), and (c) represent the reconstructed intensity distribution. Figure 4 In the diagram, (d), (e), and (f) represent the corresponding phase distributions. The columns from left to right correspond to wavelengths of 450 nm, 532 nm, and 633 nm, respectively. The [f]... k The distribution of each wavelength is labeled as E k,4f (x,y); Step 4: Convert the complex amplitude E obtained in Step 3 into... k,4f (x,y) and the eigenfield E of each mode k,LPj Perform orthogonal projection operations on (x,y) to obtain the mode complex coefficients C. k,j ; Mode complex coefficients C k,j The calculation formula is:

[0053] in, E k,LPj (x, y) represents the characteristic mode field of each LP mode at the theoretical k-th wavelength. E k,4f (x, y) represents the amplified complex amplitude distribution corresponding to the end face of the single-mode fiber 7 obtained in step 3. C k,j To correspond to the mode complex coefficients of the theoretical mode field, the magnitude value represents the mode energy proportion, and the argument represents the relative phase information; Step 5: The mode complex coefficients C obtained from Step 4 k,j Synthesize the mode field and calculate the reconstructed complex amplitude intensity.I k,rec With the strength of the synthetic mode field I k,syn Relevance Corr k This is used to evaluate the accuracy of the pattern decomposition, at which point the pattern decomposition is complete; Among them, the reconstructed complex amplitude intensity I k,rec By analyzing the complex amplitude distribution of the 4f focal plane in step 3 E k,4f (x, y) The square of the modulus is used to obtain the composite mode field strength. I k,syn The method for obtaining the complex amplitude of the mode field under test is to synthesize it through modal superposition. E k,syn Specifically, based on coefficients C k,j The formula for linearly superimposing the various transmission modes is as follows:

[0054] in, E k,LPj ( x, y ) represents the first wavelength of the optical fiber propagation field corresponding to the k-th wavelength. j Each linearly polarized eigenmode C k , j This indicates the corresponding k-th wavelength. j Modal complex coefficients of the mode field, ρ k,j It is the amplitude factor of the k-th wavelength. θ k,j It is the kth wavelength. j The phase difference between each higher-order mode and the fundamental mode; all eigenmode fields must be normalized, and then the complex amplitude is adjusted. E k,syn The square of the modulus is used to obtain the combined mode field strength I. k,syn ; The correlation calculation uses the intensity correlation operation:

[0055] in, I k,rec and I k,syn These are the reconstructed complex amplitude intensity and the synthetic mode field intensity, respectively. and These are their average values.

[0056] Figure 5 This is a schematic diagram of the modal decomposition results of the reconstructed beam of the present invention; wherein,Figure 5 In the figures (a) and (b), the modal power ratio and phase coefficient of the 450 nm beam are shown. Figure 5 In the diagram, (c) and (d) represent the modal power ratio and phase coefficient of the 532 nm beam. Figure 5 In the diagram, (e) and (f) represent the modal power ratio and phase coefficient of the 633 nm beam. The horizontal axis represents the index of the linearly polarized (LP) mode, and the vertical axis represents the modal power ratio or relative phase. Using the extracted modal power ratio and phase coefficient, the optical field on the 4f plane was synthesized according to the method described in step 4. The intensity and phase distributions at 450 nm, 532 nm, and 633 nm are shown below. Figure 6 (a) in 6 and (d) in 6 Figure 6 (b) in 6 and (e) in 6, and Figure 6 As shown in (c) in 6 and (f) in 6, and with Figure 4 The corresponding reconstruction results were compared. The synthesized light field closely matched the reconstructed light field in both intensity and phase. Quantitative analysis showed that the intensity correlation coefficients between the reconstructed light field and the synthesized light field at 450 nm, 532 nm, and 633 nm were 0.977, 0.981, and 0.986, respectively. These results confirm the accuracy of the extracted modal coefficients and further verify the reliability of the method of this invention.

[0057] In summary, this invention excites the few-mode fiber under test using a multi-wavelength laser source, modulates the mixed-wavelength optical field output from the fiber using a binary random amplitude plate (BRAP), and acquires a single-frame diffraction pattern using a detector. A low-rank mixed-state reconstruction algorithm with frequency domain constraints (MW-LF-CAMI) is used to simultaneously reconstruct the complex amplitude distribution of each wavelength component in the modulation plane from the single diffraction pattern. Subsequently, a virtual 4f system is constructed to propagate the complex amplitude values ​​of the modulation plane to the fiber endface, obtaining an amplified complex amplitude distribution at the fiber endface. Finally, a gradient descent algorithm is used to optimize spatial alignment, and modal coefficients at each wavelength are extracted through modal projection. This invention overcomes the limitations of traditional methods, which can only analyze single wavelengths or are affected by dispersion, achieving rapid, accurate, and calibration-free decomposition of multi-wavelength, multi-mode fiber optical fields in a single exposure.

[0058] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A few-mode fiber multi-wavelength and multi-mode hybrid mode decomposition device based on multi-wavelength lensless coherent modulation imaging, characterized in that, The device includes a first laser, a first beam splitter, a second beam splitter, a coupler, a single-mode fiber, a collimating lens, a polarizer, a modulation plate, and a detector, arranged sequentially along the light propagation path. A second laser is disposed on one side of the first beam splitter, and a third laser is disposed on one side of the second beam splitter. The first, second, and third lasers are used to emit laser beams of three different wavelengths, and the emission wavelengths of the three laser beams are all less than the cutoff wavelength of the single-mode fiber. The light emitted by the first and second lasers forms a first mixed beam after passing through the first beam splitter. The first mixed beam and the light emitted by the third laser form a second mixed beam after passing through the second beam splitter and are incident on the coupler. The coupler is connected to the single-mode fiber. The light-emitting end face of the single-mode fiber is placed at the front focal plane of the collimating lens. The polarizer is used to maintain the consistent polarization direction of the beam. The modulation plate is used to modulate the wavefront. The detector is used to collect the diffraction spots of the light field.

2. The device for decomposing few-mode fiber multi-wavelength and multi-mode hybrid modes based on multi-wavelength lensless coherent modulation imaging according to claim 1, characterized in that, The single-mode fiber is configured to support the propagation of multiple transverse modes under the excitation of the operating wavelengths of the first, second, and third lasers.

3. The few-mode fiber multi-wavelength and multi-mode hybrid mode decomposition device based on multi-wavelength lensless coherent modulation imaging according to claim 1, characterized in that, The polarizer is placed between the single-mode fiber and the detector.

4. The few-mode fiber multi-wavelength and multi-mode hybrid mode decomposition device based on multi-wavelength lensless coherent modulation imaging according to claim 1, characterized in that, The modulation board is an amplitude-type or phase-type modulation structure.

5. A method for decomposing multi-wavelength and multi-mode hybrid modes in few-mode fiber based on multi-wavelength lensless coherent modulation imaging, wherein the decomposition device described in any one of claims 1-4 is used for mode decomposition, characterized in that, Includes the following steps: Step 1: Acquire the intensity I of the few-mode light spot using the detector. R (x,y); Step 2: Reconstruct the complex amplitude using a multi-wavelength lensless coherent modulation imaging method, obtaining the reconstructed complex amplitude corresponding to each wavelength, and using P... k (x,y) represents the reconstructed complex amplitude of the k-th wavelength; Step 3: Based on scalar diffraction theory, construct a virtual 4f system using the focal length of the collimating lens, and apply the complex amplitude distribution P obtained in Step 2. k (x, y) is virtually transmitted through this system, and the amplified complex amplitude distribution E corresponding to the end face of the single-mode fiber is finally obtained on the back focal plane of the system. k,4f (x,y); Step 4: Convert the complex amplitude E obtained in Step 3 into... k,4f (x,y) and the eigenfield E of each mode k,LPj Perform orthogonal projection operations on (x,y) to obtain the mode complex coefficients C. k,j ; Step 5: The mode complex coefficients C obtained from Step 4 k,j Synthesize the mode field and calculate the reconstructed complex amplitude intensity. I k,rec With the strength of the synthetic mode field I k,syn Relevance Corr k The pattern decomposition is now complete.

6. The method for decomposing multi-wavelength and multi-mode hybrid modes of few-mode fiber based on multi-wavelength lensless coherent modulation imaging according to claim 5, characterized in that, The multi-wavelength lensless coherent modulation imaging method in step 2 specifically includes: Step 2-1: Generate arbitrary initial field distribution P k,0 ( x,y ) serves as the initial estimate of the incident beam in the iterative calculation; where the subscript k This indicates the wavelength number, and the subscript 0 indicates the initial iteration step. Step 2-2: In the first n In the next iteration, the first n The incident light field distribution in the next iteration P k,0 ( x,y ) and a predetermined modulation plate distribution t ( x , y Multiply by , and get the first . n Sub-iteration output light field distribution φ k,n ( x , y ): φ k,n ( x , y )= P n ( x , y )· t ( x, y ) Steps 2-3: [The text appears to be incomplete and contains several grammatical errors. A more accurate translation would require the full context.] n Sub-iteration output light field distribution φ k,n ( x , y The propagation reaches the detector plane, yielding the first... n Planar optical field distribution of the next iteration detector ψ k,n ( x, y ): ψ k,n ( x, y )= F { φ k,n ( x , y ), d} in, F {} indicates Fresnel propagation. d This represents the distance between the modulation plate and the detector; the total calculated intensity distribution is given by the following formula: Steps 2-4: Retain ψ k,n ( x, y The phase of the field is determined, and its amplitude is updated using a low-rank mixed-state algorithm that enforces energy conservation between wavelengths; the modulus of each field is determined by the measured intensity recorded by the detector. I R ( x , y The square root of ) is obtained, and the total strength is calculated. I cal ( x , y Normalization, as shown in the given formula; Steps 2-5: Backpropagation, ... ψ' k,n ( x , y The wave propagates in the reverse direction to the modulation plate plane, and the outgoing wave is updated using the following formula: φ' k,n ( x , y )= F -1 { ψ' k,n ( x , y ), d}； in, F -1 {} indicates reverse Fresnel propagation. d This indicates the distance between the modulation board and the detector. φ' k,n ( x , y ) represents the outgoing wave function of the modulation plate plane; Steps 2-6: Incident beam update. Update the incident beam on the modulation plate plane using the following formula: in, α To keep it as a constant, thus avoiding division by zero errors; Steps 2-7: Obtain the frequency domain distribution of the updated incident beam through two-dimensional Fourier transform: P’ k,n ( u , v )= f { P’ n (x, y),} in f {} represents a two-dimensional Fourier transform; Steps 2-8: Detector planar constraints, applying spatial constraints, the formula is: in, S n ( u , v ) represents a circular aperture function for a specific wavelength, whose radius is gradually increased during the iteration process to optimize the reconstruction effect; Steps 2-9: Propagate backward to the incident surface, and obtain the incident beam for the next iteration through reverse Fresnel propagation: in f -1 {} represents the inverse two-dimensional Fourier transform; Step 2-10: Convergence check, calculate the intensity on the diffraction surface. I cal ( x , y (and the intensity of the diffraction spot obtained from actual recording) I R ( x , y Error calculation is performed, and the calculation expression is as follows: When the error exponent between the two is greater than a preset value, it is considered that the iteration has converged, and the reconstructed complex amplitude spot is obtained. P k,n ( x , y ) is used for subsequent pattern decomposition processing.

7. The method for decomposing multi-wavelength and multi-mode hybrid modes of few-mode fiber based on multi-wavelength lensless coherent modulation imaging according to claim 6, characterized in that, In step 4, the mode complex coefficient C k,j The calculation formula is: in, E k,LPj (x, y) represents the characteristic mode field of each LP mode at the theoretical k-th wavelength. E k,4f (x, y) represents the amplified complex amplitude distribution corresponding to the single-mode fiber end face obtained in step 3. C k,j To correspond to the mode complex coefficients of the theoretical mode field, the magnitude value represents the mode energy proportion, and the argument represents the relative phase information.

8. The method for decomposing multi-wavelength and multi-mode hybrid modes of few-mode fiber based on multi-wavelength lensless coherent modulation imaging according to claim 7, characterized in that, In step 5, the complex amplitude intensity is reconstructed. I k,rec By analyzing the complex amplitude distribution of the 4f focal plane in step 3 E k,4f (x,y) It is obtained by taking the square of the modulus.

9. The method for decomposing multi-wavelength and multi-mode hybrid modes of few-mode fiber based on multi-wavelength lensless coherent modulation imaging according to claim 8, characterized in that, In step 5, the synthesized mode field strength I k,syn The method for obtaining the complex amplitude of the mode field under test is to synthesize it through modal superposition. E k,syn Specifically, based on coefficients C k,j The formula for linearly superimposing the various transmission modes is as follows: in, E k,LPj ( x,y ) represents the first wavelength of the optical fiber propagation field corresponding to the k-th wavelength. j Each linearly polarized eigenmode C k , j This indicates the corresponding k-th wavelength. j Modal complex coefficients of the mode field, ρ k,j It is the amplitude factor of the k-th wavelength. θ k,j It is the kth wavelength. j The phase difference between each higher-order mode and the fundamental mode; all eigenmode fields must be normalized, and then the complex amplitude is adjusted. E k,syn The square of the modulus is used to obtain the combined mode field strength I. k,syn .

10. The method for decomposing multi-wavelength and multi-mode hybrid modes of few-mode fiber based on multi-wavelength lensless coherent modulation imaging according to claim 9, characterized in that, In step 5, the correlation calculation uses intensity correlation calculation: in, I k,rec and I k,syn These are the reconstructed complex amplitude intensity and the synthetic mode field intensity, respectively. and These are their average values.

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