Optical fiber strain demodulation method and system based on scattering enhancement point position offset compensation

Abstract

The invention discloses an optical fiber strain demodulation method and system based on scattering enhancement point position offset compensation, and relates to the technical field of optical fiber strain demodulation. The method is characterized by comprising the following steps of: acquiring an optical fiber reference signal without applying strain and an optical fiber measurement signal after applying strain, and writing a plurality of scattering points with fixed intervals into an optical fiber core in advance; the method comprises the following steps: respectively carrying out fast Fourier transform on an optical fiber reference signal and an optical fiber measurement signal, respectively extracting a reflection signal between every two scattering points by adopting a sliding Hanning window, and respectively carrying out inverse Fourier transform on a reference window reflection signal and a measurement window reflection signal at the same scattering point; then calculating a cross correlation coefficient between the obtained reference RBS spectrum and the measured RBS spectrum; and calculating an optical fiber strain value of a region between every two scattering points by utilizing a wavelength drift distance in a measured RBS spectrum corresponding to the maximum value of the cross correlation coefficient. According to the method, the spatial resolution of the large strain demodulation process can be remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber strain demodulation, and in particular to an optical fiber strain demodulation method and system based on the compensation of the position offset of scattering enhancement points. Background Art

[0002] Distributed optical fiber sensors mainly rely on the Rayleigh scattering, Brillouin scattering, and Raman scattering effects in optical fibers. Analyzing their backscattered signals can continuously monitor physical information at any position along the optical fiber. Among them, the optical frequency domain reflectometry (OFDR) technology utilizes the Rayleigh scattering characteristics of optical fibers to detect signals generated by coherent optical beat interference, and demodulates the phase and amplitude changes of Rayleigh scattering to obtain strain or temperature information on the optical fiber path. Compared with traditional strain sensors, the OFDR system based on the spectral shift of Rayleigh backscattering (RBS) has high spatial resolution, high sensitivity, and distributed detection capabilities, and is widely used in fields such as structural health monitoring, pipeline deformation detection, and geological exploration.

[0003] Although the OFDR technology has made significant progress in many applications, under actual large-scale strain conditions, the existing OFDR technology still faces some technical bottlenecks: mainly the spatial position offset under large strain. When the applied strain exceeds a certain range, the change in the position of the optical fiber medium will cause a drastic shift in the Rayleigh scattering spectrum; as the proportion of misalignment increases, the accuracy of the cross-correlation operation used to demodulate the spectral frequency shift decreases, resulting in an increase in measurement error and even demodulation failure. The existing traditional demodulation technology based on the cross-correlation algorithm faces the problem of accuracy degradation caused by spatial misalignment: the similarity between the reference spectrum and the measurement spectrum decreases rapidly with the increase of strain, resulting in pseudo-peaks in the demodulation results. This severely restricts application scenarios that require large dynamic range measurements, such as structural health monitoring and composite material damage detection. Summary of the Invention

[0004] Therefore, the present invention proposes an optical fiber strain demodulation method and system based on the compensation of the position offset of scattering enhancement points, aiming to solve the problems of spatial position offset and degradation of demodulation results in existing distributed optical fiber strain sensing technologies under large strain.

[0005] According to one aspect of the present invention, an optical fiber strain demodulation method based on the compensation of the position offset of scattering enhancement points is proposed. The method includes:

[0006] Collecting an optical fiber reference signal without applied strain and an optical fiber measurement signal after applied strain; a plurality of scattering points with fixed intervals are pre-written in the optical fiber core;

[0007] Performing fast Fourier transform on the optical fiber reference signal and the optical fiber measurement signal respectively to obtain a reference distance domain signal and a measurement distance domain signal;

[0008] For the reference distance domain signal and the measured distance domain signal, a sliding Hanning window is used to extract the reflected signals between every two scattering points respectively, so as to obtain a set of reference window reflected signals corresponding to the reference distance domain signal and a set of measured window reflected signals corresponding to the measured distance domain signal;

[0009] The inverse Fourier transform is respectively performed on the reference window reflected signal and the measured window reflected signal at the same scattering point position to obtain a set of reference RBS spectra and a set of measured RBS spectra;

[0010] Calculate the cross-correlation coefficient between the reference RBS spectrum and the measured RBS spectrum;

[0011] Use the wavelength drift amount in the measured RBS spectrum corresponding to the maximum value of the cross-correlation coefficient to calculate the fiber strain value in the region between every two scattering points, so as to obtain multiple fiber strain values.

[0012] Further, an optical frequency domain reflection system is used to collect the fiber reference signal without applied strain and the fiber measurement signal with applied strain; a femtosecond laser writing technique is used to write multiple scattering points with fixed intervals in the fiber core.

[0013] Further, the fiber reference signal I(t) without applied strain and the fiber measurement signal I'(t) with applied strain are respectively expressed as:

[0014]

[0015] In the formula, E R represents the reference optical field intensity; M represents the total number of scattering points written in the fiber; r m represents the reflectivity corresponding to the m-th scattering point; E pm represents the optical field intensity reflected by the m-th scattering point under the condition of no applied strain, τ m represents the time delay caused by the reflection of the m-th scattering point under the condition of no applied strain; E pm ' represents the measured optical field intensity reflected by the m-th scattering point under the condition of applied strain, τ m ' represents the time delay caused by the reflection of the m-th scattering point under the condition of applied strain.

[0016] Further, the reference distance domain signal F(f) and the measured distance domain signal F'(f) are respectively expressed as:

[0017] F(f) = 2T sw σr m E R E Pm sinc[π(f - f bm )T sw

[0018] F'(f) = 2T​sw σr m E R E Pm 'sinc[π(f - f bm ')T sw

[0019] In the formula, T sw represents the sweep period of the laser; σ represents the detector sensitivity; f represents the frequency; f bm represents the beat frequency signal corresponding to the m-th scattering point under the condition of no applied strain; f bm ' represents the beat frequency signal corresponding to the m-th scattering point under the condition of applied strain.

[0020] Furthermore, the beat frequency signal f bm corresponding to the m-th scattering point under the condition of no applied strain and the beat frequency signal f bm ' corresponding to the m-th scattering point under the condition of applied strain are respectively expressed as:

[0021]

[0022] In the formula, c represents the speed of light in vacuum, n represents the mode refractive index in the optical fiber; γ represents the tuning rate of the laser sweep; x m represents the position of the m-th scattering point under the condition of no applied strain; x m ' represents the position of the m-th scattering point under the condition of applied strain.

[0023] Furthermore, the reference window reflection signal F window (f) and the measurement window reflection signal F' window (f) are respectively expressed as:

[0024] F window (f) = F(f) × [1 - cos(2πn / N)] / 2

[0025] = T sw σr m E R E Pm sinc[π(f - f bm )T sw × (1 - cos(2πn / N)), 0 ≤ n ≤ N

[0026] F window '(f) = F'(f) × (1 - cos(2πn / N)

[0027] = T sw σr m E R E Pm 'sinc[π(f - f bm ​')T sw ×(1 - cos(2πn / N)), 0 ≤ n ≤ N

[0028] Where N represents the total length of the window function data points, and n represents the sequence number of the data points within the window function.

[0029] Further, calculate the cross - correlation coefficient of the reference RBS spectrum and the measured RBS spectrum according to the following formula:

[0030]

[0031] Where Δλ represents the wavelength drift caused by strain; λ represents the wavelength; represents the reference RBS spectrum of the m - th scattering point; represents the measured RBS spectrum of the m - th scattering point.

[0032] Further, the calculation formula for calculating the fiber optic strain value in the region between every two scattering points by using the wavelength drift amount in the measured RBS spectrum corresponding to the maximum value of the cross - correlation coefficient is:

[0033] Δλ = C ε .Δε

[0034] Where C ε represents the single - mode fiber optic strain sensitivity; Δε represents the fiber optic strain value.

[0035] According to another aspect of the present invention, a fiber optic strain demodulation system based on the compensation of the position offset of the scattering enhancement points is proposed. The system includes:

[0036] A signal acquisition module configured to acquire the fiber optic reference signal without applied strain and the fiber optic measurement signal with applied strain; a plurality of scattering points with a fixed interval are pre - written in the fiber core;

[0037] The signal demodulation module includes a cross-correlation calculation sub-module and a strain calculation sub-module; wherein, the cross-correlation calculation sub-module is configured to: perform fast Fourier transforms on the optical fiber reference signal and the optical fiber measurement signal respectively to obtain a reference distance domain signal and a measurement distance domain signal; for the reference distance domain signal and the measurement distance domain signal, respectively use a sliding Hanning window to extract the reflection signals between every two scattering points, so as to obtain a set of reference window reflection signals corresponding to the reference distance domain signal and a set of measurement window reflection signals corresponding to the measurement distance domain signal; perform inverse Fourier transforms on the reference window reflection signal and the measurement window reflection signal at the same scattering point position respectively to obtain a set of reference RBS spectra and a set of measurement RBS spectra; calculate the cross-correlation coefficient between the reference RBS spectrum and the measurement RBS spectrum; the strain calculation sub-module is configured to: use the wavelength drift amount in the measurement RBS spectrum corresponding to the maximum value of the cross-correlation coefficient to calculate the optical fiber strain value in the region between every two scattering points, so as to obtain multiple optical fiber strain values.

[0038] Furthermore, in the signal acquisition module, an optical frequency domain reflection system is used to acquire an optical fiber reference signal without applied strain and an optical fiber measurement signal after applied strain; a femtosecond laser writing technique is used to write multiple scattering points with a fixed interval in the optical fiber core.

[0039] The beneficial technical effects of the present invention are:

[0040] The present invention provides an optical fiber strain demodulation method and system based on scattering enhancement point position deviation compensation. By repositioning the scattering enhancement points on a single-mode optical fiber, the offset compensation of the spatial position of the strain region is realized. The present invention can effectively improve the measurement accuracy of the optical fiber strain phenomenon under large strain and enhance the stability of the strain demodulation result. The present invention uses a compensation method of repositioning the scattering enhancement points in the optical fiber, which can effectively reduce the spatial dislocation caused under large strain conditions, significantly enhance the correlation of the reference spectrum and the RBS signal of the measurement spectrum, and suppress the degradation of the demodulation result. Due to the enhancement of the spectral correlation in the demodulation process, the spatial resolution of the large strain demodulation process based on the OFDR system can be significantly improved. The present invention can provide a strain demodulation with a faster response than traditional compensation means. In the demodulation of strain signals with a high signal-to-noise ratio, it is particularly suitable for monitoring applications with fast response and high precision, such as the structural health monitoring of power transmission lines, tunnels, etc. The present invention provides an effective solution for improving the spatial resolution of OFDR and realizing the feasibility of large-scale distributed strain sensing. Description of the Drawings

[0041] By reading the following detailed description with reference to the accompanying drawings, the above and other objects, features, and advantages of the exemplary embodiments of the present invention will become readily understood. In the drawings, several embodiments of the present invention are shown in an exemplary rather than restrictive manner, wherein:

[0042] Figure 1 It is a schematic diagram of the principle of optical fiber strain demodulation proposed by the existing method and the present invention;

[0043] Figure 2 It is a flowchart of a fiber optic strain demodulation method based on scattering enhanced point position offset compensation according to an embodiment of the present invention;

[0044] Figure 3 It is another schematic diagram of the process of a fiber optic strain demodulation method based on scattering enhanced point position offset compensation according to an embodiment of the present invention;

[0045] Figure 4 It is a schematic diagram of the optical path for femtosecond laser spatial inscription of scattering enhanced points;

[0046] Figure 5 It is the demodulated strain sensing result when the strain increases from 500 με to 5000 με in steps of 500 με under the condition of a spatial resolution of 6.4 mm in an embodiment of the present invention, where (a) corresponds to the demodulation region of the traditional method being 4.90 - 7.20 m, and (b) corresponds to the demodulation region of the method of the present invention being 5.20 - 7.20 m;

[0047] Figure 6 It is an example diagram of calculating the normalized cross - correlation amplitude along the SMF under a strain of 5000 με for the traditional method and the method of the present invention in an embodiment of the present invention;

[0048] Figure 7 It is a schematic diagram of the structure of a fiber optic strain demodulation system based on scattering enhanced point position offset compensation according to an embodiment of the present invention. Detailed Embodiments

[0049] Next, the principles and spirit of the present invention will be described with reference to several exemplary embodiments. It should be understood that these embodiments are given only to enable those skilled in the art to better understand and then implement the present invention, and do not limit the scope of the present invention in any way. On the contrary, these embodiments are provided to make the present disclosure more thorough and complete, and to be able to fully convey the scope of the present disclosure to those skilled in the art.

[0050] The present invention proposes a fiber optic strain demodulation method and system based on scattering enhanced point position offset compensation. Through the collaborative mechanism of partition demodulation and distance domain self-calibration, using the scattering enhanced points as built-in calibration points to continuously correct and solve the problem of position offset accumulation under large strain; automatically adjusting the demodulation window parameters according to the real-time strain distribution, effectively eliminating the multi-peak interference phenomenon in the traditional cross-correlation algorithm, while expanding the measurement range, ensuring the demodulation accuracy and high spatial resolution. The present invention studies the spatial misalignment correction method based on the repositioning of scattering enhanced points, and explores the influence laws of parameters such as the spacing and reflectivity of reflection enhanced points on the sensing accuracy and spatial resolution, so as to achieve high-precision high-temperature and large-range large-scale strain distributed sensing.

[0051] The demodulation principle is as Figure 1 shown. Figure 1 (a) shows the traditional demodulation method. The strain region (i.e., the light-colored region) is divided into four fiber segments, and the offset between the starting point of the applied strain and the first strain fiber segment is ignored. The position offset is the offset of the position after stretching from the initial position before stretching, which accumulates along the fiber and becomes significant at the end of the strain region. That is, the position offset of the third fiber segment (C to D) is three times that of the first fiber segment (A to B), and the offset increases from Δx to 3Δx. The position deviation of the fiber demodulation segment caused by the photoelastic effect and strain will accumulate throughout the sensing region, resulting in obvious inconsistencies in the fiber medium (E to the end) in the strain-free region. As Figure 1 shown by the dark part in (a), the correlation of the RBS spectrum in the sensing region will deteriorate. Because of the spatial mismatch in the fiber medium, this result degradation is particularly significant.

[0052] The demodulation principle proposed by the present invention is a distance repositioning method for the fiber medium to achieve large-range strain sensing. Compared with the traditional method where the position offset accumulates to the end and causes the degradation of the RBS spectrum cross-correlation in the sensing region, the demodulation algorithm based on the scattering enhanced point array proposed by the present invention can calibrate the position offset to zero at specific sensing points. Due to the artificial refractive index modulation, the amplitude enhancement of the RBS signal at the scattering points is higher than that of the untreated SMF. Since the built-in scattering points will spontaneously displace along the fiber medium under the applied strain, it is feasible to match the offset of the fiber medium by the positioning of the scattering points obtained from the reference signal and the measurement signal in the distance domain.

[0053] Specifically, as Figure 1As shown in (b), assume that six scattering points with a spacing of L are written in the SMF, four of which are located in the strain region, represented by the light-colored area. In this case, the amplitude of the applied strain is set to Δx / L, indicating that for each fiber segment, the stretching amount between two scattering points is Δx. Considering that the sensing region is at a distance of l from the third scattering point, the offset of the sensing region under strain relative to the initial position is Δx·l / L + 2Δx, and the position deviation relative to the third scattering point is Δx·l / L. Among them, 2Δx is the additional position deviation of the third scattering point from the initial position of the strain region, but it can be minimized through repositioning based on its own physical distance. At the same time, the surrounding optical fiber medium can be anchored according to the repositioning of each scattering point, and the adjacent optical fiber medium can be continuously guided for self-correction to suppress the spatial mismatch caused by strain, so that the position deviation of the fiber segment is continuously zeroed through distance repositioning based on the built-in distance of each scattering enhancement point.

[0054] Based on the above demodulation principle, an embodiment of the present invention proposes an optical fiber strain demodulation method based on the offset compensation of the scattering enhancement point position, as Figures 2 - 3 shown, the method includes:

[0055] S1. Collect the optical fiber reference signal without applied strain and the optical fiber measurement signal after applied strain; a plurality of scattering points with fixed intervals are pre-written in the optical fiber core;

[0056] S2. Perform fast Fourier transform on the optical fiber reference signal and the optical fiber measurement signal respectively to obtain the reference distance domain signal and the measurement distance domain signal;

[0057] S3. For the reference distance domain signal and the measurement distance domain signal, respectively use a sliding Hanning window to extract the reflection signals between every two scattering points, so as to obtain a set of reference window reflection signals corresponding to the reference distance domain signal and a set of measurement window reflection signals corresponding to the measurement distance domain signal;

[0058] S4. Perform inverse Fourier transform on the reference window reflection signal and the measurement window reflection signal at the same scattering point position respectively to obtain a set of reference RBS spectra and a set of measurement RBS spectra;

[0059] S5. Calculate the cross-correlation coefficient between the reference RBS spectrum and the measurement RBS spectrum;

[0060] S6. Use the wavelength drift amount in the measurement RBS spectrum corresponding to the maximum value of the cross-correlation coefficient to calculate the optical fiber strain value in the region between every two scattering points, so as to obtain a plurality of optical fiber strain values.

[0061] The method starts from S1, in which a fiber optic reference signal without applied strain and a fiber optic measurement signal after applied strain are collected; multiple scattering points with a fixed interval are pre-written in the fiber core.

[0062] According to an embodiment of the present invention, first, the core of a single-mode optical fiber is customized by using femtosecond laser writing technology, and scattering enhancement points are accurately written at specific positions. Figure 4 The optical path diagram of femtosecond laser spatial writing of scattering enhancement points is shown. The laser pulses generated by the femtosecond laser first pass through the aperture, and then through the half-wave plate and the Glan-Taylor prism. The combination of the two realizes the control of the output light power by rotating the half-wave plate; then the output light passes through the beam expander system to increase the spot size, so that it can finally fully enter the objective lens; then through the beam splitter prism, on the one hand, the light energy from the laser is vertically transmitted downward into the objective lens, and on the other hand, the visible light is transmitted upward to the CCD; finally, the output laser passes through the objective lens and is focused into the displacement stage, and the laser is finally focused into the core by moving the three-dimensional displacement stage. Combining the winding system with the self-focusing process, the femtosecond laser point-by-point technology is used to automatically write an array composed of M (for example, M = 20) permanent scattering points in the single-mode optical fiber, where the interval between two scattering points is l, for example, l = 12 cm. By adjusting parameters such as laser power and pulse frequency, the controllable adjustment of the morphology and distribution density of the enhancement points is realized, breaking through the limitation of the insufficient Rayleigh scattering intensity of traditional single-mode optical fibers, significantly improving the signal-to-noise ratio, and laying a physical foundation for high-spatial-resolution and high-precision distributed strain measurement.

[0063] Then, a beat signal is collected by using an optical frequency domain reflection system including a series of optical devices. Among them, the collected fiber optic reference signal without applied strain is expressed as:

[0064]

[0065] In the formula, M is the total number of scattering points written in the optical fiber, E R is the reference light field intensity, E pm is the light field intensity reflected by the m-th scattering point under the condition of no applied strain, τ m is the time delay caused by the reflection of the m-th scattering point under the condition of no applied strain; r m represents the reflectivity corresponding to the m-th scattering point.

[0066] The fiber optic measurement signal after applied strain is expressed as:

[0067]

[0068] In the formula, E pm ' is the measured light field intensity reflected by the m-th scattering point under the condition of applied strain, τ mThe time delay caused by the reflection of the m-th scattering point under the applied strain condition.

[0069] Then, S2 is executed. In S2, the fast Fourier transform is respectively performed on the optical fiber reference signal and the optical fiber measurement signal to obtain a reference distance domain signal and a measurement distance domain signal.

[0070] According to the embodiment of the present invention, the signal acquired from the data acquisition system is converted into the distance domain based on the fast Fourier transform (FFT). By using the real-time distribution of the scattering enhancement points that are globally shifted with the strain, the optical fiber in the strain loading region is divided with the scattering enhancement points as the reference, local demodulation is performed on each divided section of the optical fiber, and finally the complete distributed demodulation result is spliced.

[0071] The peak search algorithm can be used to determine the exact position of each scattering enhancement point. As a simplification, considering the beat frequency of each scattering point, the FFT result of the optical fiber reference signal without applied strain can be expressed as:

[0072]

[0073] In the formula, T sw is the sweep period of the laser in the optical frequency domain reflection system, σ is the detector sensitivity, and f represents the frequency; f bm is the beat frequency signal corresponding to the m-th scattering point under the condition of no applied strain. Since the reflectivity of the scattering point is much greater than the RBS reflectivity of the ordinary SMF, the beat frequency signal corresponding to each scattering point can be clearly seen in the distance domain, and the frequency domain is converted into the distance domain:

[0074]

[0075] In the formula, f bm is the beat frequency signal corresponding to the m-th scattering point under the applied strain condition; x m represents the position of the m-th scattering point under the condition of no applied strain; c is the speed of light in vacuum, and n is the mode refractive index in the optical fiber; γ represents the tuning rate of the laser sweep, with the unit of nm / s.

[0076] The FFT result of the optical fiber measurement signal after applying strain can be expressed as:

[0077]

[0078] In the formula, f bm ' is the beat frequency signal corresponding to the m-th scattering point under the applied strain condition, x' m is the position of the scattering point corresponding to f bm '.

[0079] Then, execute S3. In S3, for the reference distance domain signal and the measured distance domain signal, a sliding Hanning window is respectively used to extract the reflected signals between every two scattering points, so as to obtain a set of reference window reflected signals corresponding to the reference distance domain signal and a set of measured window reflected signals corresponding to the measured distance domain signal.

[0080] According to an embodiment of the present invention, assuming the position x1 of the first scattering point under the condition of no applied strain, starting from this position, a sliding Hanning window is used to extract the reflected signal on the reference domain along the optical fiber section, expressed as:

[0081]

[0082] In the formula, N represents the total length of the window function data points, and n represents the serial number of the data points within the window function.

[0083] Assuming the position x1' of the first scattering point under the condition of applied strain, starting from this position, a sliding Hanning window is used to extract the reflected signal on the measurement domain along the optical fiber section, expressed as:

[0084]

[0085] For the reference distance domain signal and the measured distance domain signal, a sliding Hanning window is respectively used to extract the reflected signals within the window. Generally speaking, the longer the length of the sliding window, the more recognizable the features contained in the window, and the more reliable the cross-correlation result. However, the longer the length of the sliding window will lead to a reduction in the system spatial resolution. Therefore, in this embodiment, the optimal value of the window length is selected as 6.4 mm. At the same time, the distance between two adjacent scattering points is affected by the applied strain. Therefore, the step size of the sliding window needs to be adjusted to match the length of each scattering point segment to ensure the same number of signal extractions during the local demodulation process. For example, assuming the number of sampling points between two scattering points in the reference state is 100, and due to the influence of strain, the number of sampling points between scattering points in the measurement state is 120; then the Hanning window step size in the measurement state is 1.2 times that in the reference state to ensure the same number of Hanning window sliding times, that is, the demodulation times, between scattering points in different states.

[0086] Then, execute S4. In S4, the inverse Fourier transform is respectively performed on the reference window reflected signal and the measured window reflected signal at the same scattering point position to obtain a set of reference RBS spectra and a set of measured RBS spectra.

[0087] According to an embodiment of the present invention, the inverse Fourier transform (IFFT) is performed on the reflected signal to obtain the corresponding RBS spectrum. The IFFT demodulation is performed on multiple reference window reflected signals at multiple positions between two scattering points to obtain the corresponding reference RBS spectra, expressed as: [RBS r 1 (λ),RBS r2 (λ), RBS r 3 (λ), ……], where λ represents the wavelength. Perform IFFT demodulation on the reflected signals of multiple measurement windows at multiple positions to obtain the corresponding measured RBS spectrum: [RBS i 1 (λ), RBS i 2 (λ), RBS i 3 (λ), ……].

[0088] Then execute S5. In S5, calculate the cross - correlation coefficient between the reference RBS spectrum and the measured RBS spectrum.

[0089] According to the embodiments of the present invention, use the following formula to perform cross - correlation on the reference spectrum and the measured spectrum at each position x of the RBS spectrum m to obtain multiple groups of cross - correlation coefficients corresponding to multiple positions:

[0090]

[0091] In the formula, Δλ represents the wavelength drift caused by strain; λ represents the wavelength; represents the reference RBS spectrum of the m - th scattering point; represents the measured RBS spectrum of the m - th scattering point.

[0092] Then execute S6. In S6, use the wavelength drift amount in the measured RBS spectrum corresponding to the maximum value of the cross - correlation coefficient to calculate the fiber strain value in the region between every two scattering points, so as to obtain multiple fiber strain values.

[0093] According to the embodiments of the present invention, for a group of cross - correlation coefficients corresponding to each position, determine the wavelength drift amount in the window - measured RBS spectrum corresponding to the maximum value in this group of coefficients, and use the following formula to calculate the fiber strain value, so as to obtain multiple fiber strain values at multiple positions:

[0094] Δλ = C ε .Δε (9)

[0095] In the formula, C ε represents the single - mode fiber strain sensitivity, generally 1.07 pm / με; Δε represents the fiber strain value. When the maximum value of the cross - correlation coefficient appears, the wavelength drift amount when the RBS spectrum at the corresponding position is subjected to strain can be obtained, and high - precision demodulation of the strain in this region can be achieved.

[0096] Different from traditional methods that use a single fixed starting point in the distance domain for distributed sensing, the present invention uses different positions of the same scattering point in different states as starting points to automatically correct the initial position offset of each local demodulation process. The optical fiber segment for local demodulation is located between two scattering points, ensuring that the position deviation at the start of each local demodulation process is cleared and will not accumulate to the end. In addition, a sliding Hanning window is used to extract the reflected signal along the optical fiber segment to obtain the corresponding RBS spectrum.

[0097] The present invention flexibly writes scattering enhancement points at different positions and with different intensities in the optical fiber by using femtosecond writing technology, thereby improving the demodulation accuracy of signals under large strain conditions. It has low difficulty in post-processing of the optical fiber and is simple to manufacture. Currently, the algorithm for offset compensation is mainly a recursive compensation method based on the distributed demodulation result, which is a traversal algorithm for finding the maximum demodulation correlation in the distance domain, seriously increasing the operation burden of the computer. The present invention uses a compensation method for repositioning the scattering enhancement points in the optical fiber, which can effectively reduce the spatial dislocation caused under large strain conditions, significantly enhance the correlation between the reference spectrum and the RBS signal of the measurement spectrum, and suppress the degradation of the demodulation result. Due to the enhancement of the spectral correlation during the demodulation process, the spatial resolution of the large strain demodulation process based on the OFDR system can be significantly improved. The present invention can provide strain demodulation with a faster response than traditional compensation means. In the demodulation of strain signals with high signal-to-noise ratio, it is particularly suitable for monitoring applications with fast response and high precision, such as structural health monitoring of power transmission lines, tunnels, etc. The present invention provides an effective solution for improving the spatial resolution of OFDR and realizing the feasibility of large-scale distributed strain sensing.

[0098] It should be noted that since the external refractive index demodulation at the scattering points fabricated by femtosecond laser writing is artificially modulated, only the optical fiber segments with natural refractive index modulation and the corresponding RBS spectra can be demodulated.

[0099] The technical effects of the present invention are further verified through experiments.

[0100] The distributed large-scale large-strain sensing performances of the traditional demodulation method and the method of the present invention are compared and studied. Figure 5 are the demodulated strain sensing results when the strain increases from 500 με to 5000 με in steps of 500 με under the condition of a spatial resolution of 6.4 mm, where Figure 5 (a) corresponds to the demodulation region of the traditional method being 4.90 - 7.20 m; Figure 5 (b) corresponds to the demodulation region of the present invention being 5.20 - 7.20 m, where the inset shows the correspondence between the wavelength shift and the strain. Figure 6 shows the calculated normalized cross-correlation amplitudes of the traditional method and the present invention along the SMF at a strain of 5000 με, corresponding to 0.309 and 0.767 respectively.

[0101] From Figure 5 As can be seen from (a), although the strain performance using the traditional method can be clearly demodulated at the initial position of the sensing region, the demodulated strain results are submerged in noise, such as the appearance of false peaks, etc. Its measurement error is mainly affected by the accumulation of position offsets. Due to the accumulation of spatial mismatch caused by large strain, the degradation of the demodulated results is caused. In contrast, the method of the present invention can clearly obtain the applied strain, as Figure 5 shown in (b). This shows that by repositioning each scattering point during the demodulation process, the sensing performance is significantly improved. From Figure 5 (b), it can also be seen that the ratio of wavelength shift to applied strain is 1.02 pm / με, and the fitting coefficient is 1.00. It should be noted that due to the external demodulation during the manufacturing process, the inherent characteristics of the RBS at the scattering points are changed, so the demodulated results of strain sensing around each scattering point are not reliable. Therefore, Figure 5 the strain sensing results in (b) contain blind spots corresponding to the positions of the scattering points, and the blind spot range is about 2 cm.

[0102] From Figure 6 (a), it can be seen that in the strain application region, especially between 5.04 and 6.24 m, the average cross-correlation coefficient obtained under a strain of 5000 με is only 0.309. In addition, it can be clearly seen that the correlation coefficient of the SMF in the strain-free region at the end is in the range of 6.24 - 7.20 m, which is affected by the position offset of the strain region, resulting in a decrease in its similarity compared with the strain-free region in the previous part before 5.04 m. On the contrary, in Figure 6 (b), after adopting the method of the present invention, the average normalized similarity coefficient is increased to 0.767 and there is no obvious degradation, indicating that the method of the present invention can effectively correct the influence of the position accumulation deviation and ensure a high correlation of the results.

[0103] Another embodiment of the present invention proposes an optical fiber strain demodulation system based on the compensation of the position offset of the scattering enhancement points, as Figure 7 shown, the system includes:

[0104] A signal acquisition module 710 configured to acquire an optical fiber reference signal without applied strain and an optical fiber measurement signal after applied strain; a plurality of scattering points with fixed intervals are pre-written in the optical fiber core;

[0105] The signal demodulation module 720 includes a cross-correlation calculation sub-module 7210 and a strain calculation sub-module 7220; wherein, the cross-correlation calculation sub-module 7210 is configured to: perform fast Fourier transform on the optical fiber reference signal and the optical fiber measurement signal respectively to obtain a reference distance domain signal and a measurement distance domain signal; for the reference distance domain signal and the measurement distance domain signal, respectively use a sliding Hanning window to extract the reflection signals between every two scattering points, so as to obtain a set of reference window reflection signals corresponding to the reference distance domain signal and a set of measurement window reflection signals corresponding to the measurement distance domain signal; perform inverse Fourier transform on the reference window reflection signal and the measurement window reflection signal at the same scattering point position respectively to obtain a set of reference RBS spectra and a set of measurement RBS spectra; calculate the cross-correlation coefficient of the reference RBS spectra and the measurement RBS spectra; the strain calculation sub-module 7220 is configured to: use the wavelength drift amount in the measurement RBS spectrum corresponding to the maximum value of the cross-correlation coefficient to calculate the optical fiber strain value in the region between every two scattering points, so as to obtain multiple optical fiber strain values.

[0106] In this embodiment, optionally, in the signal acquisition module 710, an optical frequency domain reflection system is used to acquire an optical fiber reference signal without applied strain and an optical fiber measurement signal with applied strain; a femtosecond laser writing technique is used to write multiple scattering points with a fixed interval in the optical fiber core.

[0107] For the unspecified parts of an optical fiber strain demodulation system based on scattering enhanced point position offset compensation according to an embodiment of the present invention, please also refer to the specific description of the above method embodiment.

[0108] It should be noted that although several units, modules or sub-modules are mentioned in the above detailed description, this division is merely exemplary and not mandatory. In fact, according to the embodiments of the present invention, the features and functions of the two or more modules described above can be embodied in one module. Conversely, the features and functions of one module described above can be further divided and embodied by multiple modules.

[0109] In addition, although the operations of the method of the present invention are described in a specific order in the drawings, this does not require or imply that these operations must be performed in that specific order, or that all the operations shown must be performed to achieve the desired result. Additionally or alternatively, some steps may be omitted, multiple steps may be combined into one step for execution, and / or one step may be decomposed into multiple steps for execution.

[0110] Although the spirit and principles of the present invention have been described with reference to several specific embodiments, it should be understood that the present invention is not limited to the specific embodiments disclosed, and the division of each aspect does not mean that the features in these aspects cannot be combined for benefit. Such a division is only for convenience of expression. The present invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

1. A fiber optic strain demodulation method based on scattering enhanced point position offset compensation, characterized in that Including: Collecting an optical fiber reference signal without applied strain and an optical fiber measurement signal after applied strain; A plurality of scattering points with fixed intervals are pre-written in the optical fiber core; Performing fast Fourier transform on the optical fiber reference signal and the optical fiber measurement signal respectively to obtain a reference distance domain signal and a measurement distance domain signal; For the reference distance domain signal and the measurement distance domain signal, a sliding Hanning window is respectively used to extract the reflection signals between every two scattering points, so as to obtain a set of reference window reflection signals corresponding to the reference distance domain signal and a set of measurement window reflection signals corresponding to the measurement distance domain signal; Performing inverse Fourier transform on the reference window reflection signal and the measurement window reflection signal at the same scattering point position respectively to obtain a set of reference RBS spectra and a set of measurement RBS spectra; Calculating the cross-correlation coefficient between the reference RBS spectrum and the measurement RBS spectrum; Calculating the optical fiber strain value in the region between every two scattering points by using the wavelength drift amount in the measurement RBS spectrum corresponding to the maximum value of the cross-correlation coefficient, so as to obtain a plurality of optical fiber strain values.

2. The fiber optic strain demodulation method based on scattering enhancement point position offset compensation according to claim 1, characterized in that Collecting an optical fiber reference signal without applied strain and an optical fiber measurement signal after applied strain by using an optical frequency domain reflection system; writing a plurality of scattering points with fixed intervals in the optical fiber core by using a femtosecond laser writing technique.

3. A fiber optic strain demodulation method based on scattering enhancement point position offset compensation according to claim 1, characterized in that, The optical fiber reference signal I(t) without applied strain and the optical fiber measurement signal I'(t) after applied strain are respectively expressed as: Where, E R represents the reference optical field intensity; M represents the total number of scattering points inscribed in the optical fiber; r m represents the reflectivity corresponding to the m-th scattering point; E pm represents the optical field intensity reflected by the m-th scattering point under the condition of no applied strain, τ m represents the time delay caused by the reflection of the m-th scattering point under the condition of no applied strain; E pm ' represents the measured optical field intensity reflected by the m-th scattering point under the condition of applied strain, τ m ' represents the time delay caused by the reflection of the m-th scattering point under the condition of applied strain.

4. A fiber optic strain demodulation method based on scattering enhancement point position offset compensation according to claim 3, characterized in that, The reference distance domain signal F(f) and the measurement distance domain signal F'(f) are respectively expressed as: F(f) = 2T sw σr m E R E Pm sinc[π(f - f bm )T sw ​ F'(f) = 2T sw σr m E R E Pm 'sinc[π(f - f bm )T sw ​ where, T sw represents the frequency sweep period of the laser; σ represents the detector sensitivity; f represents the frequency; f bm represents the beat frequency signal corresponding to the m-th scattering point under the condition of no applied strain; f bm ' represents the beat frequency signal corresponding to the m-th scattering point under the condition of applied strain.

5. A fiber optic strain demodulation method based on scattering enhancement point position offset compensation according to claim 4, characterized in that, The beat frequency signal \(f\) corresponding to the \(m\)th scattering point under the condition of no applied strain bm and the beat frequency signal \(f'\) corresponding to the \(m\)th scattering point under the condition of applied strain bm are respectively expressed as: Wherein, c represents the speed of light in vacuum, n represents the modal refractive index in the optical fiber; γ represents the tuning rate of the laser frequency sweep; x m represents the position of the m-th scattering point under the condition of no applied strain; x m ' represents the position of the m-th scattering point under the condition of applied strain.

6. The fiber optic strain demodulation method based on scattering enhanced point position offset compensation according to claim 5, wherein The reference window reflection signal F extracted by using a sliding Hanning window window (f) and the measurement window reflection signal F' window (f) are respectively expressed as: F window (f) = F(f) × [1 - cos(2πn / N)] / 2 = T sw σr m E R E Pm sinc[π(f - f bm )T sw × (1 - cos(2πn / N)), 0 ≤ n ≤ N F window '(f) = F'(f) × (1 - cos(2πn / N) = T sw σr m E R E Pm 'sinc[π(f - f bm ')T sw × (1 - cos(2πn / N)), 0 ≤ n ≤ N In the formula, N represents the total length of the window function data points, and n represents the data point serial number in the window function.

7. A fiber optic strain demodulation method based on scattering enhanced point position offset compensation according to claim 6, characterized in that, Calculating the cross-correlation coefficient between the reference RBS spectrum and the measurement RBS spectrum according to the following formula: Wherein, Δλ represents the wavelength drift caused by strain; λ represents the wavelength; represents the reference RBS spectrum of the m-th scattering point; represents the measured RBS spectrum of the m-th scattering point.

8. A fiber optic strain demodulation method based on scattering enhancement point position offset compensation according to claim 7, characterized in that, The calculation formula for calculating the optical fiber strain value in the region between every two scattering points by using the wavelength drift amount in the measurement RBS spectrum corresponding to the maximum value of the cross-correlation coefficient is: Δλ = C ε .Δε where C ε represents the strain sensitivity of the single-mode fiber; Δε represents the fiber strain value.

9. An optical fiber strain demodulation system based on scattering enhanced point position offset compensation, characterized in that, Including: A signal acquisition module configured to collect an optical fiber reference signal without applied strain and an optical fiber measurement signal after applied strain; A plurality of scattering points with fixed intervals are pre-written in the optical fiber core; The signal demodulation module includes a cross-correlation calculation sub-module and a strain calculation sub-module; wherein, the cross-correlation calculation sub-module is configured to: perform fast Fourier transform on the optical fiber reference signal and the optical fiber measurement signal respectively to obtain a reference distance domain signal and a measurement distance domain signal; for the reference distance domain signal and the measurement distance domain signal, respectively use a sliding Hanning window to extract the reflection signals between every two scattering points, so as to obtain a set of reference window reflection signals corresponding to the reference distance domain signal and a set of measurement window reflection signals corresponding to the measurement distance domain signal; perform inverse Fourier transform on the reference window reflection signal and the measurement window reflection signal at the same scattering point position respectively to obtain a set of reference RBS spectra and a set of measurement RBS spectra; calculate the cross-correlation coefficient between the reference RBS spectrum and the measurement RBS spectrum; the strain calculation sub-module is configured to: use the wavelength drift amount in the measurement RBS spectrum corresponding to the maximum value of the cross-correlation coefficient to calculate the optical fiber strain value in the region between every two scattering points, so as to obtain multiple optical fiber strain values.

10. A fiber optic strain demodulation system based on scattering enhanced point position offset compensation according to claim 9, characterized in that, In the signal acquisition module, an optical frequency domain reflection system is used to acquire an optical fiber reference signal without applied strain and an optical fiber measurement signal after applied strain; a femtosecond laser writing technique is used to write multiple scattering points with a fixed interval in the optical fiber core.

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