A method and system for spectral characterization of few-mode fiber Bragg gratings based on lightweight adaptive control

Abstract

This invention discloses a method and system for spectral characterization of few-mode fiber Bragg gratings based on lightweight adaptive control. The method includes: clarifying the multimode reflection characteristics and feedback mapping theoretical basis of the few-mode fiber Bragg grating; constructing an adaptive feedback closed-loop device containing the few-mode fiber Bragg grating based on the multimode reflection characteristics and feedback mapping theoretical basis; performing adaptive locking for different proportions of higher-order modes of the few-mode fiber Bragg grating through the adaptive feedback closed-loop device to characterize the evolution of the few-mode fiber Bragg grating and obtain a reflection spectrum dataset under different proportions of higher-order modes; and establishing a quantitative transfer matrix of the spectral characteristics and mode ratios of the few-mode fiber Bragg grating based on the reflection spectrum dataset.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic sensing and fiber optic testing technology, specifically relating to a few-mode fiber Bragg grating spectral characterization method and system based on lightweight adaptive control. Background Technology

[0002] With the widespread application of few-mode fiber technology, few-mode fiber Bragg gratings, as a key mode selection and sensing element, play an important role in optical communication and sensing. The reflection spectral characteristics of few-mode fiber Bragg gratings are closely related to the mode composition of the incident light field. Different proportions of the fundamental mode and higher-order mode components can cause wavelength shifts or bandwidth distortions in the grating reflection peaks.

[0003] Traditional few-mode fiber Bragg grating characterization schemes typically employ manual polarization adjustment or mechanical stress to excite higher-order modes. However, these methods have significant drawbacks: firstly, the proportion of mode components in the incident light is difficult to control precisely, continuously, and quantitatively, making it impossible to establish a precise mapping relationship between the mode proportion and spectral evolution. Secondly, existing mode control devices are susceptible to environmental temperature drift and mechanical vibration interference during long-term operation, resulting in highly unstable operation and uncontrollable changes in the incident mode field proportion, severely impacting the repeatability and accuracy of grating spectral characterization.

[0004] Therefore, how to provide a solution that can compensate for environmental disturbances in real time, accurately lock the mode component ratio, and be used for quantitative characterization of few-mode fiber Bragg gratings is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] This invention aims to address the shortcomings of existing technologies and provides the following solutions: A few-mode fiber Bragg grating spectral characterization method based on lightweight adaptive control includes the following steps: Clarify the multimode reflection characteristics of few-mode fiber Bragg gratings and the theoretical basis for feedback mapping; Based on the multimode reflection characteristics and the feedback mapping theory, an adaptive feedback closed-loop device containing a few-mode fiber Bragg grating is constructed. The adaptive feedback closed-loop device is used to perform adaptive locking for different proportions of higher-order modes in the few-mode fiber Bragg grating, and the evolution of the few-mode fiber Bragg grating is characterized to obtain a reflectance spectrum dataset under different proportions of higher-order modes. Based on the aforementioned reflectance spectral dataset, a quantitative transfer matrix of the spectral characteristics and mode ratio of a few-mode fiber Bragg grating is established.

[0006] Preferably, the adaptive feedback closed-loop device includes: a multi-source excitation unit, a mode control unit, an optical path control unit, a few-mode fiber Bragg grating characterization unit, a mode recognition unit, and an adaptive control unit; The multi-source excitation unit is used for mode field detection in the mode proportion feedback control stage and reflectance spectrum acquisition in the spectral measurement stage; The mode control unit is used to achieve energy coupling between different space modes; The optical path control unit is used to achieve directional transmission and path isolation of optical signals; The few-mode fiber Bragg grating characterization unit is used to acquire the reflection spectrum of the grating under test and extract spectral features under the input condition of controllable mode ratio, so as to establish the mapping relationship between the grating spectral response and the incident mode distribution. The pattern recognition unit and the adaptive control unit are integrated in the central processing unit. They are used to extract the real-time features of the pattern image to obtain the proportion parameters of each pattern, construct an error function, and generate a control signal based on the PID control algorithm to adjust the output parameters of the radio frequency signal source, thereby realizing closed-loop control of the pattern proportion.

[0007] Preferably, the multi-source excitation unit includes: a narrowband laser source and a broadband light source; The mode control unit includes: a piezoelectric drive assembly, an ultrasonic transducer, and a few-mode fiber coupling segment; The optical path control unit uses a few-mode fiber circulator.

[0008] Preferably, the method for obtaining the reflectance spectrum dataset includes: In the adaptive control unit, the target high-order mode ratio is set, and the narrowband laser source is turned on as a mode feedback signal source. The output mode field image is acquired in real time by a charge-coupled device, and the pattern recognition unit performs online decomposition on the output mode field image to obtain the ratio information of the current fundamental mode and the target higher-order mode. The collected proportional information is compared with the target value to generate an error signal, which is then input into an adaptive control algorithm to dynamically adjust the frequency and amplitude of the excitation power supply. This allows the piezoelectric drive component to apply high-frequency mechanical perturbations to the optical fiber, thereby continuously adjusting the mode coupling strength. Through closed-loop iteration, the proportion of the target high-order mode reaches the set value and remains stable; After the proportion of the target high-order modes stabilizes, the driving parameters remain unchanged, the excitation source is switched to the broadband light source, and the reflection spectrum of the few-mode fiber Bragg grating is collected to obtain the spectral data under the corresponding mode proportion. By changing the proportion of the target higher-order modes and repeatedly adaptively locking to the spectral acquisition process, the reflectance spectral dataset under different mode proportion conditions is obtained.

[0009] Preferably, the method for constructing the quantitative transfer matrix includes: Power information of multiple characteristic reflection peaks is extracted from the reflection spectrum dataset to construct a set of characteristic parameters corresponding to different mode proportion conditions; Using the proportion of the target high-order modes as input variables and the power of each feature reflection peak as output variables, a data-driven mapping model is constructed. During the modeling process, the input-output relationship is solved by function fitting or other regression methods to obtain the quantitative transfer matrix that describes the relationship between the spectral response of a few-mode fiber Bragg grating and the mode ratio.

[0010] The present invention also provides a few-mode fiber Bragg grating spectral characterization system based on lightweight adaptive control. The system applies the above-mentioned method and includes: a characteristic basis confirmation module, a feedback device construction module, a reflectance spectral data acquisition module, and a matrix construction module. The characteristic confirmation module is used to clarify the multimode reflection characteristics of few-mode fiber Bragg gratings and the theoretical basis for feedback mapping. The feedback device construction module constructs an adaptive feedback closed-loop device containing a few-mode fiber Bragg grating based on the multimode reflection characteristics and the feedback mapping theory. The reflectance spectral data acquisition module performs adaptive locking for different proportions of higher-order modes of the few-mode fiber Bragg grating through the adaptive feedback closed-loop device, performs evolution characterization of the few-mode fiber Bragg grating, and obtains reflectance spectral datasets under different proportions of higher-order modes. The matrix construction module establishes a quantitative transfer matrix of the spectral characteristics and mode ratios of a few-mode fiber Bragg grating based on the reflection spectrum dataset.

[0011] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) This invention, by introducing a PID closed-loop feedback control algorithm and combining it with an acoustic mode modulation unit, achieves precise locking and continuous control of the ratio of fundamental mode to higher-order mode components in a few-mode fiber. This invention can compensate for operating point drift caused by environmental disturbances in real time, ensuring the long-term stability of the incident light field ratio. This provides a stable and reliable testing environment for quantitatively characterizing the evolution of the reflection spectrum of a few-mode fiber Bragg grating with respect to mode components, effectively improving the accuracy and repeatability of grating device parameter characterization.

[0012] (2) This invention innovatively adopts a lightweight PID algorithm and an adaptive closed-loop feedback collaborative working mechanism. Through closed-loop control, it compensates for the drift of the operating point caused by environmental disturbances in real time, and locks the incident mode field components at the preset high-order mode proportion state for a long time. This solves the problem of uncontrollable modes and unstable characterization in complex environments by traditional control methods. Furthermore, it uses the spectral dataset collected under precise proportional switching to establish a quantitative transfer matrix, realizing a deep and quantitative characterization of the evolution law of the reflection spectrum of few-mode fiber Bragg gratings. Attached Figure Description

[0013] To more clearly illustrate the technical solution of the present invention, the drawings used in the embodiments are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0014] Figure 1 This is a schematic diagram of the method flow according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the few-mode Bragg grating structure and its reflection characteristics according to an embodiment of the present invention; Figure 3 This is a schematic diagram of the adaptive feedback closed-loop device for a few-mode fiber Bragg grating according to an embodiment of the present invention; Figure 4 This is a schematic diagram of the evolution of the multimode reflection spectrum of a few-mode fiber Bragg grating under different proportions of higher-order modes in an embodiment of the present invention. Among them, (a) is a schematic diagram of the reflection spectrum of a few-mode Bragg grating with different LP11 proportions, and (b) is a schematic diagram of the reflection spectrum of a few-mode Bragg grating with different LP21 proportions. Figure 5 The following is a schematic diagram of the fitting results of the characteristic reflection peaks of the few-mode fiber Bragg grating in an embodiment of the present invention: (a) is a schematic diagram of the intensity fitting of the three characteristic peaks under different input LP11 mode ratios; (b) is a schematic diagram of the comparison between the predicted peak power and the measured peak power; (c) is a schematic diagram of the intensity fitting of the five characteristic peaks under different input LP21 mode ratios; and (d) is a schematic diagram of the comparison between the predicted peak power and the measured peak power. Explanation of reference numerals in the attached figures: 1. Narrowband laser source; 2. First switching optical path; 3. Energy coupling position; 4. Few-mode light; 5. Few-mode fiber circulator; 6. Second switching optical path; 7. CCD; 8. Control terminal; 9. Spectral analysis device; 10. Few-mode fiber Bragg grating under test; 11. Signal generator; 12. RF power amplifier; 13. Broadband light source; 14. Control command line; 15. Cable; 16. Piezoelectric drive assembly; 17. Ultrasonic focusing device. Detailed Implementation

[0015] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0016] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0017] Example 1 In this embodiment, as Figure 1 As shown, a few-mode fiber Bragg grating spectral characterization method based on lightweight adaptive control includes the following steps: S1. Clarify the multimode reflection characteristics of few-mode fiber Bragg gratings and the theoretical basis for feedback mapping.

[0018] In this embodiment, the spatial distribution of wavelength and intensity mapping of few-mode fiber Bragg gratings under multimode incidence are first clarified as the physical basis for adaptive characterization. For example... Figure 2 As shown, in the few-mode fiber Bragg grating used in this embodiment, due to the difference in effective refractive index between the fundamental mode and the higher-order modes, according to the Bragg phase-matching condition, ,in, For the first m The effective refractive index of the first mode, The period of the grating indicates that different spatial modes, due to their different effective refractive indices, correspond to different reflection center wavelengths, resulting in distinct characteristic peaks for modes of different orders (m) in the reflection spectrum. Experimentally, it was observed that the relative intensity of the characteristic peaks of each mode in the reflection spectrum evolves as the proportion of mode components incident on the few-mode fiber Bragg grating changes.

[0019] The ratio of the peak power of the higher-order mode reflection peak to the peak power of the fundamental mode reflection peak in the real-time reflection spectrum is used as a feedback characteristic parameter. Experimental data show that this feedback characteristic parameter has a good monotonic correspondence with the proportion of higher-order modes in the incident mode field within the dynamic control range. This physical characteristic constitutes the logical premise of adaptive closed-loop control, namely, by monitoring the intensity evolution of the few-mode fiber Bragg grating reflection spectrum in real time, the mode component ratio of the current incident mode field can be deduced, thereby providing accurate error signal input for the subsequent adaptive control algorithm.

[0020] S2. Based on the multimode reflection characteristics and feedback mapping theory, an adaptive feedback closed-loop device containing a few-mode fiber Bragg grating is constructed.

[0021] The adaptive feedback closed-loop device includes: a multi-source excitation unit, a mode control unit, an optical path control unit, a few-mode fiber Bragg grating characterization unit, a mode recognition unit, and an adaptive control unit.

[0022] The multi-source excitation unit includes a narrowband laser source 1 and a broadband light source 13, which are selectively accessed through an optical path switcher. The narrowband laser source 1 is used for mode field detection in the mode proportion feedback control stage, and the broadband light source 13 is used for reflectance spectrum acquisition in the spectral measurement stage.

[0023] The mode control unit is used to achieve energy coupling between different spatial modes. The mode control unit includes a piezoelectric drive assembly 16, an ultrasonic transducer, and a few-mode fiber coupling section. The coating layer of the few-mode fiber is removed within a preset length of 12 cm, and it is fixed to the surface of the piezoelectric drive assembly 16 with optical adhesive. Under a certain driving frequency and driving voltage, it generates periodic perturbations, thereby achieving controllable coupling between different spatial modes. The periodic perturbations generated in a local region of the few-mode fiber under piezoelectric driving can be expressed as follows: ,in, For refractive index modulation amplitude, f For driving frequency, k The space wavenumber is used to alter the phase-matching conditions between modes, thereby enabling energy coupling between different space modes.

[0024] The optical path control unit is used to achieve directional transmission and path isolation of optical signals. The optical path control unit adopts a few-mode fiber circulator 5, whose second port is connected to the few-mode fiber Bragg grating 10 under test, and the third port is cascaded with a charge-coupled device and a spectral analysis system, respectively.

[0025] The few-mode fiber Bragg grating characterization unit is used to acquire the reflection spectrum of the grating under test and extract spectral features under input conditions with a controllable mode ratio, in order to establish a mapping relationship between the grating spectral response and the incident mode distribution. Its applications include, but are not limited to, multimode reflection characteristic analysis and mode coupling behavior characterization for few-mode or single-mode fiber Bragg gratings. Through the acquired correlation data, it is possible to evaluate the sensitivity of the grating under test in a multimode fiber sensing system or to evaluate the mode gain matching performance in a multimode fiber laser system, thereby guiding the optimized design of fiber optic devices and system compensation.

[0026] The pattern recognition unit and adaptive control unit are integrated into the central processing unit to extract features from the pattern image in real time, obtain the proportion parameters of each pattern, and construct an error function. ,in, For the proportion of the target mode, The current proportion of this mode is determined, and a control signal is generated based on the PID control algorithm to adjust the output parameters of the radio frequency signal source, thereby achieving closed-loop control of the mode proportion.

[0027] S3. Adaptive locking is performed on the few-mode fiber Bragg grating for different proportions of higher-order modes using an adaptive feedback closed-loop device, and the evolution of the few-mode fiber Bragg grating is characterized to obtain a reflectance spectrum dataset under different proportions of higher-order modes.

[0028] In this embodiment, the method for obtaining the reflectance spectrum dataset includes: In the adaptive control unit, the target high-order mode ratio is set, and the narrowband laser source 1 is turned on as the mode feedback signal source. The output mode field image is acquired in real time through a charge-coupled device (CCD), and the mode recognition unit performs online decomposition of the output mode field image to obtain the ratio information between the current fundamental mode and the target high-order mode. The acquired ratio information is compared with the target value, an error signal is generated and input into the adaptive control algorithm, and the frequency and amplitude of the excitation power supply are dynamically adjusted so that the piezoelectric drive component 16 applies high-frequency mechanical perturbation to the optical fiber, continuously adjusting the mode coupling strength. Through closed-loop iteration, the target high-order mode ratio reaches the set value and remains stable. After the target high-order mode ratio stabilizes, the drive parameters are kept unchanged, and the excitation source is switched to a broadband light source 13 to acquire the reflection spectrum of the few-mode fiber Bragg grating, obtaining spectral data under the corresponding mode ratio. By changing the target high-order mode ratio and repeating the adaptive locking to spectral acquisition process, reflection spectrum datasets under different mode ratio conditions are obtained, realizing a systematic characterization of the spectral response of the few-mode fiber Bragg grating as a function of mode input. The measurement results are as follows: Figure 4 As shown in (a).

[0029] Furthermore, when the target mode is extended from LP11 to LP21 or other higher-order modes, only the target mode type and corresponding proportion parameters need to be modified in the adaptive control unit. This allows for the reuse of the aforementioned closed-loop control and spectral acquisition process, achieving unified characterization of spectral data under multimodal conditions. Figure 4 As shown in (b).

[0030] S4. Based on the reflectance spectral dataset, establish a quantitative transfer matrix for the spectral characteristics and mode ratio of a few-mode fiber Bragg grating.

[0031] In this embodiment, the method for constructing the quantitative transfer matrix includes: extracting power information of multiple characteristic reflection peaks from the reflectance spectrum dataset, constructing a set of characteristic parameters corresponding to different mode proportion conditions; using the target higher-order mode proportion as the input variable and the power of each characteristic reflection peak as the output variable, constructing a data-driven mapping model; during the modeling process, solving the input-output relationship through function fitting or other regression methods to obtain the quantitative transfer matrix describing the relationship between the spectral response of the few-mode fiber Bragg grating and the mode proportion.

[0032] Furthermore, for different combinations of higher-order modes, only the input variables and corresponding feature parameter sets need to be adjusted to reuse the modeling method and obtain a unified mapping relationship applicable to multimode coupling scenarios. Through the quantitative transfer model, the incident mode ratio can be deduced from spectral features, or the corresponding spectral response can be predicted based on the target mode ratio, thereby completing the quantitative characterization of few-mode fiber Bragg gratings.

[0033] Example 2 In this implementation, Figure 2 This figure illustrates the multimode reflection characteristics and physical principles of few-mode fiber Bragg gratings. The left side of the figure shows a schematic diagram of the few-mode fiber Bragg grating structure, where the grating region forms a reflective structure through periodic refractive index modulation. When a composite light field containing the fundamental mode and higher-order modes is incident, different modes generate reflections under phase-matching conditions. The right side shows the corresponding multimode reflection spectrum, which includes the fundamental mode self-coupling reflection peak, the higher-order mode self-coupling reflection peak, and the mutual coupling reflection peaks between different modes, each distributed at different wavelengths. The center wavelength of different reflection peaks is related to the effective refractive index of the mode and the grating period, and their intensity changes with the power distribution of the incident mode. This figure illustrates the basic reflection characteristics of few-mode fiber Bragg gratings under multimode incident conditions, providing a basis for subsequent spectral characterization based on mode proportion modulation.

[0034] Figure 3The diagram shows the experimental setup for characterizing the evolution of few-mode fiber Bragg gratings. A narrowband laser source 1 or a broadband light source 13 is connected to the system via a first switching optical path 2. During the mode-locking phase, the narrowband laser source 1 is connected, and a signal generator 11 generates an RF signal, which drives a piezoelectric drive assembly 16 to generate ultrasonic vibration via a control command line 14, an RF power amplifier 12, and a cable 15. The vibration is amplified by an ultrasonic concentrator 17 and injected at the energy coupling position 3 with the coating removed into the transmission few-mode light 4. The excited mode is guided by a few-mode fiber circulator 5 and enters a CCD 7 via a second switching optical path 6 for mode field monitoring. The control terminal 8 is responsible for algorithm processing and feedback control to achieve target proportion locking. During the characterization phase, the system switches to the broadband light source 13, and the controlled mode enters the few-mode fiber Bragg grating 10 under test via the switching optical path for characterization. Finally, a spectral analysis device 9 is used to collect the spectral evolution data of the few-mode fiber Bragg grating 10 under test in real time.

[0035] Figure 4 A schematic diagram of the multimode reflection spectrum evolution of a few-mode fiber Bragg grating under different proportions of higher-order modes is shown. Figure 4 In Figure (a), the reflectance spectra are shown under varying LP11 mode proportions, with each curve corresponding to a steady-state spectrum with different preset LP11 proportions. It can be observed that as the LP11 mode proportion gradually increases, the relative intensities of the fundamental mode self-coupling peak, higher-order mode self-coupling peak, and mutual coupling peak in the spectrum change continuously. Figure 4 In Figure (b), the reflectance spectrum is shown under the change of LP21 mode proportion. Each curve corresponds to the steady-state spectrum with different preset LP21 proportions. As the LP21 proportion increases, in addition to the LP21 self-coupling peak and the mutual coupling peak with LP11, additional higher-order coupling peaks appear in the spectrum, reflecting the energy distribution evolution under multimode input conditions.

[0036] This figure illustrates the continuous variation of the multimode reflection spectrum of a few-mode fiber Bragg grating with the mode ratio under programmed control conditions, providing experimental basis for quantitative characterization and subsequent mapping matrix establishment.

[0037] Figure 5 The fitting results of the characteristic reflection peaks of few-mode fiber Bragg gratings are presented. Figure 5 Figure (a) shows the experimental measurements and fitting curves of the characteristic reflection peak power as a function of the incident LP11 power under LP11 mode input, reflecting the nonlinear response relationship between the fundamental mode self-coupling peak, the LP11 self-coupling peak, and the fundamental mode and LP11 mutual coupling peak. Figure 5 (b) is Figure 5 The comparison between the predicted values ​​of the fitted model and the actual measured values ​​in (a) shows that the predicted power of each characteristic peak is linearly correlated with the measured power, indicating that the fitted model can effectively reflect the grating response under the LP11 mode input condition. Figure 5In (c), the experimental measurement values and fitting curves of the characteristic reflection peak power varying with the incident LP21 power in the case of LP21 mode input are shown. It includes 5 characteristic peaks such as the LP21 self-coupling peak, the LP21-LP11 cross-coupling peak, and the LP01-related peak, demonstrating a more complex multi-mode coupling response. Figure 5 In (d), it is Figure 5 The comparison chart between the predicted values and the actual measurement values of the fitting model in (c) of the figure. The predicted peak powers and the measurement values have a high linear correlation, verifying the reliability of the fitting model in the multi-mode system.

[0038] This figure is used to illustrate that the quantitative mapping model established by the data-driven method can accurately describe the multi-mode reflection characteristics of the few-mode fiber Bragg grating under different high-order mode ratios, providing an experimental basis for spectral characterization and mapping matrix establishment.

[0039] Embodiment III In this embodiment, a few-mode fiber Bragg grating spectral characterization system based on lightweight adaptive control includes: a characteristic basis confirmation module, a feedback device construction module, a reflected spectrum data acquisition module, and a matrix construction module.

[0040] The characteristic basis confirmation module is used to clarify the multi-mode reflection characteristics of the few-mode fiber Bragg grating and the theoretical basis of feedback mapping. The feedback device construction module constructs an adaptive feedback closed-loop device including the few-mode fiber Bragg grating based on the multi-mode reflection characteristics and the theoretical basis of feedback mapping. The reflected spectrum data acquisition module performs adaptive locking for different high-order mode ratios of the few-mode fiber Bragg grating through the adaptive feedback closed-loop device, conducts the evolution characterization of the few-mode fiber Bragg grating, and obtains a reflected spectrum data set under different high-order mode ratio conditions. The matrix construction module establishes a quantitative transfer matrix between the spectral characteristics and the mode ratio of the few-mode fiber Bragg grating based on the reflected spectrum data set.

[0041] The above-described embodiments are only descriptions of the preferred embodiments of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. A few-mode fiber Bragg grating spectral characterization method based on lightweight adaptive control, characterized in that, Includes the following steps: Clarify the multimode reflection characteristics of few-mode fiber Bragg gratings and the theoretical basis for feedback mapping; Based on the multimode reflection characteristics and the feedback mapping theory, an adaptive feedback closed-loop device containing a few-mode fiber Bragg grating is constructed. The adaptive feedback closed-loop device is used to perform adaptive locking for different proportions of higher-order modes in the few-mode fiber Bragg grating, and the evolution of the few-mode fiber Bragg grating is characterized to obtain a reflectance spectrum dataset under different proportions of higher-order modes. Based on the aforementioned reflectance spectral dataset, a quantitative transfer matrix of the spectral characteristics and mode ratio of a few-mode fiber Bragg grating is established.

2. The few-mode fiber Bragg grating spectral characterization method based on lightweight adaptive control according to claim 1, characterized in that, The adaptive feedback closed-loop device includes: a multi-source excitation unit, a mode control unit, an optical path control unit, a few-mode fiber Bragg grating characterization unit, a mode recognition unit, and an adaptive control unit; The multi-source excitation unit is used for mode field detection in the mode proportion feedback control stage and reflectance spectrum acquisition in the spectral measurement stage; The mode control unit is used to achieve energy coupling between different space modes; The optical path control unit is used to achieve directional transmission and path isolation of optical signals; The few-mode fiber Bragg grating characterization unit is used to acquire the reflection spectrum of the grating under test and extract spectral features under the input condition of controllable mode ratio, so as to establish the mapping relationship between the grating spectral response and the incident mode distribution. The pattern recognition unit and the adaptive control unit are integrated in the central processing unit. They are used to extract the real-time features of the pattern image to obtain the proportion parameters of each pattern, construct an error function, and generate a control signal based on the PID control algorithm to adjust the output parameters of the radio frequency signal source, thereby realizing closed-loop control of the pattern proportion.

3. The few-mode fiber Bragg grating spectral characterization method based on lightweight adaptive control according to claim 2, characterized in that, The multi-source excitation unit includes: a narrowband laser source and a broadband light source; The mode control unit includes: a piezoelectric drive assembly, an ultrasonic transducer, and a few-mode fiber coupling segment; The optical path control unit uses a few-mode fiber circulator.

4. The few-mode fiber Bragg grating spectral characterization method based on lightweight adaptive control according to claim 3, characterized in that, The methods for obtaining the reflectance spectrum dataset include: In the adaptive control unit, the target high-order mode ratio is set, and the narrowband laser source is turned on as a mode feedback signal source. The output mode field image is acquired in real time by a charge-coupled device, and the pattern recognition unit performs online decomposition on the output mode field image to obtain the ratio information of the current fundamental mode and the target higher-order mode. The collected proportional information is compared with the target value to generate an error signal, which is then input into an adaptive control algorithm to dynamically adjust the frequency and amplitude of the excitation power supply. This allows the piezoelectric drive component to apply high-frequency mechanical perturbations to the optical fiber, thereby continuously adjusting the mode coupling strength. Through closed-loop iteration, the proportion of the target high-order mode reaches the set value and remains stable; After the proportion of the target high-order modes stabilizes, the driving parameters remain unchanged, the excitation source is switched to the broadband light source, and the reflection spectrum of the few-mode fiber Bragg grating is collected to obtain the spectral data under the corresponding mode proportion. By changing the proportion of the target higher-order modes and repeatedly adaptively locking to the spectral acquisition process, the reflectance spectral dataset under different mode proportion conditions is obtained.

5. The few-mode fiber Bragg grating spectral characterization method based on lightweight adaptive control according to claim 4, characterized in that, The method for constructing the quantitative transfer matrix includes: Power information of multiple characteristic reflection peaks is extracted from the reflection spectrum dataset to construct a set of characteristic parameters corresponding to different mode proportion conditions; Using the proportion of the target high-order modes as input variables and the power of each feature reflection peak as output variables, a data-driven mapping model is constructed. During the modeling process, the input-output relationship is solved by function fitting or other regression methods to obtain the quantitative transfer matrix that describes the relationship between the spectral response of a few-mode fiber Bragg grating and the mode ratio.

6. A few-mode fiber Bragg grating spectral characterization system based on lightweight adaptive control, wherein the system applies the method described in any one of claims 1-5, characterized in that, include: The feature is based on a confirmation module, a feedback device construction module, a reflectance spectral data acquisition module, and a matrix construction module; The characteristic confirmation module is used to clarify the multimode reflection characteristics of few-mode fiber Bragg gratings and the theoretical basis for feedback mapping. The feedback device construction module constructs an adaptive feedback closed-loop device containing a few-mode fiber Bragg grating based on the multimode reflection characteristics and the feedback mapping theory. The reflectance spectral data acquisition module performs adaptive locking for different proportions of higher-order modes of the few-mode fiber Bragg grating through the adaptive feedback closed-loop device, performs evolution characterization of the few-mode fiber Bragg grating, and obtains reflectance spectral datasets under different proportions of higher-order modes. The matrix construction module establishes a quantitative transfer matrix of the spectral characteristics and mode ratios of a few-mode fiber Bragg grating based on the reflection spectrum dataset.

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