Relative phase strain demodulation method and device based on fractional cyclic shift

Abstract

The present invention belongs to the field of optical fiber sensing technology and discloses a relative phase strain demodulation method and device based on fractional cyclic shift in optical frequency domain reflection. The optical fiber under test is fixed on a displacement table, and the Rayleigh scattering relative phase of the optical fiber under test is obtained through a distributed optical fiber sensing device. The method solves the matching problem of arbitrary phase slope on the basis of fully retaining effective information, and finally realizes distributed optical fiber strain measurement with a spatial resolution of 0.907mm. The minimum strain resolution that can be successfully measured is 0.5 microstrain, and the measurement range is 36m.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing technology, and in particular to a relative phase strain demodulation method and device based on fractional cyclic shift in optical frequency domain reflection, which is applied to optical frequency domain reflection distributed strain sensing. Background Art

[0002] Distributed fiber-optic strain sensing, thanks to its numerous advantages, including light weight, compact size, immunity to electromagnetic interference, and non-conductivity, is widely used in structural health monitoring, three-dimensional shape sensing, and biomedical applications. Traditional distributed fiber-optic strain sensors, such as phase-sensitive optical time-domain reflectometers (OTDRs), can measure strain with nanostrain accuracy, but their spatial resolution is limited to the centimeter level. Furthermore, while Brillouin optical correlation domain analysis (COD) can achieve millimeter-level spatial resolution, its sensitivity to tens of microstrains limits its use in high-precision measurement applications.

[0003] Optical Frequency Domain Reflectometry (OFDR), as a type of distributed fiber optic sensing, can be used for distributed stress and temperature sensing. Traditional OFDR uses the Rayleigh scattering spectrum cross-correlation method to achieve a minimum measurable strain of ±1 microstrain at a spatial resolution of 1 cm. However, for strain changes less than 1 microstrain, the Rayleigh scattering spectrum cross-correlation method is still difficult to achieve effective demodulation. OFDR technology based on the relative phase method has been widely used due to its high spatial resolution at the millimeter level and large strain measurement range. However, the currently reported schemes only mention the use of a cyclic shift method to non-destructively solve the phase wrapping problem caused by the starting wavelength error between the reference state and the measurement state. Moreover, since the number of cyclic shift points is an integer, the phase wrapping slope offset by the shift is discrete, which cannot meet the requirements for offsetting phase wrapping with arbitrary slopes and lacks universality in terms of application range. At the same time, there is a lack of a high-precision strain demodulation method with sub-millimeter spatial resolution within medium distances to solve the phase unwrapping problem caused by strain. Summary of the Invention

[0004] In response to the problems existing in the prior art, the present invention provides a relative phase strain demodulation method and device based on fractional cyclic shift in optical frequency domain reflection, which solves the matching problem of arbitrary phase slope on the basis of fully retaining effective information, and ultimately realizes distributed optical fiber strain measurement with a spatial resolution of 0.907mm, and can successfully measure the minimum strain resolution of 0.5 microstrain.

[0005] The present invention is implemented as follows: a relative phase strain demodulation method based on fractional cyclic shift in optical frequency domain reflectometry, the method specifically comprising:

[0006] S1: After obtaining the relative phase in the distributed strain sensing device, the number of required cyclic shift points is calculated based on the relative phase slope, zero-filling interpolation is performed on the reference state and measurement state data, and the reference state or measurement state data is cyclically shifted. Finally, redundant data is deleted to achieve fractional cyclic shifting, thereby ultimately eliminating relative phase wrapping with arbitrary slopes.

[0007] S2: Based on the fractional cyclic shift, the raw data is subjected to a fractional cyclic shift, so that the slope caused by the starting wavelength of the light source is in the opposite direction to the slope caused by the strain. Peak-finding methods can be used to find the peak and valley positions at the start and end of phase wrapping and strain. By calculating the variance near each data point, the variance of the peak and valley positions at the phase wrapping location is large, while the variance of the peak and valley positions at the start and end of strain is very small. Therefore, the peak-finding method based on variance statistics is used to obtain the start and end positions of strain.

[0008] S3: Decomposing the unwrap relative phases of the unstrained and strained regions is performed by cyclic shifting a fraction, with the number of shift points m1 and m2, respectively. A basic strain phase information map is obtained based on m1 and m2 and the strain start and end positions. The decompressed relative phases of the strained and unstrained regions are concatenated and differentiated, denoised using the bidirectional Chebyshev method, and finally superimposed on the basic strain phase information map to achieve strain demodulation.

[0009] Furthermore, the S1 specifically includes:

[0010] In the optical frequency domain reflectometry system, the different frequency components of the beat frequency interference signal are used to represent the position information of the measured optical fiber, which can be expressed as:

[0011]

[0012] Where I(t) represents the beat frequency interference signal, E0 represents the input light field of the swept frequency light source, τ z represents the time delay at any point z in the optical fiber under test, f0 represents the starting scanning frequency of the swept light source, and f b represents the beat frequency and f b =γτ z , γ represents the sweep rate of the swept light source, and ξ represents the phase noise.

[0013] Δφ=2πΔfτ z (2)

[0014] dφ=2πkm / N(3)

[0015] Where Δf is the starting wavelength error of the light source, Δφ is the relative phase change caused by the starting wavelength error Δf of the light source, N is the number of sampled data points, m is the number of active cyclic shift points, k is the number of discrete points and is linearly related to the measured fiber position z of interest, k = Nfb / f clock , dφ is the phase change caused by the active cyclic shift of m points, and in order to offset the phase winding caused by the initial wavelength error of the light source, Δφ=dφ is required.

[0016] After performing FFT on the optical frequency domain signal, M zeros are padded and then IFFT is performed, so that the total number of data becomes M+N. Therefore, the above equation is transformed into:

[0017] 2πΔfτz=2πkm' / (N+M)(4)

[0018] In order to study the slope problem, we carefully observe the changes in the slopes on the left and right sides of the equation. On the left side of the equation, when the position differs by one point, the phase change caused by the starting wavelength error is:

[0019]

[0020] Wherein, N1 and N2 are the adjacent discrete position points in the distance domain represented by the measured optical fibers z1 and z2, so N2-N1=1. is the refractive index of the optical fiber, and ΔF is the frequency sweep range of the light source.

[0021] On the right side of the equation, when cyclically shifting m' points (the number of cyclic shift points in the case of zero padding is additionally recorded as m', and the number of cyclic shift points in the case of no zero padding is recorded as m), and the total number of points is M+N, the phase change actively introduced between adjacent points k1 and k2 (k2-k1=1) is:

[0022] dφ2-dφ1=2πk2m' / (N+M)-2πk1m' / (N+M)

[0023] =2πm' / (N+M) (6)

[0024] When the left and right sides of the equation are equal, that is, the slopes are equal:

[0025]

[0026] Wherein, δφ=Δφ2-Δφ1, δφ is the phase difference between adjacent relative phase points, i.e., the relative phase slope. The sources of δφ include the starting wavelength error of the light source and the optical frequency domain shift caused by temperature, stretching, etc. Here, the source of δφ is only the starting wavelength error of the light source. When the number of cyclic shift points m' satisfies equation (7), the relative phase slope introduced by active cyclic shift is equal to the slope caused by the starting wavelength error of the light source. Δf / ΔF can be any value, and when zero padding is not performed, m' / (M+N)=m / N, m / N is discrete, and therefore cannot satisfy equation (7). Therefore, the relative phase slope introduced by active cyclic shift cannot completely offset the slope caused by the starting wavelength error of the light source. After zero padding M, M is any integer greater than 0, so fractional cyclic shift can be achieved, making equation (7) completely match. Therefore, cyclic shifting by m' points when padding with M zeros is equivalent to cyclic shifting by m points when not padding with zeros. m can be a decimal, and the expression is:

[0027]

[0028] For example, since the number of cyclic shift points is m' = 865 when the data is expanded by a factor of 10, after restoring the total amount of data, according to equation (8), the actual number of cyclic shift points is m = 86.5, thus achieving a fractional cyclic shift. Finally, the redundant data is deleted to complete the fractional cyclic shift. This relative phase eliminates the phase wrapping of any slope caused by the error in the starting wavelength of the light source. When the source of δφ is only strain, the same logic can be obtained:

[0029]

[0030] Where ΔS represents the optical frequency shift caused by strain. Equation (9) means that the optical frequency shift caused by strain is the same as the initial wavelength error of the light source, and the relative phase slope follows the same proportional relationship.

[0031] Furthermore, the S2 specifically includes:

[0032] First, the original data is subjected to a fractional cyclic shift, where the number of cyclic shift points is the average of m1 and m2 (m1 and m2 are the cyclic shift points for relative phase detangling between the strained and unstrained regions).

[0033] By using the peak-finding method, we can find the peak-valley positions of phase winding and the start and end of strain. Therefore, it is necessary to distinguish the peak-valley positions of phase winding from the peak-valley positions of the start and end of strain.

[0034] By calculating the variance around each data point and comparing them, the strain start and end locations can be distinguished. The variance of the peaks and valleys at the phase wrapping point is large, while the variance of the peaks and valleys at the strain start and end is small. Therefore, the peak-finding method based on variance statistics is used to obtain the strain start and end locations.

[0035] Furthermore, the S3 specifically includes:

[0036] The decoupling relative phase of the unstrained and strained regions is obtained by cyclic shifting of decimals, with the number of shift points m1 and m2 respectively. The basic strain phase information diagram is obtained based on m1 and m2 and the strain start and end positions. The phase slope of the strain position can be calculated using m1 and m2:

[0037] δφ=2π / N×(m2-m1)(10)

[0038] The dewrapped relative phases of the strained and unstrained regions are concatenated and differentiated, denoised using the bidirectional Chebyshev method, and finally superimposed on the basic strain phase information map.

[0039] The relative phase results at each position are windowed and summed, and the strain-related phase results are demodulated.

[0040] Another object of the present invention is to provide a distributed optical fiber sensing device using the above-mentioned relative phase strain demodulation method based on fractional cyclic shift in optical frequency domain reflectometry, the device comprising:

[0041] Tunable laser, 95:5 optical beam splitter, computer, GPIB (general purpose interface bus) control module, clock trigger device based on auxiliary interferometer, main interferometer;

[0042] The clock trigger device based on the auxiliary interferometer includes: a first balanced detector, a first 50:50 coupler, a clock shaping circuit module, a delay fiber, a first Faraday rotator mirror, a second Faraday rotator mirror and a first circulator. The clock trigger device based on the auxiliary interferometer is used to achieve equal optical frequency spacing sampling, the purpose of which is to suppress nonlinear scanning of the light source;

[0043] The main interferometer includes an 80:20 beam splitter, a polarization controller, a second circulator, a second 50:50 coupler, a second balanced detector, a collection device, a reference arm, a test arm, and a long-distance fiber Bragg grating. The main interferometer is the core of the distributed fiber optic sensing device for optical frequency domain reflection and is an improved Mach-Zehnder interferometer.

[0044] The input end of the GPIB control module is connected to a computer, the output end of the GPIB control module is connected to a tunable laser, the tunable laser is connected to a port a of a 95:5 optical beam splitter, the b port of the 95:5 optical beam splitter is connected to a port a of a first circulator (5), the c port of the 95:5 optical beam splitter is connected to a port a of an 80:20 beam splitter (95), the b port of the first circulator is connected to a port a of a first 50:50 coupler, the c port of the first circulator is connected to an input end of a first balanced detector, the b port of the first 50:50 coupler is connected to an input end of the first balanced detector, the c port of the first 50:50 coupler is connected to a first Faraday rotator mirror, the d port of the first 50:50 coupler is connected to a second Faraday rotator mirror via a delay optical fiber, and the output end of the first balanced detector is connected to a time delay optical fiber. The input end of the clock multiplication circuit module is connected to the input end of the clock shaping circuit module, the output end of the clock shaping circuit module is connected to the input end of the acquisition device, the b port of the 80:20 beam splitter is connected to the input end of the polarization controller through the reference arm, the c port of the 80:20 beam splitter is connected to the a port of the second circulator through the test arm, the output end of the polarization controller is connected to the a port of the second 50:50 coupler, the b port of the second circulator is connected to the b port of the second 50:50 coupler, the c port of the circulator is connected to the long-distance fiber Bragg grating passing through the stretching zone, the c port of the second 50:50 coupler is connected to the input end of the second balanced detector, the d port of the second 50:50 coupler is connected to the input end of the second balanced detector, the output end of the second balanced detector is connected to the input end of the acquisition device, and the output end of the acquisition device is connected to the computer.

[0045] Furthermore, when the optical frequency domain reflectometry distributed optical fiber sensing device is working, the computer controls the tuning speed, center wavelength, tuning start, etc. of the tunable laser through the GPIB control module; the output light of the tunable laser enters from the a port of the 95:5 optical beam splitter, wherein the light with a ratio of 5 enters from the b port of the 95:5 optical beam splitter through the circulator into the a port of the first 50:50 coupler, and the light enters from the a port of the first 50:50 coupler and exits from the c and d ports of the first 50:50 coupler, and is respectively transmitted by the first law of the two arms. The light is reflected by the Faraday rotator and the second Faraday rotator mirror and returns to the c and d ports of the first 50:50 coupler. The two beams interfere with each other in the first 50:50 coupler and are output from the b port of the first 50:50 coupler. The light emitted from the b port of the first 50:50 coupler enters the first balanced detector. The first balanced detector converts the detected optical signal into an interference beat signal and transmits it to the clock shaping module. The clock shaping module shapes the interference beat signal into a square wave. The shaped signal is transmitted to the acquisition device as the external clock signal of the acquisition device.

[0046] The output light of the tunable laser enters from the a port of the 95:5 optical beam splitter, the light with a ratio of 95 enters from the c port of the 95:5 optical beam splitter into the a port of the 80:20 beam splitter, passes through the b port of the 80:20 beam splitter (20), enters the polarization controller in the reference arm, and enters the a port of the second circulator on the test arm from the c port (80), the light enters from the a port of the second circulator, and enters the long-distance fiber Bragg grating in the stretching zone from the c port of the second circulator, and the backscattered light of the long-distance fiber Bragg grating enters from the c port of the second circulator. The reference light output by the polarization controller in the reference arm is combined with the backscattered light on the second circulator through the a port of the second 50:50 coupler and the b port of the second circulator, thereby forming a beat frequency interference and outputting the combined light from the c port and the d port of the second 50:50 coupler to the second balanced detector. The second balanced detector transmits the output analog electrical signal to the acquisition device, which transmits the collected analog electrical signal to the computer under the action of the external clock signal formed by the clock shaping module.

[0047] Furthermore, the GPIB control module is used for the computer to control the tunable laser;

[0048] The tunable laser is used to provide a light source for the optical frequency domain reflection system, and the optical frequency of the laser can be linearly scanned.

[0049] Further, the first circulator prevents the reflected light of the b-port of the first 50:50 coupler in the auxiliary interferometer from entering the laser;

[0050] The first 50:50 coupler is used for optical interference;

[0051] The delay optical fiber is used to realize beat frequency interference of non-equal arms and provide a clock signal.

[0052] Furthermore, the first Faraday rotator mirror and the second Faraday rotator mirror are used to provide reflection for the interferometer and can eliminate the polarization fading phenomenon of the interferometer;

[0053] The polarization controller is used to adjust the polarization state of the reference light so that the light intensity in two orthogonal directions during polarization beam splitting is substantially consistent;

[0054] The computer performs data processing on the interference signal collected by the collection device to realize distributed optical fiber stress sensing using fractional cyclic shift.

[0055] In combination with the above technical solutions and the technical problems solved, the advantages and positive effects of the technical solutions to be protected by the present invention are as follows:

[0056] First, the present invention solves the phase winding problem of arbitrary slope caused by the starting wavelength error in the relative phase through the fractional cyclic shift method. At the same time, the starting and ending positions of the strain are accurately obtained in the peak-finding method of variance statistics, realizing distributed optical fiber strain measurement with a high spatial resolution of 0.907 mm and a high sensitivity of 0.5 microstrain.

[0057] 1) Fractional cyclic shift method (S1): After obtaining the relative phase in the distributed strain sensing device, the data of the reference state and the measurement state are zero-filled and interpolated, and the interpolated data of the reference state or the measurement state are cyclically shifted. Finally, redundant data is deleted to achieve fractional cyclic shift. This method can achieve lossless phase unwrapping with arbitrary slope.

[0058] 2) Peak-finding method based on variance statistics (S2): Based on fractional cyclic shift, the raw data is subjected to fractional cyclic shift, so that the slope caused by the starting wavelength of the light source is in the opposite direction to the slope caused by the strain. This peak-finding method can be used to find the peaks and valleys at the start and end of phase wrapping and strain. By calculating the variance around each data point, the two peaks and valleys with the smallest variance are found, which are the start and end locations of strain.

[0059] 3) Strain demodulation based on fractional cyclic shift (S3): The dewrapped relative phases of the unstrained and strained regions are obtained by using fractional cyclic shifts, with the number of shift points being m1 and m2, respectively. The basic strain phase information map is obtained based on m1 and m2 and the strain start and end positions. The dewrapped relative phases of the strained and unstrained regions are concatenated and differentiated, denoised using the bidirectional Chebyshev method, and finally superimposed on the basic strain phase information map. The relative phase results at each position are windowed and summed to demodulate the strain-related phase results.

[0060] This method effectively solves the phase wrapping problem of arbitrary slope by combining multiple technologies such as fractional cyclic shift and bidirectional Chebyshev low-pass filtering, and improves the accuracy and stability of strain measurement, reflecting significant technological progress.

[0061] Second, the expected benefits and commercial value of the present invention's technical solution after transformation are as follows: This invention, based on high-precision strain measurement using OFDR relative phase, has important applications in structural health monitoring and three-dimensional shape sensing. With the continuous refinement and intelligence of technologies and equipment, the commercial value of structural health monitoring and three-dimensional shape sensing based on this invention in cutting-edge military and civilian applications, such as major national aerospace facilities, industrial robots, and medical equipment, is also increasing.

[0062] The technical solution of the present invention fills the technical gaps in the industry at home and abroad: in the relative phase demodulation method in optical frequency domain reflection, the relative phase method has attracted widespread attention due to its high precision and high spatial resolution. However, the relative phase method has a unique phase wrapping problem, and different solutions have been proposed in different research reports. In various solutions, the relative phase point information will be lost, and the algorithm is complex and not easy to equipment. The cyclic shift method can solve the phase wrapping problem losslessly, and the algorithm is extremely simple, but the number of shift points is an integer and cannot match any relative phase slope. The patent proposes a decimal cyclic shift algorithm, which provides a matching method for any slope while retaining the advantages of the original algorithm. It is easy to implement, reduces equipment requirements, and provides an equipment basis for the relative phase method.

[0063] Furthermore, currently reported approaches lack a method for phase unwrapping to demodulate small strains at submillimeter spatial resolution over medium distances. However, the present invention utilizes fractional cyclic shifting and a peak-finding method based on variance statistics to accurately obtain the strain start and end positions. The number of cyclic shift points is then used to obtain fundamental strain phase information. Finally, bidirectional Chebyshev low-pass filtering is used to achieve strain measurement with a spatial resolution of 0.907 mm, a measurement distance of 36 m, a strain accuracy of 0.5 microstrain, a maximum error of 0.2 microstrain, and a measurement standard deviation of 0.077 microstrain.

[0064] Third, the positive technical effects and significant technical advances of the present invention also include:

[0065] Improved accuracy: By eliminating phase wrapping and reducing noise interference, this method significantly improves the accuracy of measurement results.

[0066] Improved feature information extraction efficiency: Optimized the feature information extraction efficiency of complex signals in the data processing process, which can effectively obtain feature information and is particularly suitable for large-scale data analysis.

[0067] Strong adaptability: This method has better adaptability to phase winding with arbitrary slope, which improves the reliability and stability of the system.

[0068] Wide application: This method can be applied to various fields that require high-precision strain monitoring, such as health monitoring of buildings, bridges, and aerospace structures. BRIEF DESCRIPTION OF THE DRAWINGS

[0069] Figure 1 1 is a schematic structural diagram of a distributed optical fiber sensing device for optical frequency domain reflection provided by an embodiment of the present invention;

[0070] Figure 2 This is a flow chart of a relative phase strain demodulation method based on fractional cyclic shift in optical frequency domain reflectometry provided by an embodiment of the present invention;

[0071] Figure 3 Schematic diagram of relative phase unwrapping through fractional cyclic shift, zero padding and redundant information removal provided by an embodiment of the present invention;

[0072] Figure 4 Schematic diagram of obtaining the start and end positions of strain using a peak-finding method based on variance statistics provided by an embodiment of the present invention;

[0073] Figure 5 is a schematic diagram of a strain demodulation process based on fractional cyclic shift provided by an embodiment of the present invention;

[0074] Figure 6 is a schematic diagram of a strain demodulation result based on fractional cyclic shift provided by an embodiment of the present invention;

[0075] Figure 7 is a schematic diagram of a calibration curve provided by an embodiment of the present invention;

[0076] Figure 8 This is an evaluation comparison diagram of different filtering methods provided by the embodiment of the present invention.

[0077] In the figure: 1. Tunable laser; 2. First balanced detector; 3. 80:20 beam splitter; 4. 95:5 optical beam splitter; 5. 50:50 coupler; 6. Clock shaping circuit module; 7. Delay fiber; 8. First Faraday rotator; 9. Second Faraday rotator; 10. First circulator; 11. Computer; 12. Polarization controller; 13. Second circulator; 14. 50:50 coupler; 15. Stretching area; 16. Second balanced detector; 17. Acquisition device; 18. GPIB control module; 19. Reference arm; 20. Test arm; 21. Clock trigger device based on auxiliary interferometer; 22. Main interferometer; 23. Long-distance fiber Bragg grating. DETAILED DESCRIPTION

[0078] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0079] The present invention aims to improve the following problems and defects of the prior art and achieve significant technological progress:

[0080] 1. Phase wrapping problem with arbitrary slope: In traditional distributed strain sensing systems, phase wrapping often occurs due to errors in the starting wavelength of the light source. However, the existing cyclic shift method has an integer number of shift points, so the phase wrapping slope offset by the shift is discrete and cannot meet the requirements for offsetting phase wrapping with arbitrary slopes. This makes the cyclic shift method less universal in solving the phase wrapping problem, ultimately making it complex and difficult to accurately extract strain data from phase information.

[0081] 2. Noise interference: Environmental noise and the noise of the equipment itself will affect the accuracy of the measurement results, especially when extracting strain information with small changes.

[0082] 3. Low efficiency of feature information extraction: Traditional methods have low efficiency in feature information extraction when processing large amounts of data, especially in applications that require high precision and high resolution.

[0083] In view of the problems existing in the prior art, the technical solution adopted by the present invention is:

[0084] 1. Fractional cyclic shift: Through zero-padding interpolation, cyclic shift, and redundant data removal, the phase wrapping problem of arbitrary slopes is losslessly eliminated, which enhances the readability and accuracy of the data. The method is simple and easy to implement on the device.

[0085] 2. Peak finding method based on variance statistics: Through variance statistics, the phase winding position with larger variance is distinguished from the strain start and end positions with smaller variance, thus achieving accurate determination of the strain start and end positions.

[0086] 3. Bidirectional Chebyshev low-pass filtering: De-noise the relative phase information after differentiation. Compared with the normal Chebyshev low-pass filtering method, bidirectional Chebyshev low-pass filtering can effectively avoid phase information distortion.

[0087] 4. Segment summation: This method is used to obtain the final distributed strain magnitude, which improves the efficiency and accuracy of feature information extraction in data processing.

[0088] Two specific application examples of the present invention are:

[0089] Example 1: Bridge Health Monitoring

[0090] The relative phase strain demodulation method is applied to the health monitoring of bridge structures, so that the real-time health status of the bridge can be accurately monitored and potential structural problems can be discovered in a timely manner.

[0091] Device deployment: Distributed strain sensing devices are installed at key structural parts of the bridge.

[0092] Phase acquisition: First, obtain the relative phase data and calculate the required number of cyclic shift points.

[0093] Fractional cyclic shift: Performs fractional cyclic shift on the data to losslessly eliminate phase wrapping of arbitrary slope.

[0094] Peak finding method based on variance statistics: Through variance statistics, accurate determination of the start and end positions of strain is achieved.

[0095] Bidirectional Chebyshev low-pass filtering: Bidirectional Chebyshev low-pass filtering is used to denoise the relative phase information after differentiation and improve data accuracy.

[0096] Final strain analysis: Accurate bridge strain data is obtained through the fractional cyclic shift phase method and piecewise summation.

[0097] Example 2: Wind turbine blade monitoring

[0098] It is used to monitor the structural integrity and performance of wind turbine blades, monitor the strain status of wind turbine blades in real time, and promptly detect potential damage or performance degradation caused by factors such as wind force and temperature changes, thereby ensuring the safe operation and maintenance efficiency of wind power facilities.

[0099] Device deployment: Distributed strain sensing devices are installed at key locations on wind turbine blades.

[0100] Phase acquisition and cyclic shift: Similar to Example 1, the relative phase is first acquired, and then fractional cyclic shift processing is performed to losslessly eliminate phase wrapping of arbitrary slopes.

[0101] Peak finding method based on variance statistics: Through variance statistics, accurate determination of the start and end positions of strain is achieved.

[0102] Bidirectional Chebyshev low-pass filtering: Bidirectional Chebyshev low-pass filtering is used to denoise the relative phase information after differentiation and improve data accuracy.

[0103] Final strain analysis: The strain distribution of the blade is obtained through the fractional cyclic shift phase method and segmented summation.

[0104] The present invention will be described below with reference to the accompanying drawings.

[0105] The optical frequency domain reflectometry distributed optical fiber sensing device includes: a tunable laser 1, a 95:5 optical beam splitter 4, a computer 11, a GPIB control module 18, a clock trigger device 21 based on an auxiliary interferometer, and a main interferometer 22.

[0106] The auxiliary interferometer-based clock trigger device 21 includes a first balanced detector 2, a first 50:50 coupler 5, a clock shaping circuit module 6, a delay fiber 7, a first Faraday rotator mirror 8, a second Faraday rotator mirror 9, and a first circulator 10. The auxiliary interferometer-based clock trigger device 21 is used to achieve equal optical frequency spacing sampling, which aims to suppress nonlinear scanning of the light source.

[0107] The main interferometer 22 includes an 80:20 beam splitter 3, a polarization controller 12, a second circulator 13, a second 50:50 coupler 14, a second balanced detector 16, a collection device 17, a reference arm 19, a test arm 20, and a long-distance fiber Bragg grating 23. The main interferometer 22 is the core of the optical frequency domain reflectometry distributed fiber sensing device and is a modified Mach Zehnder interferometer.

[0108] The input end of the GPIB control module 18 is connected to the computer 11, the output end of the GPIB control module 18 is connected to the tunable laser 1, the tunable laser 1 is connected to the a port of the 95:5 optical beam splitter 4, the b port of the 95:5 optical beam splitter 4 is connected to the a port of the first circulator 10 (5), the c port of the 95:5 optical beam splitter 4 is connected to the a port of the 80:20 beam splitter 3 (95), the b port of the first circulator 10 is connected to the a port of the first 50:50 coupler 5, the c port of the first circulator 10 is connected to the input end of the first balanced detector 2, the b port of the first 50:50 coupler 5 is connected to the input end of the first balanced detector 2, the c port of the first 50:50 coupler 5 is connected to the first Faraday rotator mirror 8, the d port of the first 50:50 coupler 5 is connected to the second Faraday rotator mirror 9 through the delay fiber 7, and the output end of the first balanced detector 2 is connected to the input end of the clock multiplication circuit module 6 The output end of the clock shaping circuit module 6 is connected to the input end of the acquisition device 17, the b(20) port of the 80:20 beam splitter 3 is connected to the input end of the polarization controller 12 through the reference arm 19, the c(80) port of the 80:20 beam splitter 3 is connected to the a port of the second circulator 13 through the test arm 20, the output end of the polarization controller 12 is connected to the a port of the second 50:50 coupler 14, the b port of the second circulator 13 is connected to the b port of the second 50:50 coupler 14, the c port of the circulator 13 is connected to the long-distance fiber Bragg grating 23 passing through the stretching zone, the c port of the second 50:50 coupler 14 is connected to the input end of the second balanced detector 16, the d port of the second 50:50 coupler 14 is connected to the input end of the second balanced detector 16, the output end of the second balanced detector 16 is connected to the input end of the acquisition device 17, and the output end of the acquisition device 17 is connected to the computer 11.

[0109] When the device is working, the computer 11 controls the tuning speed, center wavelength, tuning start, etc. of the tunable laser 1 through the GPIB control module 18. The output light of the tunable laser 1 enters from the a port of the 95:5 optical beam splitter 4 and enters the a port of the first 50:50 coupler 5 from the b port of the 95:5 optical beam splitter 4 through the circulator 10 at a ratio of 5:95. The light enters from the a port of the first 50:50 coupler 5 and exits from the c and d ports of the first 50:50 coupler 5, and is respectively transmitted by the first Faraday mirror 8 and the second Faraday rotator 8 of the two arms. The light is reflected by the Raday mirror 9 and returns to the c and d ports of the first 50:50 coupler 5. The two beams of light interfere with each other in the first 50:50 coupler 5 and are output from the b port of the first 50:50 coupler 5. The outgoing light from the b port of the first 50:50 coupler 5 enters the first balanced detector 2. The first balanced detector 2 converts the detected light signal into an interference beat signal and transmits it to the clock shaping module 6. The clock shaping module 6 shapes the interference beat signal into a square wave. The shaped signal is transmitted to the acquisition device 17 as the external clock signal of the acquisition device 17.

[0110] The output light of the tunable laser 1 enters from the a port of the 95:5 optical beam splitter 4, enters the a port of the 80:20 beam splitter 3 from the c port (95) of the 95:5 optical beam splitter 4, passes through the b port of the 80:20 beam splitter 3 (20), enters the polarization controller 12 in the reference arm 19, and enters the a port of the second circulator 13 on the test arm 20 from the c port (80). The light enters from the a port of the second circulator 13 and enters the long-distance fiber Bragg grating 23 in the stretching zone 15 from the c port of the second circulator 13, while the backscattered light of the long-distance fiber Bragg grating 23 enters from the c port of the second circulator 13. The reference light output from the polarization controller 12 in the reference arm 19 is output from the port b of the second circulator 13 and is combined with the backscattered light on the second circulator 13 through the port a of the second 50:50 coupler 14 through the port b of the second 50:50 coupler 14 to form beat frequency interference and is output from the port c and the port d of the second 50:50 coupler 14 to the second balanced detector 16. The second balanced detector 16 transmits the output analog electrical signal to the acquisition device 17. The acquisition device 17 transmits the collected analog electrical signal to the computer 11 under the action of the external clock signal formed by the clock shaping module 6.

[0111] The GPIB control module 18 is used for the computer 11 to control the tunable laser 1 .

[0112] The tunable laser 1 is used to provide a light source for the optical frequency domain reflectometry system, and its optical frequency can be linearly scanned.

[0113] The first circulator 10 prevents the reflected light of the b-port of the first 50:50 coupler 5 in the auxiliary interferometer from entering the laser.

[0114] The first 50:50 coupler 5 is used for light interference.

[0115] The delay optical fiber 7 is used to realize the beat frequency interference of non-equal arms and provide a clock signal.

[0116] The first Faraday rotator mirror 8 and the second Faraday rotator mirror 9 are used to provide reflection for the interferometer and can eliminate the polarization fading phenomenon of the interferometer.

[0117] The polarization controller 12 is used to adjust the polarization state of the reference light so that the light intensity in two orthogonal directions during polarization splitting is substantially consistent.

[0118] The computer 11 processes the interference signal collected by the collection device 17 to realize distributed optical fiber stress sensing using fractional cyclic shift.

[0119] This distributed fiber-optic sensor device for optical frequency domain reflection achieves high-precision monitoring of stress or other external changes through the coordinated operation of core components. First, the computer controls the optical frequency scanning of the tunable laser through the GPIB control module to achieve precise tuning speed, center wavelength, etc. The light source enters the 95:5 optical beam splitter and is divided into two paths, of which 5% passes through the first circulator and enters the 50:50 coupler of the auxiliary interferometer. The light is divided into two paths within the coupler and reflected back to the coupler through the first and second Faraday rotators, generating an interference signal in the coupler. The interference light is detected by the first balanced detector and transmitted to the clock shaping module, which shapes the optical signal into a square wave as an external clock signal and transmits it to the acquisition device to ensure that the sampling is synchronized with the optical frequency.

[0120] Next, another portion (95%) of the laser light source passes through a beam splitter and enters the main interferometer. Light enters the system along the reference arm and test arm, respectively. The reference light passes through a polarization controller to adjust its polarization state, ensuring balanced interferometric light intensity. The test arm light enters a long-distance fiber Bragg grating (FBG). The backscattered light in the fiber passes through a second circulator and returns to a 50:50 coupler. The beams from the two optical paths interfere again within the coupler, and the resulting interferometric beat signal is transmitted to the acquisition device via a second balanced detector.

[0121] The acquisition device, driven by an external clock provided by a clock shaping module, synchronizes the collected interference signal, ensuring accurate data acquisition. A computer processes the collected analog signal and, through fractional cyclic shift technology, achieves high-resolution distributed stress sensing. Utilizing equal-frequency sampling control and phase unwrapping, the sensing results are extremely accurate.

[0122] Finally, the delay fiber in the additional interferometer provides a non-equal-arm structure for beat-frequency interferometry, further optimizing the interference signal quality. The first and second Faraday rotators eliminate polarization fading in the interfering light, ensuring signal stability and sensor reliability. By demodulating the fiber signals at multiple locations, a computer can extract distributed stress distribution maps, providing reliable data support for health monitoring of large structures.

[0123] The following are two specific industrial application examples based on the fractional cyclic shift relative phase strain demodulation method:

[0124] 1. Bridge and building structural health monitoring

[0125] In large-scale infrastructure such as bridges and tunnels, this relative phase strain demodulation method based on optical frequency domain reflection can be applied to monitor the strain distribution of structures. By arranging distributed fiber optic sensors on the surface or internal structure of the bridge, strain data at different locations in the structure can be acquired in real time, enabling the rapid identification of areas with abnormal strain. The high precision and real-time performance of this method support long-term monitoring of key bridge nodes, especially under complex external force conditions such as wind loads and traffic loads. After sensing the strain changes, the detection system quickly unwraps the relative phase information using a fractional cyclic shift method, removes unnecessary noise, and displays the strain changes in the bridge structure. This method provides data support for the safe operation of bridges, helps to promptly identify potential risks, and improves the efficiency of infrastructure maintenance and repair.

[0126] 2. Strain monitoring in oil drilling pipelines

[0127] In the industrial scenarios of oil drilling and pipeline transportation, the use of fiber optic sensors for strain monitoring is an important means of ensuring operational safety. A relative phase strain demodulation method based on fractional cyclic shift can be deployed at key points in drilling or oil and gas pipelines, monitoring pipeline stress changes and deformation through optical frequency domain reflectometry. In long-distance transmission pipelines, this method can effectively detect and locate areas of stress concentration, analyze the strain state of each node, identify overloads or deformations, and prevent pipeline damage and leakage accidents. Real-time, high-precision strain demodulation methods enable the oil drilling and pipeline industries to enhance monitoring capabilities and ensure the safety of downhole operations and oil and gas transportation.

[0128] This embodiment also provides a relative phase strain demodulation method based on fractional cyclic shift in optical frequency domain reflectometry, such as Figure 2 As shown, the steps of the sensing method are:

[0129] The beat frequency data of the reference state and the measurement state are obtained through the sensing system, and Fourier transform is performed to obtain the distance domain signal. Then, the two sets of distance domain information signals are padded with zeros at the end, and the two sets of signals are inversely Fourier transformed to obtain the interpolated reference data and the interpolated measurement data. By cyclically shifting the interpolated beat frequency data of one of the reference state or the measurement state, the relative phase obtained still has redundancy, such as Figure 3 (a). Delete the redundant signal, as shown in Figure 3 Finally, the relative phase at the measured fiber is intercepted to achieve fractional cyclic shift. According to formula (8), the number of fractional shift points is 86.5. Compared with the 86 and 87 points of ordinary cyclic shift, fractional cyclic shift can completely offset the phase wrapping of any slope, as shown in Figure 3 (c) shown.

[0130] In the process of peak search based on variance statistics, first, the original data is subjected to fractional cyclic shift according to the above process. Here, the number of cyclic shift points is the average value of m1 and m2 (m1 and m2 are the number of phase unwrapping cyclic shift points at the non-strained and strained positions, respectively. The relevant effect diagram can be referred to Figure 5 (a)(b)). By using the peak-finding method, the peak and valley positions of phase winding and strain start and end can be found, as shown in Figure 4 The black and red circles in (a) and (b) are shown. Then the variance near each data point is calculated to form a variance statistical graph, as shown in Figure 4 (c) By comparing the variance of the peak and valley positions, the starting and ending positions of the strain can be easily distinguished, which are the two points with the smallest variance, as shown in Figure 4 As shown in (d), the starting and ending position information of the strain is obtained.

[0131] In the strain demodulation process based on fractional cyclic shift, the phase unwrapping of the non-strained region and the strained region is achieved by fractional cyclic shift, such as Figure 5 (a) and (b) show that the number of cyclic shift points is m1 and m2 respectively. Using formula (10), as well as m1 and m2, the relative phase slope of the fiber stress area can be directly calculated (linearly related to the stress magnitude). Combined with the strain start and end position information, the basic strain phase information diagram of the fiber stress can be obtained, as shown in Figure 5 As shown in (c). Since the force on the optical fiber is not ideally uniform, and the optical fiber also contains strain disturbances, environmental changes and other factors in addition to stray noise. Therefore, in order to compensate for this information, the relative phase derivative information of the original unwinding of the optical fiber strain and non-strained area is added to the basic strain phase information. Figure 5 The non-strained area marked by the green line in (a1) Figure 5(b1) The original relative phase information of the strain area marked by the green line is spliced ​​and then derived to obtain the original lossless relative phase derivative information after removing the basic strain phase information, as shown in Figure 5 (d) is shown. Figure 5 The phase information of (d) is subjected to bidirectional Chebyshev low-pass filtering and then combined with Figure 5 (c) Add, and after taking the sum of the window in units of 10, the demodulated strain result is as follows Figure 5 As shown in (e1)(e2).

[0132] In summary, in response to the problems existing in the prior art, the present invention provides a relative phase strain demodulation method and device based on fractional cyclic shift in optical frequency domain reflection, which solves the matching problem of arbitrary phase slope on the basis of fully retaining effective information, and ultimately realizes distributed optical fiber strain measurement with a spatial resolution of 0.907 mm, and can successfully measure the minimum strain resolution of 0.5 microstrain.

[0133] The feasibility of the sensing system and sensing method is verified by combining specific experiments. Figure 6 and Figure 7 , see the following description for details:

[0134] The verification experiment of this embodiment adopts a long-distance fiber Bragg grating, which is 36m in length and consists of 3600 segments, each segment is 10mm long, wherein the grating length is 9mm and the central wavelength is 1550nm.

[0135] The final 40 cm of a long-distance fiber Bragg grating (FBG) 23 was fixed at one end and glued to a nano-displacement stage at the other. The piezoelectric nano-stage was moved within a 1 mm range with a 0.2 mm step size, thereby imparting 0.5 to 2.5 micro-strains to the fiber. During the stretching process, data acquisition was performed to obtain data corresponding to the micro-strains. In the windowed summation of the relative phase results, a window of 10 points means that the resolution of the entire system is 0.907 mm.

[0136] In the experiment, we used an OFDR system with a starting frequency of 1540.1 nm, a sweep rate of 80 nm / s, 1.5 M sampling points, and an additional interferometer fiber length of 136 m.

[0137] Figure 6 (a) Phase changes in the five cases where the microstrain increases from 0.5 to 2.5. Figure 6 (b) and (c) show the phase changes at the start and end positions of the strain, which proves that the sensing system and sensing method can achieve distributed optical fiber stress measurement with a spatial resolution of 0.907 mm and a sensitivity of 0.5 microstrain. Figure 6(d) Phase changes in five cases from 5 microstrain to 25 microstrain.

[0138] In order to verify the linearity of the phase method, Figure 7 for Figure 6 The 441 valid points in (a) are averaged and the results are fitted with linearity. The figure shows that the phase result has good linearity.

[0139] It should be noted that the embodiments of the present invention can be implemented by hardware, software or a combination of software and hardware. The hardware portion can be implemented using dedicated logic, and the software portion can be stored in a memory and executed by an appropriate instruction execution system, such as a microprocessor or dedicated design hardware. Those skilled in the art will appreciate that the above-mentioned devices and methods can be implemented using computer-executable instructions and / or contained in a processor control code, such as on a carrier medium such as a disk, CD or DVD-ROM, a programmable memory such as a read-only memory (firmware) or a data carrier such as an optical or electronic signal carrier, providing such code. The device of the present invention and its modules can be implemented by hardware circuits such as very large scale integrated circuits or gate arrays, semiconductors such as logic chips, transistors, or programmable hardware devices such as field programmable gate arrays, programmable logic devices, etc., can also be implemented by software executed by various types of processors, or can be implemented by a combination of the above-mentioned hardware circuits and software, such as firmware.

[0140] See also Figure 8 As shown in Figure 2, different filtering methods are compared. The phase integral obtained by the bidirectional Chebyshev low-pass filtering method is directly compared with the original relative phase and the cross-correlation comparison is shown in Figure 2. Figure 8 (a)(b) As shown. It can be seen that the two are highly consistent, indicating that this method can restore the phase fluctuation changes in the local area while demodulating the strain with high precision, and accurately give the overall phase change trend. In addition, since the peak of the cross-correlation result is high, it shows that the similarity between the two is high, and the cross-correlation peak is small and sharp, which shows that the local position morphology of the two is unique. The strain demodulation of the bidirectional Chebyshev filter can well retain the local strain fluctuations. In addition, the demodulation results of the FIR filtering method are shown in Figure 8 As shown in (c), there are still some errors in the fiber starting section, so the FIR filtering method is abandoned. In addition, if ordinary Chebyshev low-pass filtering is used, phase distortion will occur, such as Figure 8 As shown in (d), the normal low-pass filtering method is not used.

[0141] The following is the detailed working principle of the relative phase strain demodulation method based on fractional cyclic shift in optical frequency domain reflectometry:

[0142] 1. Fractional cyclic shift and data preprocessing

[0143] This method first acquires relative phase data from the reference and measurement states in a distributed strain sensing device and prepares for dewrapping. The initial step involves zero-padding the reference and measurement data in the distance domain to improve data resolution. Then, a cyclic shift is performed on the reference or measurement data to effectively address the relative phase difference. After the cyclic shift, lossless phase dewrapping is achieved by removing excess data, ensuring data integrity and preserving the original phase information.

[0144] 2. Slope reverse adjustment and start and end point identification

[0145] To further locate the strain onset and end points, this method adjusts the raw data based on fractional cyclic shifts so that the slope caused by the starting wavelength of the light source is in the opposite direction to the slope caused by the strain. This facilitates identification of the strain onset and end points. Subsequently, a peak-finding method is used to find and mark the peak and valley positions (including phase wrapping positions and strain onset and end positions) in the relative phase data. Calculating the variance near each peak and valley position helps further identify the peak and valley positions, ensuring that the two positions with the smallest variance are found, accurately locating the strain onset and end points, and eliminating phase wrapping positions.

[0146] 3. Phase unwrapping and establishment of basic strain phase information map

[0147] Based on the strain region start and end position information obtained in the previous step, fractional cyclic shift is used to further extract the relative phase of the unstrained and strained regions and remove the entanglement. The relative phase slopes of the unstrained and strained regions are removed by cyclic shift, and the number of shift points is recorded as m1 and m2. Based on m1, m2, equation (10), and the strain start and end positions, a basic strain phase information map is constructed. This map contains the basic distribution of the relative phase change rate, laying the foundation for subsequent analysis.

[0148] 4. Denoising and demodulation of strain-related phase

[0149] To eliminate the influence of signal noise on the demodulation results, a bidirectional Chebyshev filter was used to denoise the spliced ​​relative phase data, resulting in a smooth phase data curve. The denoised data were then overlaid on the basic strain phase information map to form a complete strain phase distribution map. Finally, a windowed summation method was applied to the relative phase data at each position, accumulating the relative phase results within different windows to demodulate the final phase result related to strain. This result reflects the strain distribution and enables high-precision strain demodulation.

[0150] To sum up, the above is only a specific implementation method of the present invention, but the scope of protection of the present invention is not limited to this. Any modifications, equivalent substitutions and improvements made by any technician familiar with this technical field within the technical scope disclosed by the present invention and within the spirit and principles of the present invention should be covered by the scope of protection of the present invention.

Claims

1. A relative phase strain demodulation method based on fractional cyclic shift in optical frequency domain reflectometry, characterized in that: The method specifically includes: S1: After obtaining the relative phase in the distributed strain sensing device, the reference state and measurement state data are zero-filled and interpolated, and the reference state or measurement state data are cyclically shifted. Finally, redundant data is deleted to achieve fractional cyclic shift. This method can achieve lossless phase unwrapping with arbitrary slope. S2: Based on the fractional cyclic shift, the original data is subjected to a fractional cyclic shift so that the slope caused by the starting wavelength of the light source is in the opposite direction to the slope caused by the strain. The peak-valley positions at the phase wrapping and the start and end of the strain can be found by using the peak-finding method. By calculating the variance near the position of each data point, the variance of the peak-valley positions at the phase wrapping is large, while the variance of the peak-valley positions at the start and end of the strain is very small. Therefore, the peak-finding method of variance statistics is used to obtain the start and end positions of the strain. S3: The dewrapped relative phases of the unstrained and strained regions are obtained by cyclic shifting of decimals, with the number of shift points being m1 and m2 respectively. The basic strain phase information diagram is obtained based on m1 and m2 and the start and end positions of the strain. The dewrapped relative phases of the strained and unstrained regions are concatenated and differentiated, denoised using the bidirectional Chebyshev method, and finally superimposed on the basic strain phase information diagram. The relative phase results at each position are windowed and summed to demodulate the strain-related phase results.

2. The relative phase strain demodulation method based on fractional cyclic shift in optical frequency domain reflectometry according to claim 1, wherein: Said S1 specifically includes: In the optical frequency domain reflectometry system, the different frequency components of the beat frequency interference signal are used to represent the position information of the measured optical fiber, which can be expressed as: Where I(t) represents the beat frequency interference signal, E0 represents the input light field of the swept frequency light source, τ z represents the time delay at any point z in the optical fiber under test, f0 represents the starting scanning frequency of the swept light source, and f b represents the beat frequency and f b =γτ z , γ represents the sweep rate of the swept light source, ξ represents the phase noise; Δφ=2πΔfτ z (2) dφ=2πkm / N(3) Where Δf is the starting wavelength error of the light source, Δφ is the relative phase change caused by the starting wavelength error Δf of the light source, N is the number of sampled data points, m is the number of active cyclic shift points, k is the number of discrete points and is linearly related to the measured fiber position z of interest, k = Nf b / f clock , dφ is the phase change caused by the active cyclic shift of m points, and in order to offset the phase winding caused by the error in the starting wavelength of the light source, Δφ = dφ is required; After performing FFT on the optical frequency domain signal, M zeros are padded and then IFFT is performed, so that the total number of data becomes M+N. Therefore, the above equation is transformed into: 2πΔfτ z =2πkm' / (N+M)(4) Because of the slope problem, carefully observe the changes in the slope on the left and right sides of the equation; on the left side of the equation, when the position differs by one point, the phase change caused by the starting wavelength error is: Wherein, N1 and N2 are the adjacent discrete position points in the distance domain represented by the measured optical fibers z1 and z2, so N2-N1=1. is the refractive index of the optical fiber, ΔF is the frequency sweep range of the light source; On the right side of the equation, when cyclically shifting m' points (the number of cyclic shift points in the case of zero padding is additionally recorded as m', and the number of cyclic shift points in the case of no zero padding is recorded as m), and the total number of points is M+N, the phase change actively introduced between adjacent points k1 and k2 (k2-k1=1) is: When the left and right sides of the equation are equal, that is, the slopes are equal: Wherein, δφ=Δφ2-Δφ1, δφ is the phase difference between adjacent relative phase points, i.e., the relative phase slope. The sources of δφ include the starting wavelength error of the light source and the optical frequency domain shift caused by temperature, stretching, etc. Here, the source of δφ is only the starting wavelength error of the light source. When the number of cyclic shift points m' satisfies equation (7), the relative phase slope introduced by active cyclic shift is equal to the slope caused by the starting wavelength error of the light source. Δf / ΔF can be any value, and when zero padding is not performed, m' / (M+N)=m / N, m / N is discrete, and therefore cannot satisfy equation (7). Therefore, the relative phase slope introduced by active cyclic shift cannot completely offset the slope caused by the starting wavelength error of the light source. After zero padding is added M, M is any integer greater than 0, so fractional cyclic shift can be achieved, so that equation (7) is completely matched. Therefore, in the case of M zero padding, cyclic shifting by m' points is equivalent to cyclic shifting by m points in the case of no zero padding, where m can be a fractional number, and its expression is: For example, since the number of cyclic shift points is m' = 865 when the data is expanded to 10 times, after restoring the total amount of data, according to formula (8), the actual number of cyclic shift points is m = 86.5, thus achieving fractional cyclic shift. Finally, the redundant data is deleted to complete the fractional cyclic shift. The relative phase has eliminated the phase wrapping of any slope caused by the error in the starting wavelength of the light source. When the source of δφ is only strain, the following can be obtained by the same logic: Where ΔS represents the optical frequency shift caused by strain; Equation (9) means that the optical frequency shift caused by strain is the same as the initial wavelength error of the light source, and the relative phase slope follows the same proportional relationship.

3. The relative phase strain demodulation method based on fractional cyclic shift in optical frequency domain reflectometry according to claim 1, wherein: The S2 specifically includes: First, the original data is subjected to fractional cyclic shift, where the number of cyclic shift points is the average of m1 and m2 (m1 and m2 are the number of cyclic shift points for relative phase detangling between the strained and unstrained regions); By using the peak-finding method, the peak-valley positions of phase winding and the start and end of strain can be found; therefore, it is necessary to distinguish the peak-valley positions of phase winding from the peak-valley positions of the start and end of strain. By calculating the variance near each data point and comparing them, the starting and ending positions of the strain can be distinguished. The variance of the peak-valley positions at the phase winding point is large, while the variance of the peak-valley positions at the starting and ending points of the strain is very small, so the starting and ending positions of the strain are obtained by the peak-finding method of variance statistics.

4. The relative phase strain demodulation method based on fractional cyclic shift in optical frequency domain reflectometry according to claim 1, wherein: The S3 specifically includes: The dewrap relative phase of the unstrained and strained regions is obtained by cyclic shifting of decimals, with the number of shift points m1 and m2 respectively. The basic strain phase information diagram is obtained based on m1 and m2 and the start and end positions of the strain. The phase height of the strain position can be calculated using m1 and m2: δφ=2π / N×(m2-m1)(10) The de-wrapped relative phases of the strained and unstrained regions are concatenated and differentiated, denoised using the bidirectional Chebyshev method, and finally superimposed on the basic strain phase information map. The relative phase results at each position are windowed and summed, and the strain-related phase results are demodulated.

5. A distributed optical fiber sensing device using the relative phase strain demodulation method based on fractional cyclic shift in optical frequency domain reflectometry according to any one of claims 1 to 4, the device comprising: Tunable laser, 95:5 optical beam splitter, computer, GPIB control module, clock trigger device based on auxiliary interferometer, main interferometer; The clock trigger device based on the auxiliary interferometer includes: a first balanced detector, a first 50:50 coupler, a clock shaping circuit module, a delay fiber, a first Faraday rotator mirror, a second Faraday rotator mirror and a first circulator. The clock trigger device based on the auxiliary interferometer is used to achieve equal optical frequency spacing sampling, the purpose of which is to suppress nonlinear scanning of the light source; The main interferometer includes an 80:20 beam splitter, a polarization controller, a second circulator, a second 50:50 coupler, a second balanced detector, a collection device, a reference arm, a test arm, and a long-distance fiber Bragg grating. The main interferometer is the core of the distributed fiber optic sensing device for optical frequency domain reflection and is an improved Mach-Zehnder interferometer. The input end of the GPIB control module is connected to a computer, the output end of the GPIB control module is connected to a tunable laser, the tunable laser is connected to a port a of a 95:5 optical beam splitter, the b port of the 95:5 optical beam splitter is connected to a port a of a first circulator (5); the c port of the 95:5 optical beam splitter is connected to a port a of an 80:20 beam splitter (95), the b port of the first circulator is connected to a port a of a first 50:50 coupler, the c port of the first circulator is connected to an input end of a first balanced detector, the b port of the first 50:50 coupler is connected to an input end of the first balanced detector, the c port of the first 50:50 coupler is connected to a first Faraday rotator mirror, the d port of the first 50:50 coupler is connected to a second Faraday rotator mirror via a delay optical fiber, and the output end of the first balanced detector is connected to a time delay optical fiber. The input end of the clock multiplication circuit module is connected to the input end of the clock shaping circuit module, the output end of the clock shaping circuit module is connected to the input end of the acquisition device, the b port of the 80:20 beam splitter is connected to the input end of the polarization controller through the reference arm, the c port of the 80:20 beam splitter is connected to the a port of the second circulator through the test arm, the output end of the polarization controller is connected to the a port of the second 50:50 coupler, the b port of the second circulator is connected to the b port of the second 50:50 coupler, the c port of the circulator is connected to the long-distance fiber Bragg grating passing through the stretching zone, the c port of the second 50:50 coupler is connected to the input end of the second balanced detector, the d port of the second 50:50 coupler is connected to the input end of the second balanced detector, the output end of the second balanced detector is connected to the input end of the acquisition device, and the output end of the acquisition device is connected to the computer.

6. The distributed optical fiber sensing device of optical frequency domain reflectometry according to claim 5, wherein: When the optical frequency domain reflectometry distributed optical fiber sensing device is working, the computer controls the tuning speed, center wavelength, tuning start, etc. of the tunable laser through the GPIB control module. The output light of the tunable laser enters from the a port of the 95:5 optical beam splitter, wherein the light with a ratio of 5 enters from the b port of the 95:5 optical beam splitter through the circulator and enters the a port of the first 50:50 coupler. The light enters from the a port of the first 50:50 coupler and exits from the c and d ports of the first 50:50 coupler, respectively, and is transmitted by the first Faraday The light is reflected by the rotating mirror and the second Faraday rotator mirror and returns to the c and d ports of the first 50:50 coupler. The two beams interfere with each other in the first 50:50 coupler and are output from the b port of the first 50:50 coupler. The light emitted from the b port of the first 50:50 coupler enters the first balanced detector. The first balanced detector converts the detected optical signal into an interference beat signal and transmits it to the clock shaping module. The clock shaping module shapes the interference beat signal into a square wave. The shaped signal is transmitted to the acquisition device as the external clock signal of the acquisition device. The output light of the tunable laser enters from the a port of the 95:5 optical beam splitter, the light with a ratio of 95 enters from the c port of the 95:5 optical beam splitter into the a port of the 80:20 beam splitter, passes through the b port of the 80:20 beam splitter (20), enters the polarization controller in the reference arm, and enters the a port of the second circulator on the test arm from the c port (80), the light enters from the a port of the second circulator, and enters the long-distance fiber Bragg grating in the stretching zone from the c port of the second circulator, and the backscattered light of the long-distance fiber Bragg grating enters from the c port of the second circulator. The reference light output by the polarization controller in the reference arm is combined with the backscattered light on the second circulator through the a port of the second 50:50 coupler and the b port of the second circulator, thereby forming a beat frequency interference and outputting the combined light from the c port and the d port of the second 50:50 coupler to the second balanced detector. The second balanced detector transmits the output analog electrical signal to the acquisition device, which transmits the collected analog electrical signal to the computer under the action of the external clock signal formed by the clock shaping module.

7. The distributed optical fiber sensing device of optical frequency domain reflectometry according to claim 5, wherein: The GPIB control module is used for a computer to control a tunable laser; The tunable laser is used to provide a light source for the optical frequency domain reflection system, and the optical frequency of the laser can be linearly scanned.

8. The distributed optical fiber sensing device of optical frequency domain reflectometry according to claim 5, wherein: The first circulator prevents the reflected light of the b-port of the first 50:50 coupler in the auxiliary interferometer from entering the laser; The first 50:50 coupler is used for optical interference; The delay optical fiber is used to realize beat frequency interference of non-equal arms and provide a clock signal.

9. The distributed optical fiber sensing device of optical frequency domain reflectometry according to claim 5, wherein: The first Faraday rotator mirror and the second Faraday rotator mirror are used to provide reflection for the interferometer and can eliminate the polarization fading phenomenon of the interferometer; The polarization controller is used to adjust the polarization state of the reference light so that the light intensity in two orthogonal directions during polarization beam splitting is substantially consistent; The computer performs data processing on the interference signal collected by the collection device to realize distributed optical fiber stress sensing using fractional cyclic shift.

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