Time delay error correction method for long-distance distributed birefringence measurement

Abstract

The invention belongs to the field of distributed optical fiber testing, and particularly relates to a time delay error correction method for long-distance birefringence measurement, which is characterized in that accumulated birefringence time delay is predicted according to a demodulation value before a to-be-demodulated point, birefringence time delay error is compensated through point-by-point loop iteration, and the time delay error is corrected. The correlation of the polarization orthogonal axis Rayleigh scattering spectrum is recovered, and the expansion of the distributed birefringence measurement distance is realized; the specific process is as follows: firstly, measuring a frequency domain signal containing birefringence information of the polarization maintaining fiber, intercepting an initial birefringence demodulation signal by taking a measurement zero point as a reference, obtaining initial birefringence, then adjusting a sliding step of a slow axis interception window according to a birefringence value, intercepting the birefringence demodulation signal again by using the sliding step, and obtaining initial birefringence; obtaining the birefringence of the section; and repeating the steps until the sliding window traverses the whole section of polarization-maintaining optical fiber, thereby obtaining birefringence distribution information of the whole section of polarization-maintaining optical fiber. According to the method, the recursion iteration thought is used, the birefringence time delay error problem in long-distance polarization maintaining optical fiber measurement is solved, the distance of distributed birefringence measurement is greatly expanded, and the method has the advantages of being high in precision and simple in calculation process.

Description

Technical Field

[0001] The invention belongs to the field of distributed optical fiber testing and relates to a time delay error correction method for long-distance distributed birefringence measurement. Background Art

[0002] Polarization Maintaining Fiber (PMF) is a specially designed single-mode optical fiber that achieves stable optical signal transmission by controlling and maintaining the polarization state of the transmitted light, ensuring the quality and transmission efficiency of the optical signal, and enabling the optical communication system to better cope with complex transmission environments. Therefore, it is widely used in optical communications, optical sensors, optical devices and other fields.

[0003] Birefringence is one of the key parameters of polarization-maintaining optical fiber. In optical fiber communication, the birefringence characteristics in the optical fiber will cause the polarization state of the optical signal to change, thus affecting the transmission and demodulation of the optical signal. A high-precision polarization-maintaining optical fiber birefringence measurement device can better study the birefringence phenomenon, control the optical signal transmission characteristics in the optical fiber, and thus improve the performance and stability of the optical fiber communication system.

[0004] Birefringence measurement technology can be divided into two types: lumped and distributed birefringence measurement technology. The existing lumped birefringence measurement technologies mainly include dynamic pressurization method, magneto-optical demodulation method, white light interferometry method, broadband light source method, Sagnac interferometry method, etc. For example, Zhao Yao and others from the 41st Institute of China Electronics Technology Group used interference technology to measure the geometric length between the two applied strain points of the polarization-maintaining optical fiber to be tested and the optical length between the corresponding coupling points, which greatly improved the measurement accuracy of the beat length of the polarization-maintaining optical fiber (A polarization-maintaining optical fiber beat length measurement device and measurement method, 202010760887.X). Although the above method can achieve high-precision birefringence measurement, it can only test the average birefringence or beat length of 1~2 meters of optical fiber, and cannot achieve distributed measurement, which is far from meeting the high-precision and distributed birefringence measurement requirements of polarization-maintaining devices such as polarization-maintaining optical fiber and optical fiber sensitive ring.

[0005] The current distributed birefringence measurement technology can be divided into distributed measurement technology based on Brillouin scattering or Rayleigh scattering. In 2012, Dong Yongkang and others from Harbin Institute of Technology used Brillouin Dynamic Gratings (BDG) measurement technology to achieve birefringence distribution measurement of polarization-maintaining optical fibers with a length of more than 500m, and overcame the problem of long measurement time (CN201210059817.7). In 2020, the same method was used to achieve birefringence measurement of polarization-maintaining optical fibers with a length of 3030m at a spatial resolution of 40cm (Optics Express. 27(2):951-964), and the measurement time was about tens of minutes.

[0006] The birefringence / beat length measurement technology based on Rayleigh scattering mainly includes: In 2019, Rao Yunjiang and others from the University of Electronic Science and Technology of China proposed a distributed birefringence measurement method based on CP-ΦOTDR (CN 201910430425.9). This method introduces chirped pulses for measurement. Only a single pulse and an incident light of any polarization state are required to be input into the sensing optical fiber to be measured to achieve distributed birefringence measurement.

[0007] Of the two methods mentioned above, the birefringence distribution measurement technology based on Brillouin scattering usually has a complex device and requires careful design of the pump pulse and the detection pulse to satisfy the conditions for sufficient Brillouin scattering in order to obtain a strong Brillouin signal. On the one hand, the spatial resolution is intrinsically limited by the phonon lifetime, and only sub-meter measurements can be achieved; on the other hand, limited by the non-local effect, the birefringence measurement length and measurement accuracy are restricted. The longer the test distance, the more serious the accuracy degradation, and even distortion at the far end. The distributed birefringence measurement technology based on CP-ΦOTDR also has the problem of insufficient spatial resolution. Limited by the time domain positioning method, it can only achieve sub-kilometer measurement lengths and meter-level spatial resolutions, which is far from meeting the current testing requirements for polarization-maintaining optical fibers and devices.

[0008] Relatively speaking, the distributed birefringence measurement technology based on Rayleigh scattering detection in the optical frequency domain has a higher spatial resolution. In 2015, Zhou et al. (Optics Communications, 2015, (345): 62-66) used the Rayleigh scattering intensity correlation to obtain the birefringence delay, thereby characterizing the distributed birefringence. Although its spatial resolution reached 7.8 cm, the measurement length was only 1.2 m and the measurement accuracy was only 2×10 -5 The main problem is that the birefringence delay between orthogonal polarization axes is not overcome, resulting in information misalignment between the P axis and the S axis during propagation constant difference demodulation, causing the birefringence measurement accuracy to degrade with the test distance, and even making it impossible to achieve distributed birefringence demodulation at the far end of the optical fiber.

[0009] The present invention is based on the improvement of the prior art. Aiming at the problem that the optical frequency domain reflectometer cannot measure the birefringence of long-distance polarization-maintaining optical fiber, a method for correcting the time delay error of long-distance birefringence measurement is disclosed. First, the frequency domain signal containing the birefringence information of the polarization-maintaining optical fiber is measured, and the initial birefringence demodulation signal is intercepted with the measurement zero point as the reference to obtain the initial birefringence. Then, the sliding step of the slow axis interception window is adjusted according to the birefringence value, and the birefringence demodulation signal is intercepted again using this sliding distance to obtain the birefringence of this section. Repeat the above steps until the sliding window traverses the entire section of polarization-maintaining optical fiber, and the birefringence distribution information of the entire section of polarization-maintaining optical fiber can be obtained. The present invention solves the problem that the birefringence time delay error restricts the measurement distance in the scenario of measuring long-distance polarization-maintaining optical fiber, and realizes the high-precision birefringence distribution measurement of long-distance polarization-maintaining optical fiber. Summary of the invention

[0010] The purpose of the present invention is to provide a method for correcting time delay errors in long-distance birefringence measurement, which solves the problem that OFDR in the background technology is difficult to achieve high-precision, long-distance polarization-maintaining optical fiber birefringence testing.

[0011] A time delay error correction method for long-distance distributed birefringence measurement is characterized by: iteratively correcting the time delay error in the birefringence measurement point by point, predicting the time delay error of the point to be demodulated by demodulating the distributed birefringence information from the starting point of the length to the point to be demodulated, compensating the sliding window position of the polarization orthogonal axis according to the predicted value of the time delay error, restoring the correlation between the Rayleigh scattering spectra of the polarization orthogonal axes, and realizing the extension of the distributed birefringence measurement distance. The specific process is:

[0012] Step 1 S01: uniformly inject interrogation light into the polarization orthogonal fast axis and slow axis of the optical fiber to be tested, and receive the Rayleigh scattering time domain signals I of the fast axis and slow axis respectively. F (t) and I S (t);

[0013] Step 2 S02: The fast axis and slow axis Rayleigh scattering time domain signals I F (t) and I S (t) Fourier transform is performed to obtain the distribution characteristics of Rayleigh scattering signal with the fiber length f F (t) and f S (t);

[0014] Step 3 S03: Starting from the measurement zero point, a sliding window with a length of △Z is used to measure the Rayleigh scattering signal f of the fast axis and the slow axis. F (t) and f S (t) is intercepted to obtain x F (0) and x P (0);

[0015] Step 4 S04: By x F(0) and x P (0) Demodulate the birefringence of the first window to obtain an initial birefringence value B0;

[0016] Step 5 S05: Demodulate the birefringence of the second window, set the sliding distance of the fast axis sliding window to D, the total number of sliding times to N, and predict the error and correct it according to the birefringence demodulation result B0 of the previous window to obtain the sliding distance of the slow axis sliding window to D. S (0); get x F (1) and x P (1) and perform birefringence demodulation to obtain a birefringence value B1;

[0017] Step 6 S06: Similarly, continue to demodulate the birefringence values ​​of N windows. When the number of step corrections n is less than the total number of sliding times N, use a sliding window with a length of △Z to adjust the birefringence values ​​of the N windows according to the sliding distances D and D between the fast axis and the slow axis. S (n-1), intercept the two-axis frequency domain signals for the nth time and get x F (n) and x P (n), where n=1,2,3,…;

[0018] Step 7 S07: By x F (n) and x P (n) Demodulate the birefringence of the nth window to obtain the birefringence value B n ;

[0019] Step 8 S08: Based on the birefringence value B of the nth window n , predict the error and correct it to get the sliding distance of the n+1th slow axis sliding window as D S (n);

[0020] Step nine S09: When the iterative correction number n is equal to the total sliding number N, the demodulation of the birefringence distribution of the optical fiber to be tested is completed; when the iterative correction number n is less than the total sliding number N, repeat steps six to eight to complete the delay error prediction and birefringence demodulation of the remaining length.

[0021] The prediction error and correction method in step 5 (S05) is as follows: the sliding distance of the second slow axis window is compensated according to the birefringence value B0 of the first window, which is defined as: D S (0) = D + B0×D.

[0022] The prediction error and correction method in step eight (S08) is as follows: the sliding distance of the n+1th slow axis window is based on the birefringence value B of the nth window. n Compensation is performed, defined as: D S (n) = D + B n ×D.

[0023] The principle of the birefringence demodulation method in step 4 S04 and step 7 S07 is as follows: Figure 2 As shown in the device, based on the generation mechanism of strain birefringence of polarization-maintaining fiber and the strain demodulation principle related to the spectrum, the internal strain between the orthogonal polarization axes of the polarization-maintaining fiber is demodulated to convert the distributed birefringence value. The specific process is: first, the interference signal of the orthogonal polarization axes P and S of the polarization-maintaining fiber to be tested is used to compensate the phase noise, and then the interference signal of the P and S axes is transformed into the frequency domain through Fourier transform; secondly, the corresponding positions of the P and S axes are intercepted respectively through a sliding window with a length of Δz; then the spatial domain signals corresponding to the spatial positions of the P and S axes are inverse Fourier transformed to transform them into the spectral domain; finally, the corresponding signals of the P and S axes are cross-correlated; then the internal strain between the P and S axes of the polarization-maintaining fiber to be tested can be calculated, and the linear relationship between it and birefringence can be used to characterize the birefringence characteristics of the polarization-maintaining fiber to be tested.

[0024] Taking the Panda-type polarization-maintaining fiber as an example, the relationship between its internal strain and birefringence is calculated according to the following formula:

[0025] (1)

[0026] Where c is the photoelastic coefficient; E is Young's modulus; R is the strain region radius; d is the distance from the strain region to the center of the fiber core; r is the fiber cladding radius; Δα is the difference in linear expansion coefficient between the strain region and the cladding material; ΔT is the difference between the fiber stretching temperature and the space temperature;

[0027] In the calculation formula of polarization-maintaining fiber birefringence, except Δα and ΔT, other parameters are structural parameters of the fiber, so only Δα and ΔT need to be measured to obtain the birefringence of the corresponding polarization-maintaining fiber. The strain on the orthogonal polarization axes P and S of the polarization-maintaining fiber can also be expressed by the product of the coefficient difference Δα and the temperature difference ΔT. The specific formula is as follows:

[0028] (2)

[0029] Substituting formula (4) into formula (3), we get the expressions of birefringence and internal strain of Panda-type polarization-maintaining fiber:

[0030] (3)

[0031] The principle of the time delay error of the long-distance distributed birefringence measurement is as follows: Figure 3As shown in the figure, in OFDR-based birefringence demodulation, a sliding window is used to slide and intercept the Rayleigh backscattering spectrum (RBS) of the F axis and the S axis, thereby achieving decoupling in the spatial domain. Its window length Δz corresponds to the spatial resolution. Since the birefringence of polarization-maintaining optical fibers is generally very high, the refractive index of the F axis and the S axis differs greatly, and there is a large time delay in the propagation of light on the two axes, which can be expressed as:

[0032] (4)

[0033] where τ FS is the time delay error between the F axis and the S axis, n S and n F are the refractive indices of the F axis and the S axis respectively, and c is the speed of light.

[0034] If the birefringence delay is ignored in the birefringence measurement, and the same optical path is used to intercept the RBS of the F axis and the S axis, each intercepted window will be affected by the delay error. The delay error between the intercepted RBS will accumulate with the increase of the test distance, resulting in a serious decrease in correlation and the inability to decouple the birefringence. This is a key factor that limits the distance of birefringence distribution measurement. However, the birefringence distribution in the polarization-maintaining fiber is uneven. Using the average birefringence for overall compensation cannot have a good compensation effect on different intercepted windows, resulting in a decrease in the overall measurement accuracy.

[0035] The present invention is based on the improvement of the prior art. Aiming at the problem that the optical frequency domain reflectometer cannot measure the birefringence of long-distance polarization-maintaining optical fiber, a method for correcting the time delay error of long-distance birefringence measurement is disclosed. First, the frequency domain signal containing the birefringence information of the polarization-maintaining optical fiber is measured, and the initial birefringence demodulation signal is intercepted with the measurement zero point as the reference to obtain the initial birefringence. Then, the sliding step of the slow axis interception window is adjusted according to the birefringence value, and the birefringence demodulation signal is intercepted again using this sliding distance to obtain the birefringence of this section. Repeat the above steps until the sliding window traverses the entire section of polarization-maintaining optical fiber, and the birefringence distribution information of the entire section of polarization-maintaining optical fiber can be obtained. The present invention solves the problem that the birefringence time delay error restricts the measurement distance in the scenario of measuring long-distance polarization-maintaining optical fiber, and realizes the high-precision birefringence distribution measurement of long-distance polarization-maintaining optical fiber.

[0036] Compared with the prior art, the advantages of the present invention are:

[0037] The present invention has a longer distributed birefringence test distance; by solving the time delay error problem of birefringence measurement, the test distance is extended from several meters to several kilometers while maintaining the measurement spatial resolution and accuracy; in a specific implementation case, the measurement of 5km polarization-maintaining optical fiber distributed birefringence is preliminarily realized, which is the distributed birefringence measurement method with the longest test distance.

[0038] The present invention has higher distributed birefringence measurement accuracy; the proposed method solves the problem that the birefringence measurement accuracy degrades with the test distance, and theoretically can maintain uniform birefringence measurement accuracy over a long distance.

[0039] The correction process of the present invention is simple, and the amount of calculation is small through iterative recursive thinking; the point-by-point correction process is precise, and the residual error is small. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 It is a summary diagram of a method for correcting time delay errors in long-distance birefringence measurements;

[0041] Figure 2 It is a schematic diagram of a high-precision distributed birefringence measurement device;

[0042] Figure 3 It is a schematic diagram of the mechanism of birefringence error;

[0043] Figure 4 It is the comparison of birefringence measurement results before and after delay error correction;

[0044] Figure 5 This is the distributed birefringence test result of 5km polarization-maintaining optical fiber. DETAILED DESCRIPTION

[0045] In order to make the purpose, technical solution and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. The specific embodiments described here are only used to explain the present application and are not used to limit the present application.

[0046] Implementation—Using OFDR-based birefringence delay error compensation method to measure and correct birefringence of 5 km polarization-maintaining fiber.

[0047] The selection and parameters of the main optoelectronic devices of this measurement system are as follows:

[0048] The light source is a narrow linewidth tunable laser source, with a relative intensity noise RIN of -145dB / Hz, a light source linewidth of 60kHz (coherent control off), a sweep range of 10nm, a wavelength tuning range of 1545nm~1555nm, a sweep rate of 10nm / s, and a sweep time of 1s;

[0049] The photosensitive materials of the main interferometer differential detector 306 and the auxiliary interferometer differential detector 204 are both InGaAs, with a light detection range of 900-1700nm, a responsivity greater than 1A / W, and a common mode rejection ratio of 25dB. If the 1817-FC balanced optical fiber receiver of Newport is used, the maximum detection bandwidth of the detector is 200MHz, the saturated differential detection power is 80uW, and the common mode rejection ratio is 25dB;

[0050] The maximum sampling rate of acquisition card 501 is 400MHz / s, and four channels can be collected simultaneously. It has two independent full-function ports, such as SPECTRUM's M4i.4471-x8 acquisition card; the number of sampling points is 400M, the sampling time is 1s, and the triggering method uses laser external triggering;

[0051] The polarization-maintaining optical fiber 4 to be tested is a Panda polarization-maintaining optical fiber with a diameter of 80 μm and a length of 2.2 km;

[0052] The auxiliary interferometer extension optical fiber 202 is a single-mode optical fiber with a diameter of 125 μm and a length of 1 km;

[0053] If the same optical path is used to intercept the RBS of the F axis and the S axis, each intercepted window will be affected by the delay error. The delay error between the intercepted RBS will accumulate with the increase of the test distance, resulting in a serious decrease in correlation and the inability to perform correlation decoupling. This is the key factor that limits the measurement distance of the birefringence distribution. -4 Taking the polarization-maintaining fiber of 100 m as an example, the delay caused by the polarization-maintaining fiber is 266 ps, which is equivalent to 8 cm optical path after conversion. In this case, if the spatial resolution of the birefringence measurement is set to 5 cm, the RBS of the intercepted data comes from different positions of the fiber, completely loses correlation, and cannot achieve the measurement of birefringence. Therefore, the birefringence delay must be compensated.

[0054] The test system used in this implementation case is as follows Figure 2As shown, it includes five parts: a tunable light source 1, an auxiliary interferometer module 2, a main interferometer 3, a polarization-maintaining optical fiber to be measured 4, and a data acquisition and demodulation module 5. The signal light is emitted from the tunable laser source 1 and injected into the auxiliary interferometer module 2 and the main interferometer module 3 through a polarization-maintaining coupler PMC1 101. After the optical signal is injected into the auxiliary interferometer module 2, it is divided into two beams by a single-mode coupler OC1 201. One beam is injected from the auxiliary interferometer measurement arm into the single-mode coupler OC2 203. The other beam is injected from the auxiliary interferometer reference arm into the delay optical fiber 202. Subsequently, the two beams converge into the single-mode coupler OC2 203 to form an auxiliary interference signal. After the optical signal is injected into the main interferometer module (3), it first passes through a 45° polarizer 102 to adjust the polarization state of the injected light to 45°. After the polarized light passes through the polarization-maintaining coupler PMC2301, one beam enters the reference arm and is directly injected into the polarization-maintaining coupler PMC3 302, a beam is injected into the polarization-maintaining fiber 4 to be tested through the polarization-maintaining circulator Cir 303, and the reflected signal light is injected back into the main interferometer module 3 through Cir 303 again, and then the light intersects at the polarization-maintaining coupler PMC3 302 and interferes. After the measured interference signal passes through PBS 304 and 305, the fast and slow axis information in the interference signal is stripped and enters BPD 306 for differential detection. BPD transmits the signals collected from the main interferometer and the auxiliary interferometer to the signal processing module 5, and after phase noise correction 501 and birefringence demodulation, the distributed birefringence of the polarization-maintaining fiber can be obtained.

[0055] A method for correcting time delay errors in long-distance birefringence measurement is implemented, comprising the following steps:

[0056] Step 1 (S01): uniformly inject interrogation light into the polarization orthogonal fast axis and slow axis of the optical fiber to be tested, and receive the Rayleigh scattering time domain signals I of the fast axis and slow axis respectively. F (t) and I S (t);

[0057] Step 2 (S02): The fast-axis and slow-axis Rayleigh scattering time domain signals I F (t) and I S (t) Fourier transform is performed to obtain the distribution characteristics of Rayleigh scattering signal with the fiber length f F (t) and f S (t);

[0058] Step 3 (S03): Starting from the starting point 5m, use a sliding window with a length of △Z = 5cm to measure the Rayleigh scattering signal f of the fast axis and the slow axis. F (t) and f S (t) is intercepted to obtain x F (0) and x P (0);

[0059] Step 4 (S04): x F (0) and x P (0) Demodulate the birefringence of the first window to obtain an initial birefringence value B0;

[0060] Step 5 (S05): Demodulate the birefringence of the second window, set the sliding distance of the fast axis sliding window to D, the total number of sliding times to N, and predict the error and correct it according to the birefringence demodulation result B0 of the previous window to obtain the sliding distance of the slow axis sliding window to D. S (0); get x F (1) and x P (1) and perform birefringence demodulation to obtain a birefringence value B1;

[0061] Step 6 (S06): Similarly, the birefringence values ​​of N windows are demodulated. When the number of step corrections n is less than the total number of sliding times N, a sliding window with a length of △Z is used to slide the fast axis and the slow axis according to the sliding distances D and D S (n-1), intercept the two-axis frequency domain signals for the nth time and get x F (n) and x P (n), where n=1,2,3,…;

[0062] Step 7 (S07): x F (n) and x P (n) Demodulate the birefringence of the nth window to obtain the birefringence value B n ;

[0063] Step 8 (S08): Based on the birefringence value B of the nth window n , predict the error and correct it to get the sliding distance of the n+1th slow axis sliding window as D S (n);

[0064] Step nine (S09): When the iterative correction number n is equal to the total sliding number N, the demodulation of the birefringence distribution of the optical fiber to be tested is completed; when the iterative correction number n is less than the total sliding number N, repeat steps six to eight to complete the delay error prediction and birefringence demodulation of the remaining length.

[0065] Figure 4The results of birefringence measurement of polarization-maintaining optical fiber from 5 m to 100 m are shown. In short-distance birefringence measurement, the measurement system can accurately measure the birefringence of the optical fiber, but as the measured length of the optical fiber increases, the measurement error of birefringence becomes larger and larger, and it is completely impossible to measure after 70 m. This result explains why the traditional OFDR is limited in measuring distance when measuring birefringence, and also shows the necessity of birefringence delay error correction. After the delay error compensation of this method, the demodulation bad point completely disappears, which proves that the birefringence delay error correction method is effective.

[0066] Figure 5 The test results of birefringence of 3km polarization-maintaining fiber are shown. It can be seen that there are no demodulation bad points in this result.

[0067] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Although the present invention has been disclosed as a preferred embodiment as above, it is not used to limit the present invention. Any technician familiar with this profession can make some changes or modify the technical contents disclosed above into equivalent embodiments without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention without departing from the content of the technical solution of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A time delay error correction method for long-distance distributed birefringence measurement, characterized in that: The delay error in the birefringence measurement is iteratively corrected point by point. The delay error of the point to be demodulated is predicted by the distributed birefringence information from the starting point of the demodulation length to the point to be demodulated. The sliding window position of the polarization orthogonal axis is compensated according to the predicted value of the delay error, and the correlation between the Rayleigh scattering spectra of the polarization orthogonal axes is restored to achieve the extension of the distributed birefringence measurement distance. The specific process is as follows: Step 1 (S01): uniformly inject interrogation light into the polarization orthogonal fast axis and slow axis of the optical fiber to be tested, and receive the Rayleigh scattering time domain signals I of the fast axis and slow axis respectively. F (t) and I S (t); Step 2 (S02): The fast-axis and slow-axis Rayleigh scattering time domain signals I F (t) and I S (t) Fourier transform is performed to obtain the distribution characteristics of Rayleigh scattering signal with the fiber length f F (t) and f S (t); Step 3 (S03): Starting from the measurement zero point, a sliding window with a length of △Z is used to measure the Rayleigh scattering signal f of the fast axis and the slow axis. F (t) and f S (t) is intercepted to obtain x F (0) and x P (0); Step 4 (S04): x F (0) and x P (0) Demodulate the birefringence of the first window to obtain an initial birefringence value B0; Step 5 (S05): Demodulate the birefringence of the second window, set the sliding distance of the fast axis sliding window to D, the total number of sliding times to N, and predict the error and correct it according to the birefringence demodulation result B0 of the previous window to obtain the sliding distance of the slow axis sliding window to D. S (0); get x F (1) and x P (1) and perform birefringence demodulation to obtain a birefringence value B1; Step 6 (S06): Similarly, the birefringence values ​​of N windows are demodulated. When the number of step corrections n is less than the total number of sliding times N, a sliding window with a length of △Z is used to slide the fast axis and the slow axis according to the sliding distances D and D S (n-1), intercept the two-axis frequency domain signals for the nth time and get x F (n) and x P (n), where n=1,2,3,…; Step 7 (S07): x F (n) and x P (n) Demodulate the birefringence of the nth window to obtain the birefringence value B n ; Step 8 (S08): Based on the birefringence value B of the nth window n , predict the error and correct it to get the sliding distance of the n+1th slow axis sliding window as D S (n); Step nine (S09): When the iterative correction number n is equal to the total sliding number N, the demodulation of the birefringence distribution of the optical fiber to be tested is completed; when the iterative correction number n is less than the total sliding number N, repeat steps six to eight to complete the delay error prediction and birefringence demodulation of the remaining length.

2. The method for correcting time delay errors in long-distance distributed birefringence measurement according to claim 1, characterized in that: The prediction error and correction method in step 5 (S05) is as follows: the sliding distance of the second slow axis window is compensated according to the birefringence value B0 of the first window, which is defined as: D S (0) = D + B0×D.

3. The method for correcting time delay error in long-distance distributed birefringence measurement according to claim 1, characterized in that: The prediction error and correction method in step eight (S08) is as follows: the sliding distance of the n+1th slow axis window is based on the birefringence value B of the nth window. n Compensation is performed, defined as: D S (n) = D + B n ×D.

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