A method for identifying faults and disturbances in few-mode fiber Raman-enhanced time-domain reflection.

Abstract

This invention belongs to the field of optical fiber testing and sensing technology, specifically relating to a method for identifying faults and disturbances in few-mode optical fibers using Raman-enhanced time-domain reflectometry (TDDR). The method includes: signal transmission based on pulse coding and mode-selective excitation; signal-to-noise ratio enhancement based on distributed Raman amplification; mode demultiplexing and mode field detection; coded demodulation and reconstruction using a multi-mode few-mode optical TDDR; multi-dimensional fault identification based on mode-dependent loss and coupling characteristics; and multi-mode differential and high-precision positioning based on digital signal processing. This invention targets high-capacity communication and distributed sensing systems using few-mode fibers. It solves the problem of detecting weak inter-mode scattering signals by coordinating pulse coding gain and distributed Raman optical gain. Subsequently, it utilizes the unique mode-dependent loss and inter-mode energy transfer characteristics of few-mode fibers for joint decision-making, providing highly sensitive, multi-dimensional optical fiber link health monitoring capabilities. This method has great potential and application prospects in the operation and maintenance of next-generation space-multiplexed optical networks.

Description

[0001] This invention relates to the field of optical communication, and specifically to a method for identifying Raman-enhanced time-domain reflection faults and disturbances in few-mode optical fibers. Background Technology

[0002] With the explosive growth in demand for fiber optic communication capacity, space division multiplexing technology based on few-mode fiber has become an important evolution direction for next-generation optical networks. In order to ensure the physical integrity and transmission quality of few-mode fiber links, it is crucial to develop high-performance distributed monitoring equipment for the characteristics of few-mode fiber. As an instrument that can simultaneously detect the fundamental mode and higher-order modes, the few-mode optical time domain reflectometer not only needs to have the traditional testing capabilities of long distance and high resolution, but is also highly anticipated by the industry in order to use the unique multimodal characteristics of few-mode fiber to solve the problems of accurate fault location and multidimensional parameter perception in complex link environments.

[0003] While existing optical time-domain reflectometry (OTDR) technology is relatively mature in single-mode fiber applications, it still faces numerous challenges in few-mode fiber applications. Although traditional single-mode OTDR technology can improve dynamic range by increasing pulse width, this inevitably leads to a decrease in spatial resolution, making it difficult to balance long-distance detection and high-precision positioning. To resolve the contradiction between signal-to-noise ratio and resolution, pulse coding technology has been introduced into few-mode OTDR, effectively improving the measurement dynamic range. Meanwhile, distributed Raman amplification technology has also been used to compensate for long-distance transmission loss. However, most of the existing solutions focus only on enhancing information in the single dimension of optical power. When dealing with few-mode fibers, they often ignore the key characteristic of intermodal differences. For example, fiber macrobending and splice loss both manifest as power attenuation on the OTDR curve. Relying solely on a single mode or simple light intensity information cannot effectively distinguish fault mechanisms, easily leading to misjudgments in operation and maintenance. In addition, existing Raman amplification schemes are mostly limited to single-mode systems and have not been optimized for the problem of high loss in higher-order modes in few-mode fibers.

[0004] In summary, there is an urgent need in the field for a comprehensive detection method. This method should not only overcome the signal-to-noise ratio limitations in long-distance few-mode fiber transmission and resolve the contradiction between detection distance and spatial resolution, but more importantly, it should fully explore and utilize the differences in sensitivity between the fundamental mode and higher-order modes in few-mode fibers to external physical quantities such as bending and stress. Developing a few-mode fiber pulse-coded Raman-enhanced few-mode optical time-domain reflectometry (TDDR) technology that can simultaneously achieve signal-to-noise ratio enhancement, high-precision positioning, and accurate fault type classification and identification has significant application value for improving the intelligent operation and maintenance level of next-generation optical networks. Summary of the Invention

[0005] This invention provides a method for identifying faults and disturbances in few-mode fiber Raman-enhanced time-domain reflection. This invention is a distributed fiber optic detection method with high signal-to-noise ratio, excellent spatial resolution, and fault classification capability, which can effectively identify and locate multidimensional faults and disturbances in long-distance few-mode fiber optic communication and sensing links.

[0006] By acquiring enhanced multimode signals through pulse-code excitation and distributed Raman amplification, the system signal-to-noise ratio (SNR) can be significantly improved, and the signal attenuation problem in long-distance transmission can be solved. Decoding the acquired data and reconstructing multimode few-mode optical time-domain reflectometer (TDDR) curves can eliminate coding interference and restore high-precision fiber optic state information. Fault identification and disturbance sensing based on multimode characteristic differences, utilizing the sensitivity differences between the fundamental mode and higher-order modes to bending and stress responses, such as mode-dependent loss and intermode coupling, can accurately distinguish static fault types such as macrobending and fracture, and locate external vibration disturbances in real time. Multimode differential and high-precision positioning can suppress common-mode noise, improve the detection SNR of weak events, and highlight fault characteristic peaks. This technology has great potential and application scenarios in the operation and maintenance of optical fiber communication networks and distributed optical fiber sensing.

[0007] A method for identifying faults and disturbances in few-mode fiber Raman-enhanced time-domain reflection includes:

[0008] Based on pulse-code emission and mode-selective excitation, the specific steps are as follows:

[0009] A signal source controlled by an arbitrary waveform generator or FPGA generates a Golay complementary pseudo-random coded sequence with high autocorrelation characteristics. This electrical signal drives an MZM high-speed electro-optic modulator to modulate the continuous light output from a narrow-linewidth laser into a coded light pulse train with high autocorrelation characteristics. Specifically, a pair of complementary Golay coded sequences are constructed. and The length of the sequence is L. This pair of sequences satisfies the ideal autocorrelation property, that is, the sum of their autocorrelation functions is a Dirac delta function. A function, mathematically expressed as: ,in and Let A and B represent the autocorrelation functions of sequence A and sequence B, respectively. As a time delay variable, this positive interactive complementarity can completely eliminate sidelobe interference in mathematical terms, achieving the ideal pulse compression effect.

[0010] The encoded optical pulse train enters the mode-selective exciter, which adopts a photonic lantern structure. By controlling the input port or phase distribution, it selectively couples optical energy into the fundamental mode or specific higher-order modes of the few-mode fiber. During this process, the excitation purity must be strictly controlled to suppress the generation of non-target modes and ensure that the initial intermode crosstalk is below -20dB.

[0011] Based on distributed Raman amplification, the specific steps are as follows:

[0012] A Raman amplification unit is placed at the end of the sensing fiber optic link. Reverse pumping technology is used to excite stimulated Raman scattering of the signal generated in step 1 within the few-mode fiber optic link. The signal power is then detected within the fiber optic link. The evolution along the distance z follows the stimulated Raman scattering equation: ,in For fiber input signal power, The signal light attenuation coefficient, The Raman gain coefficient is... The pump power is distributed along the optical fiber, and the pump power is adjusted. This allows the distributed gain generated by the integral term to effectively offset transmission losses. This balances the signal strength of each mode and improves the signal-to-noise ratio of the entire link. In order to correspond to the 13THz frequency shift of the quartz fiber, the center wavelength of the pump light is selected to be about 100nm lower than the wavelength of the probe light. The pump power is adjusted so that the high-power continuous wave pump light generates stimulated Raman scattering along the entire length of the fiber, forming a distributed optical gain that is distributed in the reverse direction along the fiber.

[0013] Based on the encoding demodulation and multi-mode optical time-domain reflectometer curve reconstruction, the specific steps are as follows:

[0014] In step 2, the backscattered Rayleigh light field returned by the few-mode fiber is separated into independent fundamental mode channels by a mode demultiplexer. ) and higher-order mode channels ( (etc.), the signals from each channel are converted into raw electrical signals by a photodetector array. , where k represents different mode channels.

[0015] The original electrical signal is processed by matched filtering. Decoding is performed, and the data is digitized by a high-speed data acquisition card. At the receiving end, the digital signal processing module utilizes the locally stored encoded sequence. and For reference, with the received signal and By performing cross-correlation and superposition operations, the impulse response function of the optical fiber link is reconstructed. : , where * denotes convolution or correlation operation, this step compresses the broadened coded signal back into a narrow pulse, thereby obtaining mode-independent few-mode optical time-domain reflectometer curves with high signal-to-noise ratio.

[0016] Based on mode-dependent loss-based fiber optic fault identification, the specific steps are as follows:

[0017] Based on the independent few-mode optical time-domain reflectometer curves obtained in step 3, a decision is made based on the loss differences and intermode coupling characteristics of each mode curve. When macrobending occurs in the fiber, the mode field diameter of higher-order modes is larger, making them more sensitive to bending. Their radiation loss is much greater than that of the fundamental mode, and the higher-order mode curves show a significant power drop step, while the fundamental mode curve shows a smaller or insignificant drop. The mode-dependent loss at z is defined as... The normalized logarithmic difference in optical power between the fundamental mode and higher-order modes: If there is a sudden loss at a certain location and , If a preset bending threshold is set, it is determined to be a macro-bending fault; if a significant power drop is detected in both the fundamental mode and higher-order modes at the same location, and the losses of each mode decrease synchronously and... If it remains stable, it is determined to be a break or weld point loss;

[0018] Based on pattern coupling-based distributed disturbance sensing, the specific steps are as follows:

[0019] Based on the independent few-mode digital signal processor curves obtained in step 3, external vibrations or stress disturbances are identified using the inter-mode coupling effect.

[0020] When an external force is applied to a part of an optical fiber, the fundamental mode energy is partially coupled to higher-order modes.

[0021] By monitoring the correlation of signals from each mode in real time, a local mode coupling coefficient is defined. As a criterion for perturbation: When the backscattered energy of the fundamental mode decreases instantaneously while the energy of the higher-order modes increases accordingly, and the algebraic sum of the energy changes of the two is approximately zero, it is determined that an external disturbance event has occurred at that location.

[0022] Based on multi-mode differential and high-precision positioning, the specific steps are as follows:

[0023] Based on the independent few-mode optical time-domain reflectometer curves obtained in step 3, differential operations and joint decisions are performed on the few-mode optical time-domain reflectometers of different modes to construct differential signals. : ,in The fundamental mode curve, For higher-order mode curves, the common-mode noise is suppressed, the signal-to-noise ratio of weak events is improved, and the fault characteristic peaks are highlighted by subtracting the fundamental mode few-mode optical time-domain reflectometer curves acquired at the same time from the higher-order mode few-mode optical time-domain reflectometer curves.

[0024] Based on the slight difference in group velocity between the fundamental mode and higher-order modes, the event location is calibrated using this known offset. Combined with the steeper rising and falling edges of the differential signal, sub-meter-level positioning accuracy is achieved, which is superior to the spatial resolution of a single-mode few-mode optical time-domain reflectometer.

[0025] A mode-selective exciter includes an optical input port, a mode conversion unit, and an optical output port. The optical input port is used to receive single-mode coded optical pulses from a modulator. The mode conversion unit is used to modulate and transform the complex amplitude distribution of the optical field of the coded optical pulses to selectively couple optical energy into a specific spatial mode of a few-mode fiber. The optical output port is used to guide the excited specific mode optical signal into the few-mode fiber link under test for distributed detection.

[0026] Optionally, the mode conversion unit is an all-fiber photonic lantern structure. The mode conversion unit is configured to independently excite the fundamental mode or higher-order modes, and the intermode crosstalk suppression ratio for non-target modes is higher than a preset threshold, so as to ensure that the optical pulse entering the few-mode fiber maintains the single-mode transmission characteristics.

[0027] An electronic device includes a memory and a processor, wherein the memory stores a computer program, and the processor runs the computer program to enable the electronic device to perform the above-described multidimensional fault and disturbance identification method based on a few-mode fiber pulse-coded Raman-enhanced few-mode optical time-domain reflectometer.

[0028] Optionally, the memory is a non-transitory computer-readable storage medium storing a computer program. When the computer program is executed by a processor, it implements the above-mentioned multi-dimensional fault and disturbance identification method for pulse-coded Raman-enhanced few-mode optical time-domain reflectometer based on few-mode fiber.

[0029] According to specific embodiments provided by the present invention, the following technical effects are disclosed: Addressing the difficulties in detecting weak backscattered signals and the limited fault type identification in long-distance few-mode fiber links, the present invention significantly improves the signal-to-noise ratio and dynamic range of the system through the synergistic effect of pulse-code excitation and distributed Raman amplification. Utilizing the distributed gain generated by reverse pumping, the transmission loss difference between higher-order modes and the fundamental mode in the few-mode fiber is effectively balanced, solving the problem of limited long-distance detection of higher-order modes. Based on the acquired multimode backscattered signals, the present invention reconstructs high-precision independent optical time-domain reflection signal curves for each mode through matched filtering decoding, and utilizes the fundamental mode and higher-order modes... By leveraging the differences in sensitivity to external physical quantities such as bending and stress, this invention enables accurate classification and identification of static faults such as macro-bending and fracture, as well as real-time perception of external dynamic disturbances. Through multi-mode differential computation and joint decision-making mechanisms, this invention suppresses Rayleigh scattering random noise while further improving event location accuracy by utilizing inter-mode group velocity differences. Compared to traditional single-mode optical time-domain reflectometry, this invention not only overcomes the contradiction between detection distance and spatial resolution but also breaks through the limitations of single-intensity detection, providing a highly sensitive, multi-dimensional fiber optic link health monitoring method that significantly improves the operation and maintenance efficiency and fault diagnosis capabilities of next-generation spatial division multiplexing optical networks. Attached Figure Description

[0030] Figure 1 This is a flowchart of the multidimensional fault and disturbance identification method for a pulse-coded Raman-enhanced few-mode optical time-domain reflectometer based on few-mode fiber according to the present invention.

[0031] Figure 2 This is a system structure diagram of the multidimensional fault and disturbance identification method of the pulse-coded-Raman enhanced few-mode optical time-domain reflectometer based on few-mode fiber in this invention.

[0032] Figure 3 The following is a schematic diagram of the matched filtering and correlation operation provided in this invention: (a) Matched filtering result of path A; (b) Matched filtering result of path B; (c) Final demodulation result.

[0033] Figure 4 This is a diagram of the internal structure of the detection and processing module provided in this invention. Detailed Implementation

[0034] 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.

[0035] The purpose of this invention is to provide a method, system, and device for multidimensional fault and disturbance identification based on a pulse-coded Raman-enhanced few-mode optical time-domain reflectometer (TDDR) using few-mode fiber. This method has a high signal-to-noise ratio, excellent spatial resolution, and fault classification capability, and can effectively identify and locate multidimensional faults and disturbances in long-distance few-mode fiber communication and sensing links.

[0036] 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.

[0037] Example 1:

[0038] To address the growing operational and maintenance demands of space-division multiplexing optical networks and to solve the problems of high high-order mode loss in few-mode fiber links, the inability of traditional optical time-domain reflectometers to distinguish fault types, and insufficient signal-to-noise ratio for long-distance detection, this invention designs a pulse-coded Raman-enhanced few-mode optical time-domain reflectometry multidimensional fault identification method based on few-mode fiber.

[0039] The overall system architecture of this embodiment includes: a narrow linewidth laser source, Golay pulse code modulation, a photonic lantern structure mode-selective excitation, a distributed Raman amplification transmission link, a mode demultiplexing receiver module, and a detection and processing digital signal processing unit. Its core innovation lies in combining the gain advantage of pulse coding with the loss compensation characteristics of Raman amplification, and utilizing the unique inter-mode differences such as mode-dependent loss and mode coupling of few-mode fiber to achieve accurate classification of link faults and real-time perception of weak disturbances.

[0040] like Figure 1 As shown, the specific implementation steps and algorithm flow are as follows:

[0041] An arbitrary waveform generator is used to generate a Golay coded sequence A and its complementary coded sequence B. This drives an electro-optic modulator to modulate narrow-linewidth continuous light into a sequence of optical pulses. The transmitted complementary Golay sequence has a length of L, and the sequence elements... Its ideal autocorrelation characteristics satisfy the formula: , where * indicates related operations, Using the Dirac function, the optical pulse is excited by mode polling in different time slots through a mode-selective exciter, which can independently excite the fundamental mode. and higher-order models For example, the intermodal extinction ratio of the exciter must be greater than 20dB to ensure the mode purity of the initial emitted light field.

[0042] To compensate for the large attenuation coefficient of higher-order modes in few-mode fibers, a backpump light with a wavelength reduced by 100 nm is used. This backpump light is coupled into the fiber via a wavelength division multiplexer. Based on stimulated Raman scattering, the signal light acquires distributed gain during transmission, increasing optical power. With distance The evolution follows the following differential equation: ,in The signal light attenuation coefficient, The Raman gain coefficient is... The effective mode field area of ​​a few-mode fiber. The pump light power is adjusted by... This ensures that the signal optical power at the end of the link remains above the detection sensitivity threshold.

[0043] At the receiving end, the backscattered Rayleigh light returned by the fiber optic link contains mixed mode field information. A mode demultiplexer is used to spatially separate the mixed optical field into independent fundamental mode channels and higher-order mode channels. The power of the backscattered light from each separated mode is then determined. It can be represented as: , where subscript Indicates the pattern type. This represents the capture coefficient of the mode. The pulse width. This represents the group velocity of the mode.

[0044] Response to the acquired sequence A and Sequence B response Matched filtering and correlation operations are performed to demodulate the impulse response function of the fiber optic link. : ,like Figure 3 As shown, Figure 3 (a) shows the result of the matched filter for path A; Figure 3 (b) shows the B-path matched filtering result; Figure 3 (c) is the final demodulation result. Subsequently, a logarithmic transformation is performed on the demodulated data to reconstruct the independent few-mode optical temporal reflectance curves for each mode: By comparing and analyzing the fundamental mode and higher-order mode curves, the mode loss characteristics distributed along the optical fiber can be obtained.

[0045] like Figure 4 As shown, after entering the detection and processing digital signal processing unit, for static faults, the system uses mode-dependent loss characteristics to distinguish macro-bending from weld points and break points. First, the cumulative mode-dependent loss at each location is calculated: Then calculate the local loss difference at the time point. The fault identification logic is as follows: If a certain point The local attenuation of the modulus is significantly greater than If the two modes show attenuation steps and the attenuation amounts are similar, it is determined to be a macro-bending fault.

[0046] When an optical fiber is subjected to external dynamic disturbances such as vibration or sound waves, minute changes in the fiber's geometry can lead to intermodal energy coupling, which is defined as the "local mode coupling coefficient". As a criterion for perturbation: By calculating the change in coupling coefficient between adjacent time points ,when Exceeding the preset threshold At that time, it was determined that there was a dynamic disturbance at that location. Since different mode groups have different velocities, differential time delay was used. It can further correct the position of disturbances, and the positioning accuracy can reach the meter level.

[0047] Differential operations and joint decision-making are performed on the optical temporal reflectance curves of different modes to achieve high-sensitivity fault detection. The reconstructed fundamental mode curve... and higher order mode curves Spatial alignment and power normalization preprocessing are performed to eliminate time axis offset and injection power differences caused by inter-modal dispersion. Subsequently, inter-modal differential signals are constructed. , In the formula, z represents the location information of the fiber optic link, and N is the moving average window size or the number of repeated measurements. and These are the logarithmic domain optical power values ​​of the fundamental mode and higher-order modes at the corresponding positions. Finally, an adaptive threshold is set. ,when When the threshold is exceeded, it is judged as an abnormal event, and a joint judgment is made by combining the slope change characteristics of the original curve, which greatly improves the signal-to-noise ratio and positioning accuracy of weak fault events.

[0048] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that, unless otherwise defined, the technical or scientific terms used in this invention should be understood in their ordinary sense as understood by one of ordinary skill in the art to which this invention pertains. Terms such as "comprising" or "including" as used in this invention mean that the element or object preceding the word encompasses the element or object listed after the word and its equivalents. Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for identifying faults and disturbances in few-mode fiber Raman-enhanced time-domain reflection, characterized in that, include: (1) Generate a pulse-coded optical sequence and directionally couple it into a specific mode in a minimum-mode fiber through mode-selective excitation to achieve coded modulation and mode multiplexing transmission; (2) Stimulated Raman scattering is excited in a few-mode fiber link by using reverse pumping technology, and the transmitted signal is distributedly amplified throughout the process to improve the signal-to-noise ratio and generate non-excited mode back Rayleigh scattering signal. (3) Separate the backscattered light field into independent mode channels and perform parallel photoelectric conversion to obtain the original electrical signals corresponding to each mode; (4) The original electrical signal is decoded by the matched filtering algorithm to reconstruct the independent high dynamic range optical time-domain reflectance curves of each mode; (5) Based on the loss differences and inter-mode coupling characteristics of each mode curve, the classification and identification of link fault types and the distributed location of external disturbances are realized; (6) Perform differential operations and joint decision-making on different modes of few-mode optical time-domain reflectometers to suppress Rayleigh scattering random noise and improve the reliability of event detection.

2. The method for identifying faults and disturbances in few-mode fiber Raman-enhanced time-domain reflection according to claim 1, characterized in that... Pulse-coded transmission and mode-selective excitation achieve coded modulation and mode-selective transmission, specifically including: (1) Coding sequence generation and modulation: The signal generator generates a pseudo-random coding sequence to drive the optical modulator, which modulates the continuous light into a coding optical pulse train with high autocorrelation characteristics, so as to improve the average optical power of a single transmission; (2) Mode-selective excitation: The working state of the mode exciter is controlled by a photonic lantern structure. The mode polling excitation strategy is used to selectively excite the fundamental mode or specific higher-order modes in the few-mode fiber as probe light in different time slots, and suppress the generation of non-target modes to reduce initial intermode crosstalk.

3. The method for identifying faults and disturbances in few-mode fiber Raman-enhanced time-domain reflection according to claim 1, characterized in that... Distributed Raman amplification and signal-to-noise ratio enhancement achieve end-to-end distributed amplification, specifically including: (1) Reverse pump injection: High-power continuous wave Raman pump light is injected at the end of the few-mode fiber link through a wavelength division multiplexer. The Raman frequency shift difference between the pump light wavelength and the probe light wavelength satisfies the Raman frequency shift difference of the fiber material to ensure that the Raman gain spectrum covers the probe light band. (2) Distributed gain equalization: Adjust the power level of the pump light to generate stimulated Raman scattering effect over the entire length of the optical fiber, and use the generated distributed gain to offset the transmission loss of the few-mode fiber, equalize the signal strength of each mode and improve the overall signal-to-noise ratio of the system.

4. The method for identifying faults and disturbances in few-mode fiber Raman-enhanced time-domain reflection according to claim 1, characterized in that... Mode demultiplexing and mode field detection are used to acquire the original electrical signal, specifically including: (1) Spatial mode separation: The back-scattered mixed optical field returned by the few-mode fiber is spatially separated into independent fundamental mode channels and higher-order mode channels by using a mode demultiplexer that is symmetrical with the mode selective exciter structure at the transmitter, so as to ensure that the optical signals of each mode do not interfere with each other. (2) Optical preprocessing: Each mode channel passes through an optical bandpass filter in sequence to suppress residual pump noise, and then enters the low-noise optical amplification unit or directly enters the photoelectric detection unit. (3) Multi-channel parallel detection: The separated optical signals of each mode are coupled to the corresponding photodetector arrays, and the weak optical signals are converted into analog electrical signals through photoelectric conversion, and then quantized into multi-channel digital signal sequences by a high-speed data acquisition card.

5. The method for identifying faults and disturbances in few-mode fiber Raman-enhanced time-domain reflection according to claim 1, characterized in that... Curve reconstruction is achieved through encoding demodulation and multi-mode few-mode optical time-domain reflectometry reconstruction, specifically including: (1) Matched filtering decoding: Using the locally stored transmission coding sequence as a reference, cross-correlation or deconvolution is performed with the received multi-channel digital signal to compress the broadened coded pulse back to the single pulse response width; (2) Curve reconstruction and averaging: Extract the peak value and timing information of the decoded signal, reconstruct the fundamental mode and higher-order mode independent few-mode optical time domain reflectometer curves that reflect the backscattering intensity at various locations along the optical fiber, and further suppress random noise by averaging multiple times to obtain test curves with high dynamic range.

6. The method for identifying faults and disturbances in few-mode fiber Raman-enhanced time-domain reflection according to claim 1, characterized in that... Multidimensional fault identification and distributed disturbance sensing enable fault classification and localization, specifically including: (1) Static fault classification and identification: Calculate the loss difference between the fundamental mode and the higher-order mode and the few-mode optical time domain reflectometer curves at the same location, i.e., the mode-dependent loss. If there is a sudden loss change at a certain location and the mode difference dependent loss value exceeds the preset threshold, it is determined to be a macro-bending fault; if the loss of each mode decreases synchronously and the mode difference dependent loss value remains stable, it is determined to be a break or weld point loss. (2) Real-time perception of dynamic disturbance: Real-time monitoring of the time correlation and energy distribution of backscattered signals of each mode. If the energy of the fundamental mode decreases and the energy of the higher-order modes increases at a certain location, and the total energy is basically conserved, it is determined that there is an external vibration disturbance at that location, and the location of the disturbance is determined based on the signal delay.

7. The multi-mode differential and high-precision positioning of the pulse-coded-Raman enhanced few-mode optical time-domain reflectometer based on few-mode fiber according to claim 1, characterized in that... Multi-mode differential and high-precision positioning, specifically including: (1) Mode difference operation: The reconstructed few-mode optical time-domain reflectometer curves of different modes are subjected to point-by-point difference operation. By utilizing the lack of correlation of Rayleigh scattering signals between modes, coherent fading noise is effectively suppressed and event characteristics are highlighted. (2) Multi-mode joint decision: Logical joint decision is made using the original curves and differential results of multiple modes, cross-validation of the existence of events, and confirmation of faults only when multiple features match, thereby reducing false alarm rate and improving the reliability of event detection. (3) Improved positioning accuracy: Based on the signal characteristics after multi-mode joint processing, the position calibration is performed by utilizing the steeper edge characteristics of the differential signal or the velocity difference of the multi-mode group, achieving event positioning accuracy and higher spatial resolution that is superior to that of a single-mode few-mode optical time domain reflectometer.

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