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

Abstract

The invention belongs to the technical field of optical fiber sensing, and discloses a relative phase strain demodulation method and device based on decimal cyclic shift in optical frequency domain reflection.The measured optical fiber is fixed to a displacement table, and the Rayleigh scattering relative phase of the measured optical fiber is obtained through a distributed optical fiber sensing device; on the basis that effective information is completely reserved, the matching problem of any phase slope is solved, distributed optical fiber strain measurement with the spatial resolution reaching 0.907 mm is finally achieved, the minimum strain resolution reaching 0.5 microstrain can be successfully measured, and the measurement range is 36 m.

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 stress sensing is widely used in structural health detection, three-dimensional shape sensing, and biomedicine due to its advantages such as light weight, small size, anti-electromagnetic interference, and non-conductivity. Traditional distributed fiber optic strain sensors include phase-sensitive optical time domain reflectometers, which can achieve strain accuracy measurement at the nanostrain level, but their spatial resolution is at the centimeter level. In addition, although Brillouin optical correlation domain analysis can obtain a spatial resolution of millimeters, its sensitivity of tens of microstrains limits it in high-precision measurement situations.

[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 1cm. 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, in the currently reported scheme, it is only mentioned that the phase winding problem caused by the starting wavelength error between the reference state and the measurement state is non-destructively solved by the cyclic shift method, and because the number of cyclic shift points is an integer, the phase winding slope offset by the shift is discrete, which cannot meet the offset of the phase winding of any slope, 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 for the phase unwinding problem caused by strain. Summary of the invention

[0004] In view of 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 finally 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 reflection, the method specifically comprising:

[0006] S1: After the relative phase is obtained in the distributed strain sensing device, the number of required cyclic shift points is calculated by the relative phase slope, zero-filling interpolation is performed on the reference state and the measurement state data, and the reference state or the measurement state data is cyclically shifted, and finally redundant data is deleted to achieve decimal cyclic shift, and finally eliminate relative phase winding of arbitrary slope;

[0007] S2: On the basis of fractional cyclic shift, the original 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. The peak-to-valley positions at the start and end of phase winding and strain can be found by the peak-finding method. By calculating the variance near the position of each data point, the variance of the peak-to-valley position at the phase winding is large, while the variance of the peak-to-valley position at the start and end of strain is very small, so the start and end positions of strain are obtained by the peak-finding method of variance statistics.

[0008] S3: Obtain the de-wrapped relative phase of the non-strained and strained regions by cyclic shifting of decimals, with the shift points m1 and m2 respectively. Obtain the basic strain phase information graph based on m1 and m2 and the start and end positions of the strain. Concatenate the de-wrapped relative phases of the strained and non-strained regions and then take the derivative, denoise them by the bidirectional Chebyshev method, and finally superimpose them on the basic strain phase information graph to achieve strain demodulation.

[0009] Further, the S1 specifically includes:

[0010] In the optical frequency domain reflection 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, and τ z represents the time delay at any point z of 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 scanning 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 the optical frequency domain signal is transformed by FFT, M zeros are filled and then IFFT is performed to achieve that the total number of data becomes M+N, so the above equation is transformed into:

[0017] 2πΔfτz=2πkm' / (N+M)(4)

[0018] Since we are studying 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 there are m' points of cyclic shift (here, 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 movement 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 the 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 the active cyclic shift cannot completely offset the slope caused by the starting wavelength error of the light source. After zero padding is added to 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 without zero padding. m can be a decimal, and its expression is:

[0027]

[0028] For example, when the data is expanded to 10 times, the number of cyclic shift points is m'=865. Therefore, after restoring the total amount of data, according to formula (8), the actual number of cyclic shift points is m=86.5, thereby realizing the fractional cyclic shift. Finally, the redundant data is deleted to complete the fractional cyclic shift. The relative phase has eliminated the phase winding of any slope caused by the initial wavelength error 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] Further, 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 number of cyclic shift points for relative phase detangling between strained and non-strained regions).

[0033] 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.

[0034] By calculating the variance near each data point and comparing them, the start and end positions of the strain can be distinguished. The variance of the peak and valley positions at the phase winding is large, while the variance of the peak and valley positions at the start and end of the strain is small, so the peak finding method of variance statistics is used to obtain the start and end positions of the strain.

[0035] Further, the S3 specifically includes:

[0036] The de-wrapping relative phase of the non-strained and strained regions is obtained by cyclic shifting of decimals, and the number of shift points is m1 and m2 respectively. The basic strain phase information diagram is obtained according to m1 and m2 and the start and end positions of the strain. Among them, the phase slope of the strain position can be calculated by 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 to demodulate the strain-related phase results.

[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 optical 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 realize equal optical frequency spacing sampling, and its purpose 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 grating. The main interferometer is the core of the distributed fiber optic sensing device of optical frequency domain reflection, which 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 of a 95:5 optical beam splitter, the b port of the 95:5 optical beam splitter is connected to a port of a first circulator (5), the c port of the 95:5 optical beam splitter is connected to a port of a 80:20 beam splitter (95), the b port of the first circulator is connected to a port 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, 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 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, 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 The light is reflected by the Faraday rotator mirror and the second Faraday rotator mirror and returns to the c and d ports of the first 50:50 coupler. The two beams of light interfere in the first 50:50 coupler and are output from the b port of the first 50:50 coupler. The outgoing light 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 frequency signal and transmits it to the clock shaping module. The clock shaping module shapes the interference beat frequency signal into a square wave. The shaped signal is transmitted to the acquisition device as an 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 80:20 beam splitter (20) and enters from the b port into the polarization controller in the reference arm, and enters from the c port (80) into the a port of the second circulator on the test arm, the light enters from the a port of the second circulator, and enters from the c port of the second circulator into the long-distance fiber Bragg grating in the stretching zone, 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 50:50 coupler to form beat frequency interference and is output 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, and the acquisition device transmits the collected analog electrical signal to the computer under the action of the external clock signal formed by the clock shaping module.

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

[0048] The tunable laser is used to provide a light source for an optical frequency domain reflection system, and its optical frequency 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 the 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 is basically consistent during polarization beam splitting;

[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, and at the same time accurately obtains the starting and ending positions of the strain 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 the relative phase is obtained 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, and finally the redundant data is deleted to realize the fractional cyclic shift. This method can realize lossless phase unwrapping of arbitrary slope.

[0058] 2) Peak finding method based on variance statistics (S2): Based on fractional cyclic shift, the original 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. Through the peak finding method, the peak and valley positions of phase winding and the start and end of strain can be found. By calculating the variance near each data point position, the two peak and valley positions with the smallest variance are found, which are the start and end positions of strain.

[0059] 3) Strain demodulation based on fractional cyclic shift (S3): The de-wrapped relative phase of the non-strained and strained regions is obtained by fractional cyclic shift, and the number of shift points is m1 and m2 respectively. The basic strain phase information diagram is obtained according to m1 and m2 and the start and end positions of the strain. The de-wrapped relative phases of the strain and non-strained regions are spliced ​​and differentiated, and denoised by 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.

[0060] This method effectively solves the phase wrapping problem of arbitrary slope by combining multiple techniques 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 technical solution of the present invention after transformation are as follows: The present invention is based on OFDR relative phase high-precision strain measurement, and has important applications in structural health monitoring, three-dimensional shape sensing, etc. With the continuous refinement and intelligence of technology and equipment, the commercial value of the application of structural health monitoring and three-dimensional shape sensing based on the present invention in cutting-edge military and civilian fields 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 gap in the industry at home and abroad: in the relative phase demodulation method in optical frequency domain reflection, the relative phase method has received widespread attention due to its high precision and high spatial resolution. However, the relative phase method has a unique phase winding 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 difficult to instrument. The cyclic shift method can solve the phase winding 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, and is easy to implement, reducing equipment requirements, and providing an instrumentation basis for the relative phase method.

[0063] In addition, in the current reported scheme, there is a lack of a method for demodulating micro-strains at a medium-distance submillimeter spatial resolution in the phase unwinding problem. In the present invention, the starting and ending positions of the strain are accurately obtained by using the cyclic shift of decimals and the peak search method based on variance statistics, and the basic strain phase information is obtained by using the number of cyclic shift points. Finally, the strain measurement with a spatial resolution of 0.907mm, a measurement distance of 36m, a strain accuracy of 0.5 microstrain, and a maximum error of 0.2 microstrain is achieved through bidirectional Chebyshev low-pass filtering, and the measurement standard deviation is 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 greatly 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 It is a flow chart of a relative phase strain demodulation method based on fractional cyclic shift in optical frequency domain reflection provided by an embodiment of the present invention;

[0071] Figure 3 is a 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 is a schematic diagram of obtaining the start and end positions of strain using a peak-finding method based on variance statistics provided in 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 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 solution 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 used to limit the present invention.

[0079] The present invention mainly improves the following problems and defects of the prior art and achieves significant technical progress:

[0080] 1. Phase wrapping problem with arbitrary slope: In traditional distributed strain sensing systems, phase wrapping often occurs due to the error in the starting wavelength of the light source. However, the number of shift points in the existing related cyclic shift method is an integer, so the phase wrapping slope offset by the shift is discrete and cannot satisfy the offset of phase wrapping with arbitrary slope. This makes the cyclic shift method less universal in solving the phase wrapping problem, which ultimately makes it complicated 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-filling interpolation, cyclic shift and redundant data deletion, the phase wrapping problem of arbitrary slope 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 and the strain start and end positions with smaller variance are distinguished, 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. Segmental summation: This method is used to obtain the final distributed strain size, which improves the efficiency and accuracy of feature information extraction in data processing.

[0088] Two specific application examples of the embodiment 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 time.

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

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

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

[0094] Peak finding method based on variance statistics: Through the statistics of variance, 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 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 state 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 of wind turbine blades.

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

[0101] Peak finding method based on variance statistics: Through the statistics of variance, 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 by the fractional cyclic shift phase method and segment summation.

[0104] The present invention will be described below in conjunction with the accompanying drawings.

[0105] The distributed optical fiber sensing device of optical frequency domain reflection 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 clock trigger device 21 based on the auxiliary interferometer includes: a first balanced detector 2, a first 50:50 coupler 5, a clock shaping circuit module 6, a delay optical fiber 7, a first Faraday rotator mirror 8, a second Faraday rotator mirror 9 and a first circulator 10. The clock trigger device 21 based on the auxiliary interferometer is used to achieve equal optical frequency spacing sampling, and its purpose is 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 grating 23. The main interferometer 22 is the core of the distributed fiber optic sensing device of optical frequency domain reflection, which is an improved 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 collection 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 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 collection device 17, and the output end of the collection 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 from the b port of the 95:5 optical beam splitter 4 through the circulator 10 into the a port of the first 50:50 coupler 5 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 light beams interfere 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 from the c port (95) of the 95:5 optical beam splitter 4 into the a port of the 80:20 beam splitter 3, passes through the 80:20 beam splitter 3 (20), enters from the b port of the polarization controller 12 in the reference arm 19, enters from the c port (80) into the a port of the second circulator 13 on the test arm 20, enters from the a port of the second circulator 13, enters from the c port of the second circulator 13 into the long-distance fiber Bragg grating 23 in the stretching zone 15, and the backscattered light of the long-distance fiber Bragg grating 23 enters from the c port of the second circulator 13. The reference light outputted from the port b of the second circulator 13, the reference light outputted by the polarization controller 12 in the reference arm 19 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 outputted 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 reflection 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 function of the polarization controller 12 is to adjust the polarization state of the reference light so that the light intensity in two orthogonal directions is substantially consistent during polarization beam splitting.

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

[0119] The distributed fiber optic sensor device of optical frequency domain reflection realizes 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 accurate 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 in the coupler and is reflected back to the coupler through the first and second Faraday rotator mirrors, generating an interference signal in the coupler. The interference light is detected by the first balanced detector and transmitted to the clock shaping module, and the optical signal is shaped into a square wave as an external clock signal and transmitted to the acquisition device to ensure that the sampling is synchronized with the optical frequency.

[0120] Secondly, another part (95%) of the laser light source enters the main interferometer through the beam splitter. The light enters the system along the reference arm and the test arm respectively, where the reference light passes through the polarization controller to adjust the polarization state to ensure the balance of the interfering light intensity; the test arm light enters the long-distance fiber grating, and the backscattered light in the optical fiber returns to the 50:50 coupler through the second circulator. The light beams passing through the two optical paths interfere again in the coupler, and the generated interference beat frequency signal is transmitted to the acquisition device through the second balanced detector.

[0121] The acquisition device can synchronize the collected interference signals under the action of the external clock provided by the clock shaping module to ensure the accuracy of data acquisition. The computer processes the collected analog signals and realizes high-resolution distributed stress sensing through fractional cyclic shift technology. The sensing results have extremely high accuracy by using equal optical frequency spacing sampling control and phase unwrapping method.

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

[0125] In large 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 the structure. By arranging distributed optical fiber sensors on the surface or internal structure of the bridge, the strain data at different positions in the structure can be obtained in real time, and the areas with abnormal strain can be quickly identified. The high precision and real-time performance of this method support long-term monitoring of key nodes of the bridge, especially under complex external force conditions such as wind load and traffic load. After sensing the strain change, the detection system quickly unwraps the relative phase information through the method of decimal cyclic shift, removes unnecessary noise, and displays the strain change of the bridge structure. This method provides data support for the safe operation of the bridge, helps to timely detect risks and hidden dangers, 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 to ensure operational safety. The relative phase strain demodulation method based on fractional cyclic shift can be deployed at key points in drilling or oil and gas pipelines to monitor the force changes and deformation of the pipelines through optical frequency domain reflection technology. In long-distance transmission pipelines, this method can effectively detect and locate stress concentration areas, analyze the strain state of each node, identify overload or deformation, and prevent pipeline damage and leakage accidents. The real-time high-precision strain demodulation method enables the oil drilling and pipeline industry to improve 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 reflection, 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 ends of the two sets of distance domain information signals are padded with zeros, 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). The redundant signal is deleted, as shown in Figure 3 Finally, the relative phase at the measured optical fiber is intercepted to realize fractional cyclic shift. According to equation (8), the number of fractional shift points is 86.5. Compared with 86 and 87 points of ordinary cyclic shift, fractional cyclic shift can completely offset the phase winding of arbitrary slope, as shown in Figure 3 (c) as shown.

[0130] In the process of peak finding method based on variance statistics, firstly, the original data is subjected to fractional cyclic shift according to the above process, where 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 non-strain and strain, 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 the start and end of strain 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 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 (d) As shown in Figure 3, the start and end 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) are shown, and the number of cyclic shift points is counted as m1 and m2 respectively. Using formula (10), as well as m1 and m2, the relative phase slope of the fiber stress area (linearly related to the stress magnitude) can be directly calculated. 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 without 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) After adding the sum of the windows 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 finally 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 end 40cm of the long-distance fiber grating 23 is fixed at one end and glued to the nano displacement stage at the other end. The piezoelectric nano platform is moved within a range of 1mm with a step length of 0.2mm, thus giving the optical fiber 0.5 to 2.5 micro strains. During the stretching process, data acquisition is performed to obtain the corresponding micro strain data. In the window summation of the relative phase results, when 10 points are taken as the window, it means that the resolution of the entire system is 0.907mm.

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

[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) are 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 the 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 effective points in (a) are averaged and the results are fitted linearly. It can be seen from the figure 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 part can be implemented using dedicated logic, and the software part can be stored in a memory and executed by an appropriate instruction execution system, such as a microprocessor or a dedicated design hardware. It can be understood by a person of ordinary skill in the art that the above-mentioned device and method can be implemented using computer executable instructions and / or contained in a processor control code, such as 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. Such code is provided on the 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. The device of the present invention and its modules can be implemented by hardware circuits such as ultra-large-scale integrated circuits or gate arrays, semiconductors such as logic chips, transistors, etc., 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, and can also 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 1, different filtering methods are compared. The phase result 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 1. 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 value of the cross-correlation result is high, it means that the similarity between the two is high, and the cross-correlation peak is small and sharp, which means 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] The method first obtains the relative phase data of the reference state and the measurement state in the distributed strain sensing device and prepares for the unwrapping process. The initial step includes zero padding the reference state and the measurement state data in the distance domain to improve the resolution of the data, and then performs a cyclic shift operation on the reference state or the measurement state data, thereby effectively solving the relative phase difference change problem. After the cyclic shift is performed, lossless phase unwrapping is achieved by deleting the redundant data, ensuring that the data remains intact and the original phase information is retained.

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

[0145] In order to further locate the starting and ending positions of the strain, this method adjusts the original data based on the fractional cyclic shift so that the slope caused by the starting wavelength of the light source is opposite to the slope caused by the strain, which can facilitate the identification of the starting and ending positions of the strain. Subsequently, the peak-finding method is used to find and mark the peak and valley positions (including the phase winding position and the starting and ending positions of the strain) in the relative phase data. The variance calculation near each peak and valley position helps to further identify between the peak and valley positions, ensuring that the two positions with the smallest variance are found, accurately locating the starting and ending positions of the strain, and excluding the phase winding position.

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

[0147] According to the starting and ending position information of the strain region obtained in the previous step, the relative phase of the non-strain region and the strain region is further extracted by fractional cyclic shift to remove the winding. The relative phase slopes of the non-strain region and the strain region are removed by cyclic shift, and the number of shift points is recorded as m1 and m2. The basic strain phase information diagram is constructed based on m1, m2, formula (10) and the starting and ending positions of the strain. This diagram contains the basic distribution of the relative phase change rate, which lays the foundation for subsequent analysis.

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

[0149] In order to eliminate the influence of signal noise on the demodulation results, the bidirectional Chebyshev filtering method is used to denoise the spliced ​​relative phase data to obtain a smooth phase data curve. The denoised data is then superimposed on the basic strain phase information diagram to form a complete strain phase distribution diagram. Finally, the window summation method is applied to the relative phase data at each position to accumulate the relative phase results in different windows to demodulate the final phase result related to the strain. This result reflects the strain distribution and achieves high-precision strain demodulation.

[0150] To sum up, the above is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by any technician familiar with the technical field within the technical scope disclosed by the present invention and within the spirit and principles of the present invention should be covered within the protection scope 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 data of the reference state and the measurement state are zero-filled and interpolated, and the data of the reference state or the measurement state are cyclically shifted, and finally the redundant data is deleted to realize the cyclic shift of the decimal. This method can realize lossless phase unwrapping with arbitrary slope; S2: On the basis of fractional cyclic shift, the original 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. The peak-finding method can be used to find the peak-valley positions of phase winding and the start and end of strain; by calculating the variance near the position of each data point, the variance of the peak-valley position at the phase winding is large, while the variance of the peak-valley position at the start and end of strain is very small, so the start and end positions of strain are obtained by the peak-finding method of variance statistics. S3: Obtain the de-wrapped relative phase of the non-strained and strained regions by cyclic shifting of decimals, with the shift points m1 and m2 respectively. Obtain the basic strain phase information diagram based on m1 and m2 and the start and end positions of the strain. Concatenate the de-wrapped relative phases of the strained and non-strained regions and then derive them, denoise them by the bidirectional Chebyshev method, and finally superimpose them on the basic strain phase information diagram; sum the relative phase results at each position by taking a window, and 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, characterized in that: The S1 specifically includes: In the optical frequency domain reflection 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, and 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 frequency light source, and f b represents the beat frequency and f b =γτ z , γ represents the scanning rate of the swept light source, and 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 initial wavelength error of the light source, Δφ=dφ is required. After the optical frequency domain signal is transformed by FFT, M zeros are filled and then IFFT is performed to achieve that the total number of data becomes M+N, so the above equation is transformed into: 2πΔfτ z =2πkm' / (N+M)(4) Since we are studying 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: 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. On the right side of the equation, when there are m' points of cyclic shift (here, 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 movement 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 the 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 the active cyclic shift cannot completely offset the slope caused by the starting wavelength error of the light source. After zero padding is added to 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 without zero padding. m can be a decimal, and its expression is: For example, when the data is expanded to 10 times, the number of cyclic shift points is m'=865. Therefore, after restoring the total amount of data, according to formula (8), the actual number of cyclic shift points is m=86.5, thereby realizing the fractional cyclic shift. Finally, the redundant data is deleted to complete the fractional cyclic shift. The relative phase has eliminated the phase winding of any slope caused by the initial wavelength error of the light source. When the source of δφ is only strain, the same logic can be obtained: 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, characterized in that: The S2 specifically includes: 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 number of cyclic shift points for relative phase detangling between strained and non-strained 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 start and end positions of the strain can be distinguished. The variance of the peak and valley positions at the phase winding is large, while the variance of the peak and valley positions at the start and end of the strain is small, so the peak finding method of variance statistics is used to obtain the start and end positions of the strain.

4. The relative phase strain demodulation method based on fractional cyclic shift in optical frequency domain reflectometry according to claim 1, characterized in that: The S3 specifically includes: The de-wrapping relative phase of the non-strained and strained regions is obtained by cyclic shifting of decimals. The shift points are 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. Among them, the phase height of the strain position can be calculated by m1 and m2: δφ=2π / N×(m2-m1)(10) 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.

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 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 optical 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 realize equal optical frequency spacing sampling, and its purpose 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 grating. The main interferometer is the core of the distributed fiber optic sensing device of optical frequency domain reflection, which 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 of a 95:5 optical beam splitter, the b port of the 95:5 optical beam splitter is connected to a port of a first circulator (5); the c port of the 95:5 optical beam splitter is connected to a port of a 80:20 beam splitter (95), the b port of the first circulator is connected to a port 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, 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 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 reflection as claimed in claim 5, characterized in that: When the optical frequency domain reflectometry distributed optical fiber sensor device is working, the computer controls the tuning speed, central 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, respectively, and is transmitted by the first Faraday The light is reflected by the rotating mirror and the second Faraday rotating mirror and returns to the c and d ports of the first 50:50 coupler. The two beams of light interfere in the first 50:50 coupler and are output from the b port of the first 50:50 coupler. The outgoing light 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 frequency signal and transmits it to the clock shaping module. The clock shaping module shapes the interference beat frequency signal into a square wave. The shaped signal is transmitted to the acquisition device as an 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 80:20 beam splitter (20) and enters from the b port into the polarization controller in the reference arm, and enters from the c port (80) into the a port of the second circulator on the test arm, the light enters from the a port of the second circulator, and enters from the c port of the second circulator into the long-distance fiber Bragg grating in the stretching zone, 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 50:50 coupler to form beat frequency interference and is output 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, and the acquisition device 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 reflection as claimed in claim 5, characterized in that: 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 an optical frequency domain reflection system, and its optical frequency can be linearly scanned.

8. The distributed optical fiber sensing device of optical frequency domain reflection as claimed in claim 5, characterized in that: 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 the beat frequency interference of non-equal arms and provide a clock signal.

9. The distributed optical fiber sensing device of optical frequency domain reflection as claimed in claim 5, characterized in that: 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 is basically consistent during polarization beam splitting; 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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