Anti-fading optical frequency domain reflectometry distributed measurement method and device for multi-core optical fiber

Abstract

This invention relates to the field of distributed optical fiber sensing technology, specifically to a distributed measurement method and apparatus for anti-fading optical frequency domain reflectance of multi-core optical fibers. The method includes: separating a strain-free region near the end of the multi-core optical fiber from a sensing region where strain may exist; extracting the demodulated phase of each fiber core in the strain-free region and unfolding it along time, quantizing and fitting it to obtain fitting coefficients, and using the fitting coefficients to reconstruct the curve; using the fitted curve of one fiber core as a reference curve, and subtracting the fitted curves of all other fiber cores from the reference curve to obtain an alignment term; introducing the alignment term into the demodulation of the sensing region to perform inter-core alignment; and restoring, superimposing, and converting the inter-core aligned phases to obtain the strain signal of the sensing region. This invention employs a time-domain alignment method for multi-degree-of-freedom measurement demodulated phases, effectively suppressing the time accumulation of phase errors during long-term measurements, and improving the time stability and long-term measurement capability of phase-type OFDR distributed measurements.

Description

Technical Field

[0001] This invention relates to the field of distributed optical fiber sensing technology, and more specifically to a distributed measurement method and apparatus for anti-fading optical frequency domain reflectance of multi-core optical fibers. Background Technology

[0002] Distributed fiber optic sensing (DFOS) technology based on back-scattering Rayleigh (RBS) utilizes a probe laser with a specific waveform to measure internal and external environmental information along the entire fiber optic cable. It boasts numerous superior characteristics, including long distance, large measurement range, high sensitivity, and strong anti-interference capabilities, and can be widely applied in distributed sensing, link status monitoring, intrusion detection, security inspection of structural components such as highways / bridges / vehicles, and marine / seismic wave monitoring. During fiber manufacturing, imperfections in processes and materials lead to non-uniform density distributions in the radial and axial directions. The resulting random fluctuations in refractive index are the primary cause of Rayleigh scattering during light injection. For a single fiber, its refractive index has a relatively stable distribution after manufacturing; therefore, Rayleigh scattering can be considered an inherent characteristic of optical fibers. Typically, Rayleigh scattering exists in all directions, with the scattered light, directed opposite to the injection direction, returning along the fiber to the injection end—this is the RBS signal.

[0003] In the field of DFOS (Diverterless Fiber Optic System), Optical Frequency Domain Reflectometer (OFDR) technology based on Resonant Short-Range Laser (RBS) injects a linearly swept probe laser into an optical fiber, collects the echo signal, and performs optical frequency deskewing demodulation. Then, the RBS signal along the fiber can be obtained through frequency domain analysis. By utilizing the delay difference, amplitude, and phase information of the RBS signal returning to the injection end at different locations in the fiber, the OFDR system can achieve distributed sensing with spatial positioning and measurement capabilities.

[0004] In recent years, phase-based OFDR technology has gained widespread attention due to its high sensitivity and linearity in sensing parameters such as strain and vibration. It achieves the sensing of strain and vibration measurements by extracting the phase of the Resonant Baseline Filter (RBS) at different locations along the optical fiber. However, limited by spatial resolution, all RBS signals within any spatial resolution will coherently superimpose at the receiver. Due to the random fluctuations in refractive index, the phase of a single RBS echo signal exhibits a random distribution, leading to drastic fluctuations in signal intensity after coherent superposition. Furthermore, in regions of coherent destructive interference, the receiver signal-to-noise ratio is severely degraded, making it impossible to extract the phase information of strain or vibration at that location. This results in a random detection blind zone within a certain spatial region, a phenomenon known as coherent fading. The resulting coherent fading noise is a key factor limiting the performance of RBS-based DFOS systems, such as phase-sensitive OFDR.

[0005] During phase demodulation, since the amplitude and phase of the RBS signal are closely related to the wavelength of the probe signal and the intrinsic distribution of the fiber refractive index, different coherent superposition phenomena are exhibited at different probe laser frequencies. The phase coherence cancellation characteristics within the fiber with independent refractive index distribution are also different. It can be considered that the probe signal light of different frequencies and the statistically independent fiber are independent measurement degrees of freedom.

[0006] In recent years, with the development of optical fiber technology, space division multiplexing (SDM) technology based on multi-core and multi-mode fibers has developed rapidly. Based on the multi-degree-of-freedom superposition method of multi-core fiber SDM, this method utilizes the characteristic that each fiber core can synchronously measure the RBS signal at the same spatial location within the same time frame. Each fiber core can be treated as a degree of freedom, and the measurement data for each core can be vector-summed with reference to the first time period. This method exhibits high spatiotemporal consistency and is considered an important development direction for anti-fading noise technology.

[0007] However, due to the differences between degrees of freedom and random noise during the measurement process, the superposition of multiple degrees of freedom inevitably faces noise accumulation over time as the measurement time increases. Specifically, as the measurement time gradually increases, the noise floor after the superposition of multiple degrees of freedom gradually deteriorates, becoming a key factor restricting long-term measurement capabilities. This noise accumulation has a significant impact on the RBS phase. Therefore, it is urgent to address the problem of RBS phase noise accumulation over measurement time in anti-fading measurement methods, enhance the system's temporal stability, and improve long-term measurement performance. Summary of the Invention

[0008] In view of this, the present invention provides a distributed measurement method and device for anti-fading optical frequency domain reflection of multi-core optical fiber, which adopts a time-domain alignment method for multi-degree-of-freedom measurement demodulation phase, effectively suppressing the time accumulation of phase error in long-term measurement, and improving the time stability and long-term measurement capability of phase-type OFDR distributed measurement.

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] In a first aspect, the present invention provides a distributed measurement method for anti-fading optical frequency domain reflectance of multi-core optical fibers, comprising the following steps:

[0011] Phase demodulation: The demodulated data of the RBS signal of each core in the multi-core optical fiber over a period of time are preprocessed, and the strain-free region near the end of the multi-core optical fiber and the sensing region where strain may exist are separated.

[0012] Quantization characterization: Select and extract the demodulated phase of each fiber core at a location in the strain-free region and expand it along time to quantize it;

[0013] Trend fitting: Fit the quantized phase to obtain fitting coefficients, and use the fitting coefficients to reconstruct the curve according to the length of the sensing area;

[0014] Alignment correction: The fitted curve of a certain fiber core is used as the reference curve, and the fitting curves of all other fiber cores are subtracted from the reference curve to obtain the alignment term; the alignment term is introduced into the demodulation phase of the sensing region where strain may exist to perform inter-core alignment.

[0015] Vector summation: In the sensing region, the phases after core alignment are restored into vectors and superimposed. After superposition, the phase is expanded in the time domain and strain is calculated along the frequency dimension to obtain the strain signal in the sensing region.

[0016] Furthermore, in the phase demodulation step, the preprocessing includes: period segmentation, windowing, Fourier transform, phase spectrum extraction, unwinding differential, and vector rotation.

[0017] Furthermore, in the quantization characterization step, the demodulated phase of each fiber core at a certain location in the strain-free region, after being expanded along time, is represented as follows: f∈(f0,f nan_end ), where f and t represent the frequency axis and time axis, respectively, f0 represents the starting position of the near-end strain-free region, f nan_end This indicates the termination position of the near-end strain-free region, and k represents the core number of the multi-core fiber.

[0018] Furthermore, in the quantization characterization step, the demodulated phase is quantized by finding discrete points, removing extreme values, and averaging or modulating the values.

[0019] Furthermore, in the alignment correction step, the method for performing inter-core alignment in the sensing region where strain may exist is as follows:

[0020]

[0021] in, This indicates the phase after core alignment within the sensing region; Denotes the demodulation phase in the sensing region, f∈(f nan_end f end ), f nan_end f represents the starting position of the sensing area. end Indicates the end position of the sensing area; Indicates alignment item; f str_n Indicates the quantized position.

[0022] Furthermore, in the vector summation step, within the sensing region, the phase and amplitude data after core alignment are combined and restored to a vector form:

[0023]

[0024] Among them, S f (f, t, k) represents the restored vector containing amplitude and phase information; E(f, t, k) represents the amplitude information within the sensing area; i represents the complex unit imaginary root;

[0025] Summing the restored vectors, assuming the multi-core fiber contains a total of 4 cores, the calculation formula is as follows:

[0026] S f (f, t) = [S f (f, t, 1) + S f (f, t, 2) + S f (f, t, 3) + S f (f, t, 4)]

[0027] Among them, S f (f, t, 1), S f (f, t, 2), S f (f, t, 3) and S f (f, t, 4) represent the vector forms of fiber core 1, fiber core 2, fiber core 3 and fiber core 4 in a multi-core optical fiber, respectively.

[0028] Furthermore, the relationship between phase and strain is expressed as:

[0029]

[0030] Where λ represents the swept laser frequency, n represents the fiber core refractive index, κ represents the strain constant, and L represents the fiber length in the sensing region.

[0031] φ(f,t) represents the phase expanded in the time domain, and it is calculated as follows:

[0032] φ(f, t) = arctan[S f (f, t)].

[0033] In a second aspect, the present invention provides a distributed measurement device for anti-fading optical frequency domain reflection of multi-core optical fiber, comprising: a demodulation subsystem and an analysis subsystem;

[0034] The demodulation subsystem is used to apply stress to the multi-core optical fiber and collect the RBS signal of each fiber core for data demodulation to obtain demodulated data.

[0035] The analysis subsystem is used to measure the demodulated data of the multi-core optical fiber using the above measurement method to obtain the strain signal of the multi-core optical fiber.

[0036] Furthermore, the demodulation subsystem includes: a laser emitting module, a first polarization coupler for the signal path, a polarization coupler for the reference path, a circulator, a fan-in terminal, a polarization controller, a second polarization coupler for the signal path, a balanced photodetector, an oscilloscope, an optical fiber under test, and a cantilever beam;

[0037] The laser emitting module emits a laser beam, which is connected to the first polarization coupler of the signal path and the polarization coupler of the reference path through multiple single-mode optical fibers, forming a signal path and a reference path.

[0038] The laser in the reference path is divided into multiple parts. Each part of the reference path laser is connected to the second polarization coupler of the signal path, waiting to be coupled and beat with the corresponding signal path optical frequency.

[0039] The laser in the signal path is divided into multiple parts that are equivalent to the laser in the reference path. Each part of the laser is connected to a single-mode fiber through the circulator and then connected to the corresponding fiber core of the multi-core fiber under test through the fan-in end.

[0040] A section of the optical fiber to be tested is attached to the cantilever beam, and the strain is applied by pressing the cantilever beam.

[0041] After strain is applied, the fiber under test captures the vibration signal. The RBS signal of each fiber core enters the corresponding single-mode fiber through the fan-in terminal, and then passes through the circulator, the polarization controller, the second polarization coupler of the signal path and the balanced photodetector in sequence before entering the corresponding channel of the oscilloscope to obtain the demodulation data of each fiber core.

[0042] As can be seen from the above technical solution, compared with the prior art, the present invention has the following beneficial effects:

[0043] 1. This invention studies the characteristics of accumulated noise during the summation calculation of multi-core optical fibers and uses an algorithm to quickly achieve time-domain core alignment, thus suppressing accumulated noise. Compared with traditional solutions, the technical solution proposed in this invention solves the problem of time-domain noise accumulation that can even overwhelm the strain signal, enabling it to handle strain measurements over longer periods and providing a certain degree of real-time detection capability.

[0044] 2. The technical solution proposed in this invention can arbitrarily select a suitable quantization mode according to the needs of actual applications, and adapt to different degrees of coherent fading.

[0045] 3. Compared with the phase-type OFDR distributed sensing system that requires high precision and long time, the technical solution proposed in this invention does not require additional hardware facilities. The algorithm is fast and easy to operate, effectively reducing measurement noise, reducing time error accumulation, and improving the long-term stability of measurement. Attached Figure Description

[0046] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0047] Figure 1 A flowchart of the measurement method provided by the present invention;

[0048] Figure 2 This is a schematic diagram of the demodulation subsystem in the measuring device provided by the present invention;

[0049] Figure 3 is a comparison of the effects of different algorithms provided by the present invention.

[0050] Figure 3(a) shows the calculation results of the summation and averaging algorithm;

[0051] Figure 3(b) shows the calculation results of the method of the present invention;

[0052] Figure 3(c) shows the calculation results for single cores of fibers 1, 2, 3, and 4, respectively. Detailed Implementation

[0053] 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.

[0054] like Figure 1 As shown in the figure, this invention discloses a distributed measurement method for anti-fading optical frequency domain reflectance of multi-core optical fibers, comprising the following steps:

[0055] S1. Phase demodulation: The demodulated data of the RBS signal of each core in the multi-core optical fiber are preprocessed over a period of time, and the strain-free area near the end of the multi-core optical fiber and the sensing area where strain may exist are separated.

[0056] S2. Quantization characterization: Select and extract the demodulated phase of each fiber core at a location in the strain-free region and expand it along time to quantize it;

[0057] S3. Trend Fitting: Fit the quantized phase to obtain fitting coefficients, and use the fitting coefficients to reconstruct the curve according to the length of the sensing area;

[0058] S4. Alignment Correction: The fitted curve of a certain fiber core is used as the reference curve. The fitting curves of all other fiber cores are subtracted from the reference curve to obtain the alignment term. The alignment term is introduced into the demodulation phase of the sensing region where strain may exist to perform inter-core alignment.

[0059] S5. Vector Summation: In the sensing area, the phases after core alignment are restored into vectors and superimposed. After superposition, the phase is expanded in the time domain and strain is calculated along the frequency dimension to obtain the strain signal in the sensing area.

[0060] The following is a further explanation of each of the above steps.

[0061] S1, Phase Demodulation:

[0062] For the demodulated RBS signal data of each core in a multi-core optical fiber acquired over a period of time, basic operations such as period segmentation, windowing, Fourier transform, phase spectrum extraction, unwinding differential, and vector rotation are performed. The near end of the multi-core optical fiber, which remains strain-free, is selected as the strain-free region (non-measurement region), and the remaining locations are selected as the sensing regions where strain may occur (measurement regions). The phase of the strain-free region is calculated. Calculate phase f∈(f str0 ,f end ).

[0063] S2, Quantitative Characterization:

[0064] The single-core phase of the strain-free region is calculated and unfolded along the time axis to obtain the phase, which is then projected onto the phase-time plane. Where: f and t represent the frequency axis (i.e., the distance axis) and the time (measurement period) axis, respectively; f0 represents the starting position of the near-end strain-free region, f nan_end This indicates the termination position of the near-end strain-free region, and k represents the core number of the multi-core fiber.

[0065] Subsequently, the phase of each fiber core unfolding along the time axis in the unstrained region was quantized by finding discrete points and removing extreme values, or by averaging or mode, to obtain... This is to prevent excessive noise or related fading at individual frequency points from causing the next fitting step to fail.

[0066] S3. Trend Fitting: The quantization phase obtained in the previous step is fitted using a quadratic or cubic polynomial to obtain the fitting coefficients P. The curve is then reconstructed in the test area using the coefficients P.

[0067] S4. Alignment Correction: Due to the presence of strain in the test area, the phase-time change trend cannot be directly described. Therefore, in the strain-free region, the fitted curve of a certain fiber core is used as the reference curve, and the fitting curves of all other fiber cores are subtracted from the reference curve to obtain the alignment term.

[0068]

[0069] in, This represents the time-domain alignment term for each fiber core. This represents the phase curve reconstructed in the sensing area. This represents the phase curve (reference curve) recovered in the sensing area of ​​fiber core 1.

[0070] Phase in the sensing region where strain may exist An alignment item is introduced to perform core-to-core alignment. The specific alignment method is as follows:

[0071]

[0072] in, This indicates the phase after core alignment within the sensing region; Denotes the demodulation phase in the sensing region, f∈(f str0 f end ), f str0 f represents the starting position of the sensing area. end Indicates the end position of the sensing area; Indicates alignment item; f str_n Indicates the quantized position.

[0073] S5, Vector Summation:

[0074] In the region to be measured, the phase and amplitude data after core alignment are combined and restored to a vector form:

[0075]

[0076] Among them, S f (f, t, k) represents the restored vector containing amplitude and phase information; E(f, t, k) represents the amplitude information within the sensing area; i represents the complex unit imaginary root;

[0077] Continuing with the vector summation step in the typical rotational vector superposition operation, assuming the multi-core fiber contains a total of 4 cores, the calculation formula is as follows:

[0078] S f (f, t) = [S f (f, t, 1) + S f (f, t, 2) + S f(f, t, 3) + S f (f, t, 4)]

[0079] Among them, S f (f, t, 1), S f (f, t, 2), S f (f, t, 3) and S f (f, t, 4) represent the vector forms of fiber core 1, fiber core 2, fiber core 3 and fiber core 4 in a multi-core optical fiber, respectively.

[0080] Then, the phase is expanded in the time domain and strain is calculated along the frequency dimension to obtain the strain signal in the sensing area.

[0081] The relationship between phase φ(f,t) and strain ε(f,t) is expressed as:

[0082]

[0083] Where λ represents the swept laser frequency, n represents the fiber core refractive index, κ represents the strain constant, and L represents the fiber length in the sensing region.

[0084] φ(f,t) represents the phase expanded in the time domain, and it is calculated as follows:

[0085] φ(f, t) = arctan[S f (f, t)].

[0086] Because coherent fading is random, the probability of coherent fading occurring at the same time and location in each fiber core is much lower than that in a single fiber core. During the rotation vector summation process, coherent fading signals with smaller magnitudes have less impact on the results. This can effectively reduce time-domain accumulated noise and suppress coherent fading noise in long-term measurement scenarios.

[0087] This invention also provides a distributed measurement device for anti-fading optical frequency domain reflection of multi-core optical fiber, characterized in that it includes: a demodulation subsystem and an analysis subsystem;

[0088] The demodulation subsystem is used to apply stress to the multi-core optical fiber and to acquire the RBS signal of each fiber core for data demodulation to obtain demodulated data.

[0089] The analysis subsystem is used to measure the demodulated data of the multi-core optical fiber using the above measurement method to obtain the strain signal of the multi-core optical fiber.

[0090] Specifically, such as Figure 2As shown, the demodulation subsystem includes: a laser emission module 11, a first polarization coupler for the signal path 1, a polarization coupler for the reference path 2, a circulator 3, a fan-in terminal 4, a polarization controller 5, a second polarization coupler for the signal path 6, a balanced photodetector 7, an oscilloscope 8, an optical fiber under test 9, and a cantilever beam 10.

[0091] The laser emitting module 11 emits a laser, which is connected to the first polarization coupler 1 of the signal path and the polarization coupler 2 of the reference path through multiple single-mode optical fibers, forming the signal path and the reference path respectively.

[0092] The laser in the reference path is divided into multiple parts. Each part of the reference path laser is connected to the second polarization coupler 6 of the signal path, waiting to be coupled and beat with the corresponding signal path optical frequency.

[0093] The laser in the signal path is divided into multiple parts that are equivalent to the laser in the reference path. Each part of the laser is connected to a single-mode fiber through a circulator 3, and then connected to the corresponding fiber core of the multi-core fiber 9 under test through a fan-in end 4.

[0094] A section of the optical fiber 9 to be tested is pasted on the cantilever beam 10, and the strain is applied by pressing the cantilever beam 10.

[0095] After strain is applied, the fiber under test 9 captures the vibration signal. The RBS signal of each fiber core enters the corresponding single-mode fiber through the fan-in terminal 4, and then passes through the circulator 3, polarization controller 5, signal path second polarization coupler 6 and balanced photodetector 7 in sequence before entering the corresponding channel of oscilloscope 8 to obtain the demodulation data of each fiber core.

[0096] The analysis subsystem includes: a preprocessing module, a quantization module, a fitting module, an alignment module, and a vector summation module.

[0097] The measurement process of this measuring device will be further explained below.

[0098] 1. The laser emission module transmits laser signals. The first polarization coupler 1 and the reference polarization coupler 2 are divided into two parts. One part of the laser enters the signal path, and the other part of the laser enters the reference path. The reference path is divided into 4 parts and enters 4 single-mode optical fibers, waiting to be coupled with the signal path to beat the frequency.

[0099] 2. The laser in the signal path is divided into 4 parts, which enter 4 single-mode optical fibers through circulator 3, and then enter 4 cores of the multi-core optical fiber under test through Fan-in 4, numbered 1, 2, 3 and 4 respectively. The total length of the multi-core optical fiber is about 50m. One part of it is pasted on the cantilever beam structure. Press down on the suspended end of the cantilever beam, and after releasing it, the cantilever beam vibrates on its own.

[0100] 3. The vibration signal generated in step 2 is captured by the multi-core optical fiber. Its RBS signal returns to Fan-in4 along the original path, and then enters the four single-mode optical fibers in sequence through circulator 3, polarization controller 5, signal path second polarization coupler 6 and balanced photodetector 7 before being output to the four channels of oscilloscope 8. Finally, the data from the oscilloscope is read into the analysis subsystem in the computer.

[0101] 4. The preprocessing module sequentially imports global parameters into the data of each fiber core and extracts the demodulated data of the RBS signal of each fiber core in the multi-core fiber within a certain period of time. It performs basic operations such as period segmentation, Hanning windowing, Fourier transform, phase spectrum extraction, unwinding differential and vector rotation, and separates the strain-free area and the sensing area that may have strain in the near end of the multi-core fiber.

[0102] 5. The demodulated phase of each fiber core at a location within the strain-free region is selected and extracted using the quantization module and expanded along time for quantization. Specifically, for each fiber core, the vector signal of the first time cycle is used as a reference. The vector signals of all subsequent time intervals are multiplied by their normalized conjugate signals to calculate the phase. The data from the near-strain-free region is projected onto the phase-time plane, and quantization is performed in the frequency dimension using one of three methods: discrete point quantization, extreme value removal averaging, or mode quantization.

[0103] 6. The quantization phase is fitted using a quadratic or cubic polynomial through the fitting module to obtain fitting coefficients. The curve is then reconstructed using the fitting coefficients according to the length of the sensing area.

[0104] 7. In the strain-free region, the fitting curve of a certain fiber core is used as the reference curve by the alignment module. The fitting curves of all other fiber cores are subtracted from the reference curve to obtain the alignment term. The alignment term is introduced into the phase curve of the sensing region where strain may exist to perform inter-core alignment.

[0105] 8. The phases of the core-to-core aligned regions in the sensing area are restored into vectors by the vector summation module and superimposed according to the traditional demodulation method. After superposition, the phase is expanded in the time domain and strain is calculated along the frequency dimension to obtain the strain signal in the sensing area.

[0106] To further verify the effectiveness of the method of the present invention, it was also compared with existing summation averaging algorithms and single-core demodulation algorithms. The comparison results are as follows: Figure 3a - Figure 3c As shown, the method of the present invention can be seen to suppress coherent fading while solving the problem of time-domain noise accumulation during long-term measurements.

[0107] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the apparatus disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple; relevant parts can be referred to the method section.

[0108] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A distributed measurement method for anti-fading optical frequency domain reflectance of multi-core optical fibers, characterized in that, Includes the following steps: Phase demodulation: The demodulated data of the RBS signal of each core in the multi-core optical fiber over a period of time are preprocessed, and the strain-free region near the end of the multi-core optical fiber and the sensing region where strain may exist are separated. Quantization characterization: Select and extract the demodulated phase of each fiber core at a location in the strain-free region and expand it along time to quantize it; Trend fitting: Fit the quantized phase to obtain fitting coefficients, and use the fitting coefficients to reconstruct the curve according to the length of the sensing area; Alignment correction: Take the fitted curve of a certain fiber core as the reference curve, and subtract the fitted curves of all other fiber cores from the reference curve to obtain the alignment term; An alignment term is introduced into the demodulation phase of the sensing region where strain may exist to perform inter-core alignment; the method for inter-core alignment in the sensing region where strain may exist is as follows: in, This indicates the phase after core alignment within the sensing region; This indicates the demodulation phase in the sensing region. , Indicates the starting position of the sensing area. Indicates the end position of the sensing area; Indicates alignment items; Indicates the quantized position; Vector summation: In the sensing area, the phases after core alignment are restored into vectors and superimposed. After superposition, the phase is expanded in the time domain and strain is calculated along the frequency dimension to obtain the strain signal in the sensing area. In the vector summation step, within the sensing region, the phase and amplitude data after core alignment are combined and restored to a vector form: in, This represents the restored vector containing amplitude and phase information; It represents the amplitude information within the sensing area; Represents the imaginary root of the complex number unit; Summing the restored vectors, assuming the multi-core fiber contains a total of 4 cores, the calculation formula is as follows: in, , , and These represent the vector forms of fiber core 1, fiber core 2, fiber core 3, and fiber core 4 in a multi-core optical fiber, respectively.

2. The anti-fading optical frequency domain reflectance distributed measurement method for multi-core optical fibers according to claim 1, characterized in that, In the phase demodulation step, the preprocessing includes: period segmentation, windowing, Fourier transform, phase spectrum extraction, unwinding differential, and vector rotation.

3. The anti-fading optical frequency domain reflectance distributed measurement method for multi-core optical fibers according to claim 1, characterized in that, In the quantization characterization step, the demodulated phase of each fiber core at a certain location in the strain-free region, after being expanded along time, is represented as follows: , ,in, and Representing the frequency axis and time axis respectively. Indicates the starting position of the strain-free region near the end. Indicates the termination position of the near-strain-free region. This indicates the core number of a multi-core optical fiber.

4. The anti-fading optical frequency domain reflectance distributed measurement method for multi-core optical fibers according to claim 1, characterized in that, In the quantization characterization step, the demodulated phase is quantized by finding discrete points, removing extreme values, and averaging or mode.

5. The anti-fading optical frequency domain reflectance distributed measurement method for multi-core optical fibers according to claim 1, characterized in that, The relationship between phase and strain is expressed as: in, Indicates the frequency of the swept laser. Indicates the refractive index of the fiber core. Represents the strain constant. Indicates the length of the optical fiber in the sensing area; The phase, as expanded in the time domain, is calculated as follows: 。 6. A distributed measurement device for anti-fading optical frequency domain reflection of multi-core optical fiber, characterized in that, include: Demodulation subsystem and analysis subsystem; The demodulation subsystem is used to apply stress to the multi-core optical fiber and collect the RBS signal of each fiber core for data demodulation to obtain demodulated data. The analysis subsystem is used to measure the demodulation data of the multi-core optical fiber using the measurement method described in any one of claims 1-5, to obtain the strain signal of the multi-core optical fiber.

7. The anti-fading optical frequency domain reflection distributed measurement device for multi-core optical fibers according to claim 6, characterized in that, The demodulation subsystem includes: a laser emitting module, a first polarization coupler for the signal path, a polarization coupler for the reference path, a circulator, a fan-in terminal, a polarization controller, a second polarization coupler for the signal path, a balanced photodetector, an oscilloscope, an optical fiber under test, and a cantilever beam; The laser emitting module emits a laser beam, which is connected to the first polarization coupler of the signal path and the polarization coupler of the reference path through multiple single-mode optical fibers, forming a signal path and a reference path. The laser in the reference path is divided into multiple parts. Each part of the reference path laser is connected to the second polarization coupler of the signal path, waiting to be coupled and beat with the corresponding signal path optical frequency. The laser in the signal path is divided into multiple parts that are equivalent to the laser in the reference path. Each part of the laser is connected to a single-mode fiber through the circulator and then connected to the corresponding fiber core of the multi-core fiber under test through the fan-in end. A section of the optical fiber to be tested is attached to the cantilever beam, and the strain is applied by pressing the cantilever beam. After strain is applied, the fiber under test captures the vibration signal. The RBS signal of each fiber core enters the corresponding single-mode fiber through the fan-in terminal, and then passes through the circulator, the polarization controller, the second polarization coupler of the signal path and the balanced photodetector in sequence before entering the corresponding channel of the oscilloscope to obtain the demodulation data of each fiber core.

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