Fast OFDR strain measurement method and device based on backscattering enhanced optical fiber

Abstract

The application discloses a kind of based on backscattering enhanced optical fiber fast OFDR strain measurement method and its device, the method includes: obtaining the time-domain signal of optical frequency domain reflectometer output, time-domain signal is divided into reference signal and test signal, sensing optical fiber is backscattering enhanced optical fiber with enhanced intensity multiple a;Reference signal and test signal are respectively Fourier transformed, take window, amplitude division, determine data section length to obtain the reference local feature data section and test local feature data section of different window positions of sensing optical fiber;Reference local feature data section and test local feature data section of the same window position are matched and calculated, to obtain the wavelength shift of reference signal and test signal at corresponding window position;The strain distribution of sensing optical fiber is calculated based on the wavelength shift at different window positions.The application can realize the fast demodulation to strain, improve system real-time performance.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of optical fiber sensing, and more particularly relates to a fast OFDR strain measurement method and device based on backscattering enhanced optical fiber. BACKGROUND

[0002] Optical fiber strain sensors have high sensitivity, large dynamic range, and anti-electromagnetic interference, and have made great progress in structural health monitoring and shape sensing, and have shown excellent application prospects. Optical frequency domain reflectometry (OFDR) based on Rayleigh scattering is a high-resolution optical fiber measurement technology, which has a spatial resolution of up to centimeters or even millimeters and high measurement accuracy. In the strain demodulation process based on OFDR technology, the strain sensing resolution, accuracy and measurement dynamic range are severely dependent on the intensity of the Rayleigh backscattering (RBS) signal. Generally, the intensity of RBS in a single-mode optical fiber is weak, which makes the signal-to-noise ratio of a single-mode optical fiber generally low. Backscattering enhanced optical fiber is used to improve the signal-to-noise ratio of RBS in the OFDR system, so as to realize high spatial resolution and large strain measurement range of the OFDR system at the same time.

[0003] However, whether using a standard single-mode optical fiber or a backscattering enhanced optical fiber, in the strain demodulation process based on OFDR technology, a large amount of data is involved, and the calculation amount is large, resulting in slow strain demodulation speed. Therefore, how to improve the strain demodulation speed based on OFDR technology is an important research direction at present. SUMMARY

[0004] In view of the above defects or improvement needs of the prior art, the present application provides a fast OFDR strain measurement method and device based on backscattering enhanced optical fiber, which aims to improve the strain demodulation speed based on OFDR technology.

[0005] To achieve the above-mentioned purpose, according to one aspect of the present application, a fast OFDR strain measurement method based on backscattering enhanced optical fiber is provided, comprising:

[0006] Obtaining a time domain signal output by an optical frequency domain reflectometer, the time domain signal being divided into a reference signal obtained without applying strain to a sensing optical fiber and a test signal obtained by the sensing optical fiber in an actual environment, the sensing optical fiber being a backscattering enhanced optical fiber with an enhanced intensity multiple of a;

[0007] Performing feature extraction based on the reference signal and the test signal respectively to obtain reference local feature data segments and test local feature data segments of different window positions of the sensing optical fiber;

[0008] The reference local feature data segment and the test local feature data segment at the same window position are matched to obtain the wavelength shift of the reference signal and the test signal at the corresponding window position;

[0009] The strain distribution of the sensing optical fiber is calculated based on the wavelength shifts at different window positions;

[0010] The process of feature extraction includes:

[0011] The time domain signal is subjected to Fourier transform to obtain an optical frequency domain signal;

[0012] The optical frequency domain signal is converted into a distance domain signal;

[0013] The distance domain signal is subjected to sliding window to intercept the distance domain window signal of the sensing optical fiber at the corresponding window position;

[0014] The distance domain window signal is subjected to short-time Fourier transform to obtain an optical wavelength domain window signal;

[0015] The amplitude of the optical wavelength domain window signal is subjected to normalization to obtain an optical wavelength domain normalized window signal;

[0016] The amplitudes of the optical wavelength domain normalized window signal are divided into a equal parts to form a amplitude interval;

[0017] The total distribution frequency points of the last a-1 amplitude intervals are counted as the data segment length M;

[0018] The highest peak of the amplitude of the optical wavelength domain normalized window signal is taken as the center to intercept M frequency points to form the corresponding local feature data segment.

[0019] In one embodiment, the length of the window is represented by the number of frequency points, and the length of another window is h. After the distance domain signal with the length of h is intercepted, the length of the window with the length of h is expanded to N by the method of zero padding at the end to obtain the final distance domain window signal, wherein the range of h is 200<h<2000, and the range of N is 1000<N<20000.

[0020] In one embodiment, the length of the reference local feature data segment is defined as M ref , and the length of the test local feature data segment is defined as M test The process of calculating the wavelength shift of the reference signal and the test signal at the same window position includes:

[0021] When M ref ≠M test , the end of the local feature data segment with the smaller length is zero-padded to make the lengths of the reference local feature data segment and the test local feature data segment equal;

[0022] correlation processing is performed on the reference local feature data segment and the test local feature data segment, a frequency point offset d1 corresponding to a peak of the cross-correlation is calculated, and a wavelength shift ws1 corresponding to the frequency point offset d1 is calculated;

[0023] A frequency point offset d2 between a highest peak of the reference local feature data segment and a highest peak of the test local feature data segment is calculated, and a wavelength shift ws2 corresponding to the frequency point offset d2 is calculated.

[0024] The wavelength shift ws1 and the wavelength shift ws2 are added to obtain a wavelength shift of the reference signal and the test signal at the current window position.

[0025] In one of the embodiments, a calculation formula of the wavelength shift ws according to the frequency point offset d is as follows:

[0026]

[0027] wherein, ΔF is a sweep range, and N is a frequency point number of the distance domain window signal.

[0028] In one of the embodiments, the backscattering enhanced optical fiber is a single-mode optical fiber with periodic backscattering enhancement.

[0029] In one of the embodiments, the enhancement intensity multiple a ranges from 10 to 30.

[0030] In one of the embodiments, a conversion relationship of converting the optical frequency domain signal into the distance domain signal is as follows:

[0031]

[0032] wherein, z is a distance, f represents a beat frequency, c is a light speed, n is a refractive index of the sensing optical fiber, and γ is a sweep rate of a laser in the optical frequency domain reflectometer.

[0033] In one of the embodiments, the strain distribution of the sensing optical fiber is calculated based on the wavelength shifts at different window positions, including: calculating a strain size at each window position based on the wavelength shift at the window position, and obtaining the strain distribution of the sensing optical fiber according to the strain sizes of all the window positions, wherein,

[0034] A calculation formula of the strain size according to the wavelength shift is as follows:

[0035] ε s =k*wavelength_shift

[0036] wherein, ε s is the strain size, k is a strain coefficient of the backscattering enhanced optical fiber, and wavelength_shift is the wavelength shift.

[0037] According to another aspect of the present application, there is provided a fast OFDR strain measurement device based on backscattering enhanced optical fiber, comprising a data processing system, the data processing system comprising

[0038] a feature extraction unit configured to perform feature extraction based on a time domain signal output by an optical frequency domain reflectometer to obtain a local feature data segment of the sensing optical fiber at a different window position;

[0039] a wavelength shift calculation unit configured to, after obtaining a reference local feature data segment and a test local feature data segment, perform matching calculation on the reference local feature data segment and the test local feature data segment at the same window position to obtain a wavelength shift of a reference signal and a test signal at the corresponding window position, wherein the reference local feature data segment is a local feature data segment obtained by the feature extraction unit when the time domain signal is a reference signal, and the test local feature data segment is a local feature data segment obtained by the feature extraction unit when the time domain signal is a test signal;

[0040] a strain distribution calculation unit configured to calculate a strain distribution of the sensing optical fiber based on the wavelength shifts at different window positions;

[0041] wherein the feature extraction unit comprises:

[0042] a Fourier transform sub-unit configured to perform Fourier transform processing on the input time domain signal to obtain an optical frequency domain signal;

[0043] a distance domain conversion sub-unit configured to convert the optical frequency domain signal into a distance domain signal;

[0044] a window intercepting sub-unit configured to slide a window on the distance domain signal to intercept a distance domain window signal of the sensing optical fiber at a corresponding window position;

[0045] a short-time Fourier transform sub-unit configured to perform short-time Fourier transform on the distance domain window signal to obtain an optical wavelength domain window signal;

[0046] a normalization processing sub-unit configured to perform normalization processing on an amplitude of the optical wavelength domain window signal to obtain an optical wavelength domain normalized window signal;

[0047] an amplitude division sub-unit configured to divide the amplitude of the optical wavelength domain normalized window signal into a equal parts to form a amplitude interval;

[0048] a data segment length calculation sub-unit configured to count a total distribution frequency point number of the last a-1 amplitude intervals as a data segment length M;

[0049] The local feature data segment output subunit is configured to take the amplitude maximum peak of the optical wavelength domain normalized window signal as the center, intercept M frequency points, and form a corresponding local feature data segment.

[0050] In one embodiment, the OFDR strain measurement device further comprises an optical frequency domain reflectometer and a sensing optical fiber, and the sensing optical fiber is a backscattering enhanced optical fiber with an enhanced intensity multiple a.

[0051] Overall, compared with the prior art, the above technical solutions conceived by the present application can achieve the following beneficial effects:

[0052] The present application first sets the sensing optical fiber in the optical frequency domain reflectometer as a backscattering enhanced optical fiber, which can enhance the backscattering signal in a partial area. The inventors have found through analysis that when a backscattering enhanced optical fiber is used, strain demodulation mainly depends on this small part of information of the scattering enhanced signal, and a large amount of information of the unenhanced part is not useful for the overall calculation result. Based on this, the present application extracts features from the original signal, determines the data segment length M through amplitude division, further extracts key data features with a length of M from the intercepted optical wavelength domain window signal, reduces a large amount of calculation under the premise of ensuring correctness, improves the strain measurement speed of the system, and improves the real-time performance of the system. BRIEF DESCRIPTION OF DRAWINGS

[0053] Figure 1 The structure diagram of the OFDR strain measurement device of one embodiment;

[0054] Figure 2 The step flow chart of the fast OFDR strain measurement method based on the backscattering enhanced optical fiber of one embodiment;

[0055] Figure 3 The structure diagram of the backscattering enhanced optical fiber of one embodiment;

[0056] Figure 4 The step flow chart of extracting the local feature data segment of one embodiment;

[0057] Figure 5(a) is a schematic diagram of the optical wavelength domain normalized window signal when the reference signal is a strain of 0με and the test signal is a strain of 4000με of one embodiment;

[0058] Figure 5(b) is a statistical distribution histogram after amplitude division of the optical wavelength domain normalized window signal corresponding to the reference signal of one embodiment;

[0059] Figure 6 The schematic diagram of the local feature data segment of one embodiment, wherein (a) is a schematic diagram of the reference local feature data segment, and (b) is a schematic diagram of the test local feature data segment;

[0060] Figure 7 a schematic diagram of strain measurement results of a sensing fiber for an embodiment;

[0061] Figure 8 a schematic diagram of a structure of a fast OFDR strain measurement device based on backscattering enhanced fiber for an embodiment. DETAILED DESCRIPTION

[0062] In order to make the objects, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and not used to limit the present application. In addition, the technical features involved in the various embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0063] In order to facilitate understanding of the present application, first, the OFDR strain measurement device is introduced.

[0064] As Figure 1The structure of the OFDR strain measurement device in an embodiment is shown, which includes an optical frequency domain reflectometer, a sensing optical fiber 19 and a host computer 23 for data analysis. The optical frequency domain reflectometer includes a tunable laser 1, a 90:10 coupler 2, an auxiliary interferometer 9 and a main interferometer 21, a data acquisition card 22 and a radio frequency connection line 24. The auxiliary interferometer 9 includes a circulator 3, a 2*2 coupler 4, a delay optical fiber 5, Faraday rotators 6 and 7, and a photodetector 8. The main interferometer 21 includes a 90:10 coupler 10, a polarization controller 11, a circulator 12, polarization beam splitters 13 and 14, 2*2 couplers 15 and 16, and photodetectors 17 and 18. The output end of the tunable laser 1 is connected to the input end a of the 90:10 coupler 2, and the output end of the 90:10 coupler 2 includes the b end accounting for 10% of the input end optical signal and the c end accounting for 90% of the input end optical signal, wherein the b end is connected to the input end a of the circulator 3 in the auxiliary interferometer 9, and the c end is connected to the input end a of the 90:10 coupler 10 in the main interferometer 21. The circulator 3 in the auxiliary interferometer 9 includes the a end, the b end and the c end, wherein the b end is connected to the a end in the 2*2 coupler 4, and the c end is connected to the input end b of the photodetector 8. The c end in the 2*2 coupler 4 is connected to the delay optical fiber 5, and then to the Faraday rotator 6. The b end in the 2*2 coupler 4 is connected to the input end a of the photodetector 8, and the d end in the 2*2 coupler 4 is connected to the Faraday rotator 7. The 90:10 coupler 10 in the main interferometer 21 includes the b end accounting for 10% of the input end a optical signal and the c end accounting for 90% of the input end optical signal. The b end is connected to the polarization controller 11, and the c end is connected to the input end a of the circulator 12. The polarization controller 11 is connected to the input end a of the polarization beam splitter 13, and the output ends b and c of the polarization beam splitter are respectively connected to the a ends of the two 2*2 couplers 15 and 16. The output ends c and d of the 2*2 coupler 16 are respectively connected to the input ends a and b of the photodetector 18. The c end in the circulator 12 in the main interferometer 21 is divided into two paths through the polarization beam splitter 14, and is respectively connected to the b ends of the two 2*2 couplers 15 and 16. The output ends c and d of the 2*2 coupler 15 are respectively connected to the input ends a and b of the photodetector 17. The c end in the circulator 12 is connected to the sensing optical fiber 19. The photodetector 8 in the auxiliary interferometer 9 and the photodetectors 17 and 18 in the main interferometer 21 are all connected to the input end of the data acquisition card 22, and the output end of the data acquisition card 22 is finally connected to the host computer 23. The radio frequency connection line 24 is respectively connected to the tunable laser 1 and the data acquisition card 22.

[0065] With the OFDR strain measurement device, the strain distribution of the sensing fiber 19 can be measured. In operation, the tunable laser 1 provides light with tunable wavelength, which enters the 90:10 coupler 2 through the a end, 10% of which enters the circulator 3a end in the auxiliary interferometer 9, and the light from the b end of the circulator 3 enters the two arms through the 2*2 coupler 4 respectively, is reflected back to the c and d ends of the 2*2 coupler 4 through the Faraday rotating mirror 67 respectively, and performs beat interference in the 2*2 coupler 4; one of the interference light signals returns to the c end of the circulator 3 through the a end, and then enters the photodetector 8, and the other directly enters the photodetector 8, and is transmitted to the host computer 23 after digital-to-analog conversion by the data acquisition card 22. In addition, the light of the tunable laser 1 enters the 90:10 coupler 2a end, 90% of which enters the 90:10 coupler 2a end in the main interferometer 21, and 10% of which exits from the b end as reference light, which is adjusted by the polarization controller 11 and enters the polarization beam splitter 13, and is divided into two lights p and s; 90% of the light from the c end enters the a end of the circulator 12, and is sent into the sensing fiber 19 through the b end, and the sensing light reflected back by the sensing fiber 19 is emitted from the c end of the circulator 12 and enters the polarization beam splitter 14, which is also divided into two lights p and s; the p and s lights in the two polarization beam splitters 13, 14 enter the two 2*2 couplers 15, 16 respectively to perform beat interference, and the final interference light is detected by the photodetectors 17, 18 and outputted as time domain signals after digital-to-analog conversion by the data acquisition card 22 and transmitted to the host computer 23, and the strain of the sensing fiber 19 is analyzed. Finally, the shot bias connection line 24 is used to trigger the acquisition work of the data acquisition card 22 by taking the electrical signal emitted by the tunable laser 1 in operation as a trigger signal. It should be noted that when the present application is verified, the deformation of the sensing fiber 19 can be controlled by the displacement table 20, so that a strain force of a certain size is applied to a certain position of the sensing fiber.

[0066] In the prior art, the data is large and the calculation amount is large when the data is processed by the host computer, the strain analysis speed is slow, and the strain demodulation speed is affected. Based on this, the present application proposes the following technical solutions.

[0067] As shown in Figure 2 The step flow chart of the fast OFDR strain measurement method based on the backscattering enhanced optical fiber in the embodiment of the present application is shown in the figure, which mainly includes steps S100-S400, and the steps will be described below.

[0068] Step S100: Obtain the time domain signal output by the optical frequency domain reflectometer, which is divided into a reference signal obtained without applying strain to the sensing fiber and a test signal obtained by the sensing fiber in the actual environment, and the sensing fiber is a backscattering enhanced optical fiber with an enhanced intensity multiple of a.

[0069] The selected sensing optical fiber is a backscattering enhanced optical fiber, which can be obtained by continuously exposing a standard single-mode optical fiber to ultraviolet light. The backscattering Rayleigh signal of the optical fiber is greatly enhanced, and the signal-to-noise ratio and measurement sensitivity of the system are improved. Compared with the standard single-mode optical fiber before processing, the scattering enhancement intensity of the backscattering enhanced optical fiber obtained after processing is a times that of the standard single-mode optical fiber before processing, and the value of a can be adjusted according to specific requirements when the backscattering enhanced optical fiber is manufactured. Compared with the standard single-mode optical fiber, the reflectivity of the backscattering enhanced optical fiber is higher, so the intensity of the RBS signal is higher, thereby having better signal-to-noise ratio and sensitivity. In a specific embodiment, the sensing optical fiber can be a periodically backscattering enhanced single-mode optical fiber, that is, the backscattering is periodically enhanced. As Figure 3 The structure of the backscattering enhanced optical fiber is shown in FIG. 1. The large circles represent the scattering points of the backscattering enhanced optical fiber, and the small circles represent the scattering points of the standard single-mode optical fiber. The reflectivity of the backscattering enhanced optical fiber is much higher than that of the standard single-mode optical fiber. When designing the backscattering enhanced optical fiber, the enhancement intensity a in the range of 10 < a < 30 can meet the requirements, and the enhancement intensity a can be designed as 20.

[0070] The time-domain signal output by the selected sensing optical fiber is measured by using an optical frequency domain reflectometer. In addition to measuring the test signal actually output by the sensing optical fiber in the actual environment, a group of reference signals are also needed to be measured, that is, the corresponding time-domain signal of the sensing optical fiber without strain is taken as the reference signal. The strain of the sensing optical fiber to be measured is obtained based on the data processing of the reference signal and the test signal. It should be noted that the sensing optical fibers used to obtain the reference signal and the test signal are the same structure, only the strain forces are different.

[0071] Step S200: feature extraction is performed based on the reference signal and the test signal respectively, to obtain the reference local feature data segment and the test local feature data segment of different window positions of the sensing optical fiber.

[0072] The process of obtaining the reference local feature data segment when the time-domain signal is the reference signal and the process of obtaining the test local feature data segment when the time-domain signal is the test signal are the same, as shown in FIG. 2. Figure 4 The process of obtaining the reference local feature data segment when the time-domain signal is the reference signal and the process of obtaining the test local feature data segment when the time-domain signal is the test signal are the same, as shown in FIG. 2.

[0073] Step S210: Fourier transform is performed on the time-domain signal to obtain an optical frequency domain signal.

[0074] Step S220: The optical frequency domain signal is converted into a distance domain signal.

[0075] The distance domain signal is a signal that varies with distance, and the distance refers to the distance from the starting point of the sensing optical fiber to the position on the sensing optical fiber. The conversion relationship between the distance domain signal and the optical frequency domain signal is:

[0076]

[0077] wherein z is the distance, f represents the beat frequency, c is the speed of light, n is the refractive index, and γ is the sweep rate of the laser.

[0078] Step S230: sliding window processing is performed on the distance domain signal to obtain a distance domain window signal of the sensing optical fiber at a corresponding window position.

[0079] The length of the sliding window is set to h, and the number of frequency points corresponds to the distance. Therefore, the distance can be expressed by the number of frequency points, and the window length can also be expressed by the number of frequency points. In an embodiment, a signal with a length of h can be directly obtained from the distance domain signal as the distance domain window signal. In another embodiment, a signal with a length of h can be first obtained from the distance domain signal, and then the length of the window with a length of h is expanded to N by zero padding at the end to obtain the final distance domain window signal. The range of h is 200<h<2000, and the range of N is 1000<N<20000. The purpose of the zero padding at the end to expand the window is to improve the wavelength resolution without changing the high spatial resolution, thereby improving the resolution of the strain measurement. It can be understood that by moving the sliding window, multiple window signals can be obtained from the distance domain signal. For example, taking h=1000 as an example, the first segment is a signal at the (0, 1000] frequency point, the second segment is at (1000, 2000], and so on, until the last frequency point in the distance domain signal is obtained.

[0080] Step S240: performing short-time Fourier transform on the distance domain window signal to obtain an optical wavelength domain window signal.

[0081] The short-time Fourier transform is performed on each distance domain window signal to obtain a corresponding optical wavelength domain window signal. At this time, the width of the local distance domain window signal and the local optical wavelength domain signal segment is N.

[0082] Step S250: performing normalization processing on the amplitude of the optical wavelength domain window signal to obtain an optical wavelength domain normalized window signal.

[0083] After normalization, the amplitude information of the signal is converted into the interval [0, 1].

[0084] As shown in Fig. 5(a), the wavelength domain information result of the reference signal is the strain of 0με, and the wavelength domain information result of the test signal is the strain of 4000με, wherein the length N of the data segment is 5000. At this time, the reference signal and the test signal are both normalized, and the highest peak point is converted to 1.

[0085] Step S260: The amplitude of the optical wavelength domain normalized window signal is divided into a equal parts to form a amplitude intervals.

[0086] It can be understood that a here is an integer.

[0087] The amplitude of the optical wavelength domain normalized window signal is divided into a equal parts to form a amplitude intervals, which are [0, 1 / a), [1 / a, 2 / a), … (1-1 / a, 1] respectively. The corresponding frequency points in each amplitude interval can be counted to form an amplitude distribution statistical histogram, as shown in Fig. 5(b), wherein the vertical axis represents the distribution frequency of data in each interval. Taking a = 20 as an example, the data is divided into 20 equal intervals. It can be seen that the frequency points in the [0, 1 / a) amplitude interval are as high as 4500 or more, and the frequency points in the 19 intervals (bar columns in the dashed box) of the (1 / a, 1) part are very few. By amplifying the 19 intervals of the (1 / a, 1) part, it can be seen that the frequency points of most data are less than 50. Specifically, for the reference data, the frequency of each interval can be recorded as N ref_1 ,N ref_2 ,…N ref_a , and for the test data, the frequency of each interval can be recorded as N test_1 ,N test_2 ,…N test_a .

[0088] Step S270: The total distribution frequency points of the last a-1 amplitude intervals are counted as the data segment length M.

[0089] Through analysis, it is found that after dividing the amplitude intervals in the above manner, the frequency points in the first amplitude interval [0, 1 / a) have a large proportion, but the characteristic information has little effect on strain analysis, so a large amount of redundant calculation will be caused. Therefore, the frequency points in this amplitude interval are only counted, and the total distribution frequency points of the last a-1 amplitude intervals are counted as the data segment length M. Taking a = 20 as an example, the data is divided into 20 equal intervals, and only the total frequency points of the 19 intervals of the (1 / a, 1) part are counted as the data segment length M. For the reference signal, the obtained data segment length is recorded as M ref , and for the test signal, the obtained data segment length is recorded as M test , M ref =N-N ref_1 , M test =N-N test_1 , the data segment length Mref and M test Adapts to the value of a.

[0090] Step S280: Taking the peak amplitude of the normalized window signal in the optical wavelength domain as the center, extract M frequency points to form the corresponding local feature data segments.

[0091] After determining the length of the effective data segment, a signal segment of length M is taken from the optical wavelength domain normalized window signal as the data for subsequent analysis offset. Specifically, the frequency point corresponding to the highest peak of the optical wavelength domain normalized window signal is taken as the center, and the same number of frequency points are taken on both sides to form a local feature data segment of length M. It should be noted that when M is odd, the (M+1) / 2th frequency point can be taken as the center, and (M-1) / 2 frequency points can be taken on both sides; when M is even, the (M / 2)th frequency point can be taken as the center, and M / 2 frequency points can be taken on the left and M / 2-1 frequency points on the right, or the (M / 2-1)th frequency point can be taken as the center, and M / 2-1 frequency points can be taken on the left and M / 2 frequency points on the right. Figure 6 As shown, with M ref =M test For example, with a frequency of 350, (a) is a reference local feature data segment obtained by extracting 350 frequency points from the optical wavelength domain normalized window signal corresponding to the reference signal, and (b) is a test local feature data segment obtained by extracting 350 frequency points from the optical wavelength domain normalized window signal corresponding to the test signal. Subsequent offset analysis will be based on the extracted local feature data segments, which can greatly reduce the analysis data and improve the analysis speed.

[0092] Step S300: Perform matching calculations on the reference local feature data segment and the test local feature data segment at the same window position to obtain the wavelength offset of the reference signal and the test signal at the corresponding window position.

[0093] In step S200, the distance domain window signal at a certain position on the sensing fiber is first determined by a sliding window. Then, the corresponding optical wavelength domain normalized window signal is obtained through data processing. Subsequently, effective data of length M is extracted to obtain a local feature data segment. Therefore, the wavelength offset obtained by analyzing the reference local feature data segment and the test local feature data segment based on the same sliding window (i.e., the same position) is the wavelength offset of the sensing fiber at the corresponding sliding window position.

[0094] In one embodiment, the process of calculating the wavelength offset between the reference signal and the test signal at the same window location includes:

[0095] Step S310: When M ref ≠M testAt this time, the end of the local feature data segment with small length is padded with zeros to make the length of the reference local feature data segment and the test local feature data segment equal.

[0096] The length of the reference local feature data segment M ref and the test local feature data segment M test is compared, if M ref <M test , the end of the reference local feature data segment is padded with zeros to expand, so that the expanded length M' ref =M test ; otherwise, the end of the test local feature data segment is padded with zeros, so that the expanded length M' test =M ref . For convenience, the new data segment after the alignment operation is named M' ref and M' test .

[0097] Step S320: Cross-correlation processing is performed on the reference local feature data segment and the test local feature data segment, the offset frequency point number d1 corresponding to the peak of the cross-correlation is calculated, and the corresponding wavelength shift ws1 is calculated.

[0098] The new reference data and test data segments M' ref and M' test are cross-correlated, the offset point number d1 corresponding to the peak of the cross-correlation is obtained, that is, the wavelength shift ws1 is obtained.

[0099] Step S330: The offset frequency point number d2 between the highest peak of the reference local feature data segment and the highest peak of the test local feature data segment is calculated, and the corresponding wavelength shift ws2 is calculated.

[0100] The highest peak P ref and P test between the two sets of data are matched to calculate the offset point distance d2, and the wavelength shift ws2 is obtained.

[0101] Step S340: The wavelength shift ws1 and the wavelength shift ws2 are added to obtain the wavelength shift of the reference signal and the test signal at the current window position.

[0102] The final matching calculation wavelength shift between the reference data and the test data is: wavelength_shift=ws1+ws2.

[0103] Specifically, the wavelength shift wavelength_shift is related to the sweep range ΔF, the offset point numbers d1 and d2, and the total number of data points N, and is calculated as follows:

[0104]

[0105] In addition, the amount of calculation data is 2*M when demodulating the strain at each position at present ref -1. And before the local feature extraction and matching, the amount of calculation data is 2*N-1 under the same condition. Wherein, due to the enhanced intensity a of the backscattering enhanced optical fiber, M is much smaller than N.

[0106] Step S400: calculating the strain distribution of the sensing optical fiber based on the wavelength shift at different window positions.

[0107] By sliding the window and repeating the above steps, the wavelength shift at each position on the sensing optical fiber is obtained, and the distributed strain change on the sensing optical fiber is calculated according to the shift. Wherein, the strain size epsilon s The relationship between the strain size epsilon and the wavelength shift is shown in the formula, and the proportional coefficient is the strain coefficient k of the backscattering enhanced optical fiber.

[0108] epsilon s =k*wavelength_shift

[0109] As Figure 7 The strain measurement result on the sensing optical fiber is represented. After the local feature adaptive extraction and matching calculation, the strain of 4000με is correctly demodulated, which is consistent with the strain size applied on the optical fiber, and the strain measurement has the advantages of high spatial resolution, large strain measurement and high strain accuracy.

[0110] Correspondingly, the application also relates to a kind of fast OFDR strain measurement device based on backscattering enhanced optical fiber, it includes data processing system, such as Figure 8As shown, the data processing system comprises a feature extraction unit, a wavelength shift calculation unit and a strain distribution calculation unit. The feature extraction unit is configured to perform feature extraction based on the time-domain signal output by the optical frequency domain reflectometer to obtain a local feature data segment of the sensing optical fiber at different window positions. The wavelength shift calculation unit is configured to, after obtaining a reference local feature data segment and a test local feature data segment, perform matching calculation on the reference local feature data segment and the test local feature data segment at the same window position to obtain the wavelength shift of the reference signal and the test signal at the corresponding window position, wherein the reference local feature data segment is a local feature data segment obtained by the feature extraction unit when the time-domain signal is a reference signal, and the test local feature data segment is a local feature data segment obtained by the feature extraction unit when the time-domain signal is a test signal. The strain distribution calculation unit is configured to calculate the strain distribution of the sensing optical fiber based on the wavelength shift at different window positions. When performing OFDR strain measurement, the reference signal and the test signal output by the optical frequency domain reflectometer are input into the feature extraction unit to obtain the reference local feature data segment and the test local feature data segment, respectively, and then the wavelength shift of the reference signal and the test signal at each window position is calculated by the wavelength shift calculation unit, and finally the strain distribution of the sensing optical fiber is calculated based on the wavelength shift at different window positions by the strain distribution calculation unit.

[0111] The feature extraction unit comprises:

[0112] a Fourier transform sub-unit configured to perform Fourier transform on the input time-domain signal to obtain an optical frequency domain signal;

[0113] a distance domain conversion sub-unit configured to convert the optical frequency domain signal into a distance domain signal;

[0114] a window intercepting sub-unit configured to slide a window on the distance domain signal to intercept a distance domain window signal of the sensing optical fiber at a corresponding window position;

[0115] a short-time Fourier transform sub-unit configured to perform short-time Fourier transform on the distance domain window signal to obtain an optical wavelength domain window signal;

[0116] a normalization processing sub-unit configured to perform normalization processing on the amplitude of the optical wavelength domain window signal to obtain an optical wavelength domain normalized window signal;

[0117] an amplitude division sub-unit configured to divide the amplitude of the optical wavelength domain normalized window signal into a equal parts to form a amplitude intervals;

[0118] a data segment length calculation sub-unit configured to count the total distribution frequency points of the last a-1 amplitude intervals as a data segment length M;

[0119] The local feature data segment output subunit is configured to take the amplitude maximum peak of the optical wavelength domain normalized window signal as the center, intercept M frequency points, and form a corresponding local feature data segment.

[0120] Specifically, the data processing system can be located in the host computer, and the fast OFDR strain measurement device based on the backscattering enhanced optical fiber further comprises an optical frequency domain reflectometer and a sensing optical fiber, and the sensing optical fiber is a backscattering enhanced optical fiber with an enhanced intensity multiple a.

[0121] Those skilled in the art will easily understand that the above description is only the preferred embodiment of the present application, and is not used to limit the present application, and any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A fast OFDR strain measurement method based on backscatter enhanced optical fiber, characterized in that, The method comprises the following steps: acquiring a time domain signal output by an optical frequency domain reflectometer, the time domain signal being divided into a reference signal obtained without applying strain to a sensing optical fiber and a test signal obtained by the sensing optical fiber in an actual environment, the sensing optical fiber being a backscattering enhanced optical fiber with an enhanced strength multiple a; performing feature extraction based on the reference signal and the test signal respectively to obtain reference local feature data segments and test local feature data segments of different window positions of the sensing optical fiber; performing matching calculation on the reference local feature data segments and the test local feature data segments of the same window position to obtain wavelength shift amounts of the reference signal and the test signal at the corresponding window position; calculating a strain distribution of the sensing optical fiber based on the wavelength shift amounts at different window positions; wherein the process of performing feature extraction comprises the following steps: performing Fourier transform on the selected time domain signal to obtain an optical frequency domain signal; converting the optical frequency domain signal into a distance domain signal; sliding a window on the distance domain signal to intercept a distance domain window signal of the sensing optical fiber at a corresponding window position; performing short-time Fourier transform on the distance domain window signal to obtain an optical wavelength domain window signal; performing normalization processing on the amplitude of the optical wavelength domain window signal to obtain an optical wavelength domain normalized window signal; dividing the amplitude of the optical wavelength domain normalized window signal into a equal parts to form a amplitude interval; counting the total distribution frequency points of the last a-1 amplitude intervals as a data segment length M; taking the highest peak of the amplitude of the optical wavelength domain normalized window signal as the center to intercept M frequency points to form a corresponding local feature data segment. performing cross-correlation processing on the reference local feature data segment and the test local feature data segment to calculate a shift frequency point number d1 corresponding to the peak value of the cross-correlation and to calculate a corresponding wavelength shift amount ws1; 2. The backscatter-enhanced optical fiber-based fast OFDR strain measurement method of claim 1, wherein, The length of the window is expressed by the number of frequency points, and the length of another window is h; after intercepting the distance domain signal with the length of h, the window size with the length of h is expanded to N by the end zero padding method to obtain the final distance domain window signal, wherein the range of h is 200<2000, and the range of N is 1000 <20000.

3. The backscatter-enhanced optical fiber-based fast OFDR strain measurement method of claim 1, wherein, The length of the reference local feature data segment is defined as M ref The length of the test local feature data segment is M test The process of calculating the wavelength offset of the reference signal and the test signal at the same window position comprises: When M ref ≠ M test , the end of the local feature data segment with smaller length is zero-padded to make the length of the reference local feature data segment and the test local feature data segment equal. calculating a shift frequency point number d2 between the highest peak of the reference local feature data segment and the highest peak of the test local feature data segment and calculating a corresponding wavelength shift amount ws2; adding the wavelength shift amount ws1 and the wavelength shift amount ws2 to obtain the wavelength shift amount of the reference signal and the test signal at the current window position. The calculation formula of the wavelength shift amount ws based on the shift frequency point number d is as follows:

4. The backscatter-enhanced optical fiber-based fast OFDR strain measurement method of claim 1, wherein, wherein, ΔF is a sweep range, and N is a frequency point number of the distance domain window signal. The backscattering enhanced optical fiber is a single-mode optical fiber with periodic backscattering enhancement.

5. The backscatter-enhanced optical fiber-based fast OFDR strain measurement method of claim 1, wherein, The enhanced strength multiple a ranges from 10 to 30.

6. The backscatter-enhanced optical fiber-based fast OFDR strain measurement method of claim 1, wherein, The conversion relationship of converting the optical frequency domain signal into the distance domain signal is as follows:

7. The backscatter-enhanced optical fiber-based fast OFDR strain measurement method of claim 1, wherein, wherein, z is a distance, f represents a beat frequency, c is a light speed, n is a refractive index of the sensing optical fiber, and γ is a sweep rate of a laser in the optical frequency domain reflectometer. The calculation of the strain distribution of the sensing optical fiber based on the wavelength shift amounts at different window positions comprises: calculating a strain size at a corresponding window position based on the wavelength shift amount at each window position and obtaining the strain distribution of the sensing optical fiber according to the strain sizes of all window positions, wherein 8. The backscatter-enhanced optical fiber-based fast OFDR strain measurement method of claim 1, wherein, The calculation formula of the strain size according to the wavelength shift amount is as follows: The data processing system comprises e s = k * wavelength_shift where ε s is the strain magnitude, k is the strain coefficient of the backscattering enhanced fiber, and wavelength_shift is the wavelength shift.

9. A fast OFDR strain measurement device based on backscatter enhanced optical fiber, characterized in that, ​ The feature extraction unit is configured to perform feature extraction based on the time-domain signal output by the optical frequency domain reflectometer to obtain a local feature data segment of the sensing optical fiber at a different window position; The wavelength shift calculation unit is configured to, after obtaining the reference local feature data segment and the test local feature data segment, perform matching calculation on the reference local feature data segment and the test local feature data segment at the same window position to obtain a wavelength shift of the reference signal and the test signal at the corresponding window position, wherein the reference local feature data segment is a local feature data segment obtained by the feature extraction unit when the time-domain signal is a reference signal, and the test local feature data segment is a local feature data segment obtained by the feature extraction unit when the time-domain signal is a test signal; The strain distribution calculation unit is configured to calculate the strain distribution of the sensing optical fiber based on the wavelength shifts at different window positions. The feature extraction unit includes: The Fourier transform sub-unit is configured to perform Fourier transform on the input time-domain signal to obtain an optical frequency domain signal; The distance domain conversion sub-unit is configured to convert the optical frequency domain signal into a distance domain signal; The window intercepting sub-unit is configured to slide a window on the distance domain signal to intercept a distance domain window signal of the sensing optical fiber at a corresponding window position; The short-time Fourier transform sub-unit is configured to perform short-time Fourier transform on the distance domain window signal to obtain an optical wavelength domain window signal; The normalization processing sub-unit is configured to normalize the amplitude of the optical wavelength domain window signal to obtain an optical wavelength domain normalized window signal; The amplitude division sub-unit is configured to divide the amplitude of the optical wavelength domain normalized window signal into a equal parts to form a amplitude intervals; The data segment length calculation sub-unit is configured to count the total distribution frequency points of the last a-1 amplitude intervals as a data segment length M; The local feature data segment output sub-unit is configured to take the highest peak of the amplitude of the optical wavelength domain normalized window signal as the center to intercept M frequency points to form a corresponding local feature data segment.

10. The backscatter-enhanced optical fiber based fast OFDR strain measurement apparatus of claim 9, wherein, The optical frequency domain reflectometer and the sensing optical fiber are also included, and the sensing optical fiber is a backscattering enhanced optical fiber with an enhanced strength multiple of a.

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