Distributed optical fiber strain / temperature detection method, device, equipment and medium

Abstract

The invention discloses a distributed optical fiber strain / temperature detection method, device, equipment and medium, and belongs to the technical field of distributed optical fiber sensing detection, and the method comprises the steps: obtaining a reference signal and a measurement signal; re-sampling the obtained signal, and mapping the signal to a distance domain; demodulating the strain / temperature at the FBG position by adopting a spectrum method; adopting a phase method to obtain a winding phase; calculating the average value of the strain / temperature of the adjacent FBGs as the strain / temperature rough estimation value of the middle section optical fiber of the adjacent FBG position, and obtaining the rough estimation phase of the middle section optical fiber by combining the strain / temperature sensitivity coefficient and the length of the middle position section of the adjacent FBGs; and performing phase unwrapping by using the coarse estimation phase to obtain a phase value of the middle section of the optical fiber after unwrapping, and performing demodulation by combining a strain / temperature sensitivity coefficient and the length of the middle position section of the adjacent FBG. According to the invention, effective fusion of a phase method and a spectrum demodulation method based on the OFDR sensing system is realized, a sensing blind area between adjacent FBGs is eliminated, and the effective spatial sampling rate of the system is improved.

Description

Technical Field

[0001] This invention relates to the field of distributed optical fiber sensing and detection technology, specifically to a distributed optical fiber strain / temperature detection method, device, equipment, and medium. Background Technology

[0002] OFDR (Optical Frequency Domain Reflectometry) is an advanced fiber optic sensing technology. OFDR systems locate scattered signals by measuring the frequency of the Rayleigh scattering signal generated by modulated probe light, offering extremely high spatial resolution and sensing accuracy. Its operating principle is based on coherent detection of continuously modulated light waves, measuring strain / temperature changes along the fiber's length by analyzing the frequency changes of the light waves reflected back from the fiber.

[0003] The main methods for measuring strain / temperature in OFDR systems are the spectroscopic method and the phase method. The spectroscopic method is used to demodulate the strain at the FBG location. It utilizes the fact that changes in external strain / temperature cause changes in the grating pitch and effective refractive index of the fiber grating core, resulting in a corresponding shift in the center wavelength of the FBG. Therefore, by obtaining the spectral shift of the FBG reflection spectrum at each location and multiplying it by the corresponding strain conversion coefficient, the strain value at each FBG location can be obtained. The phase method is used to detect the strain in the fiber at the middle section of adjacent FBGs. First, the phase difference between the corresponding gratings in the distance domain of the reference signal and the measurement signal is extracted to obtain the time difference phase. Then, the phase difference between the front and rear points of the weak FBG at adjacent locations on the sensing fiber is calculated, for example, according to... Figure 1 The positions of the two points shown allow us to obtain the differential phase of the fiber segment between FBG2 and FBG3. By sequentially performing phase differential analysis on all intermediate positions of adjacent FBGs on the fiber under test using the method described above, we can obtain the phase difference of the fiber at all positions in the intermediate sections of adjacent FBGs, which represents the phase change caused by strain at the intermediate positions of adjacent FBGs. Multiplying the obtained phase change by the corresponding conversion factor yields the strain in the dead zone of adjacent FBGs. This method allows for the monitoring of strain at the dead zone location.

[0004] Phase-based demodulation is fast and accurate, but the inherent periodicity of the phase limits the demodulated phase range to within 2π, thus restricting the strain / temperature measurement range of the system. Spectroscopic demodulation is not limited by the range of light frequency ν, and its demodulation range is much larger than that of phase-based methods, allowing for large-range static parameter measurements. However, spectroscopic methods require cross-correlation calculations, resulting in a larger computational load and longer demodulation time compared to phase-based methods.

[0005] In the prior art, patent CN111678456A discloses an OFDR device and its measurement method for simultaneous temperature and strain measurement. This patent connects a dual-parameter sensing fiber and a temperature sensing fiber deployed on the test piece to their respective optical paths, and obtains the corresponding frequency shift by cross-correlation calculation of the reference signal and the measurement signal spectrum. The temperature frequency shift is used to compensate for the dual-parameter frequency shift, and combined with the frequency shift coefficient, the temperature change and strain are obtained respectively. This method is based on the cross-correlation calculation of the spectrum to obtain strain, which, as mentioned above, suffers from a long system demodulation time. Patent CN112082498A discloses a noise-suppressed sensing method for OFDR strain and temperature based on the phase measurement method. This method is an OFDR strain sensing method based on the phase measurement data statistics of the phase method, but the system has a limited strain measurement range. Summary of the Invention

[0006] This invention aims to address the shortcomings of existing OFDR systems that rely solely on spectral and phase methods for strain / temperature measurement. It proposes a distributed fiber optic strain / temperature detection method, device, equipment, and medium. This invention effectively integrates phase and spectral demodulation methods based on OFDR sensing systems. Without altering the traditional system structure or increasing system costs, it eliminates the sensing blind zone between adjacent fiber optic groups by fully utilizing the effective information obtained from demodulation, thereby improving the system's effective spatial sampling rate.

[0007] To achieve the above-mentioned objectives, the technical solution of the present invention is as follows: A distributed optical fiber strain / temperature detection method includes the following steps: S1. Obtain the time-domain signals before and after the environmental parameters to be measured are applied to the OFDR-based distributed optical fiber sensor, and denote them as the reference signal and the measurement signal, respectively. S2. Resample the reference signal and the measurement signal to obtain the resampled reference signal and measurement signal; S3. Map the reference signal and the measurement signal onto the range domain using a fast Fourier transform to obtain the range domain distribution of the FBG; S4. The strain / temperature at the FBG location is demodulated using the spectrum of the FBG using the spectral method; at the same time, the strain / temperature of the fiber at the middle section of the adjacent FBG is detected using the phase method to obtain the phase difference of the fiber at the middle section of all adjacent FBGs. S5. For the strain / temperature signals at each FBG location obtained by the spectral method, calculate the average strain / temperature of adjacent FBGs as a coarse estimate of the strain / temperature of the fiber in the middle section of adjacent FBGs. Combine the strain / temperature sensitivity coefficient and the length of the middle section of adjacent FBGs to obtain a coarse estimate of the phase of the fiber in the middle section. S6. Using the coarsely estimated phase, the phase difference of the fiber at the middle section is unwound to obtain the unwound phase value at the middle position of the FBG. Then, combined with the corresponding strain / temperature sensitivity coefficient and the length of the middle section of the adjacent FBG, the strain / temperature at the middle position of the adjacent FBG can be demodulated.

[0008] Furthermore, resampling the reference signal and the measurement signal includes: splitting the continuous wave laser output from the tunable laser into a clock signal or wavelength correction signal in the output system of the auxiliary interferometer, and using the wavelength correction signal from each measurement to perform equal-frequency resampling on the time domain signal output by the main interferometer to obtain the corrected time domain signal.

[0009] Furthermore, the method of using spectral analysis to demodulate the strain / temperature at the FBG location using the spectrum of the FBG includes: Divide the signal into N equal parts by setting the window size C to the length of FBG; Zero-padding and fast inverse Fourier transform are performed on the range domain information of the corresponding grating positions of the reference signal and the measurement signal to obtain the grating reference spectrum and measurement spectrum at each position respectively; One-dimensional cross-correlation results are obtained by cross-correlation calculation of the reference and measured spectra after fast inverse Fourier transform at each location; When the external strain / temperature changes, the center wavelength of the FBG shifts accordingly. The maximum value of the cross-correlation curve is found, and multiplied by the corresponding strain / temperature conversion coefficient to obtain the strain / temperature value at each FBG location.

[0010] Furthermore, the method of obtaining the phase difference of the optical fiber at the intermediate segment of adjacent FBGs using the phase method is described as the entanglement phase, including: For the range domain signal obtained from S3, the time difference phase is obtained by subtracting the phases of the corresponding gratings in the range domain of the reference signal and the measurement signal. ; Next, the phases of the FBG points before and after adjacent positions on the sensing fiber are differentially divided, and the differential phase at a certain position is obtained according to the differential method described above. ; The phase difference is performed sequentially at the middle section of all FBGs of the fiber under test using the above method to obtain the phase difference of the fiber at the middle section of all adjacent FBGs, which is the phase change caused by strain / temperature at the middle position of adjacent FBGs.

[0011] Further, the method of obtaining a coarse estimate of the phase of the intermediate fiber segment of adjacent FBGs using the strain / temperature signals at each FBG location obtained by the spectral method in S4 includes: calculating the average strain / temperature of adjacent FBGs as a coarse estimate of the strain / temperature of the intermediate fiber segment at each FBG location using the strain / temperature signals obtained by the spectral method in S4; combining the strain / temperature sensitivity coefficient and the length of the intermediate segment of adjacent FBGs to obtain the coarse estimate of the phase of the intermediate fiber segment, denoted as... φ rough。

[0012] Further, the step of using the coarsely estimated phase to unwrap the phase difference obtained by the phase method in S4 includes: First, according to the formula judge The phase difference between the actual phase and the phase value is an integer multiple of 2π; where, φ rough This provides a coarse estimate of the phase for the middle section of the optical fiber. The phase difference of the optical fiber at the midpoint of adjacent FBGs; Reuse formula Calculate and obtain the phase value after unwinding. By combining the corresponding strain / temperature sensitivity coefficient and the length of the middle section of adjacent FBGs, the strain / temperature of the optical fiber at the middle section of adjacent FBGs can be demodulated.

[0013] This invention also proposes a distributed optical fiber strain / temperature detection device, comprising: The signal acquisition module is configured to acquire the time-domain signals before and after the environmental parameters under test are applied to the OFDR-based distributed optical fiber sensor, which are denoted as the reference signal and the measurement signal, respectively. The first processing module is configured to resample the reference signal and the measurement signal to obtain the resampled reference signal and the measurement signal. The second processing module is configured to map the reference signal and the measurement signal onto the range domain through a fast Fourier transform to obtain the range domain distribution of the FBG. The third processing module is configured to use a spectral method to demodulate the strain / temperature at the FBG location using the spectrum of the FBG; at the same time, it uses a phase method to detect the strain / temperature of the optical fiber at the middle section of adjacent FBGs and obtain the phase difference of the optical fiber at the middle section of all adjacent FBGs. The fourth processing module is configured to calculate the average strain / temperature of adjacent FBGs as a coarse estimate of the strain / temperature of the fiber in the middle section of adjacent FBGs, based on the strain / temperature sensitivity coefficient and the length of the middle section of adjacent FBGs. The module then combines this with the length of the middle section of adjacent FBGs to obtain a coarse estimate of the phase of the fiber in the middle section. The fifth processing module is configured to use the coarsely estimated phase to unwrap the phase difference obtained by the phase method, and obtain the unwrapped phase value. By combining the corresponding strain / temperature sensitivity coefficient and the length of the middle section of adjacent FBGs, it is possible to demodulate the strain / temperature of the optical fiber at the middle section of adjacent FBGs. The output module is configured to use the strain / temperature at the FBG location obtained by demodulation by spectroscopy, combined with the strain / temperature at the middle position of the FBG obtained by unwinding by the fifth processing module, to monitor the strain / temperature of the fiber at the middle section of adjacent FBGs in the sensing fiber.

[0014] Furthermore, when the fifth processing module uses the coarsely estimated phase to unwrap the phase difference obtained by the phase method, it first uses the formula... judge The phase difference between the actual phase and the phase value is an integer multiple of 2π; where, φ rough This provides a coarse estimate of the phase for the middle section of the optical fiber. The phase difference of the fiber at the midpoint of adjacent FBGs; then using the formula Calculate the phase value after unwinding. By combining the corresponding strain / temperature sensitivity coefficient and the length of the middle section of adjacent FBGs, the strain / temperature of the optical fiber at the middle section of adjacent FBGs can be demodulated.

[0015] The present invention also proposes a distributed optical fiber strain / temperature detection device, including a memory, a processor, and a strain / temperature detection program stored in the memory and executable on the processor, wherein the strain / temperature detection program, when executed by the processor, implements the steps in the method described above.

[0016] The present invention also proposes a medium, which is a computer-readable storage medium storing a strain / temperature detection program, which, when executed by a processor, implements the steps of the method described above.

[0017] In summary, the present invention has the following advantages: 1. This invention is a combined algorithm based on the vernier effect of two demodulation schemes: spectral method and phase method. Without changing the traditional system structure or increasing the system cost, it eliminates the sensing blind zone in the middle of adjacent FBG by making full use of the effective information obtained by demodulation, increases the effective spatial sampling rate of the system, and improves the strain / temperature sensing accuracy.

[0018] 2. This invention doubles the system spatial sampling rate by optimizing the demodulation algorithm. In OFDR-based strain sensing systems, the increase in spatial sampling rate has a significant advantage for applications in the field of shape sensing, enabling the system to have greater advantages and broader applications in high-precision shape monitoring fields such as aerospace and medical equipment.

[0019] 3. Since the demodulation scheme of the phase method has a fast demodulation speed, and the combination method proposed in this invention only performs basic addition and subtraction operations, this algorithm basically does not increase the extra computational load of the system. Attached Figure Description

[0020] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments, wherein: Figure 1 Two demodulation methods for sensing optical fibers etched with FBG arrays: spectral method and phase method; Figure 2 This invention provides a demodulation step that combines spectral and phase methods. Figure 3 This is a schematic diagram of an OFDR sensing system; In the picture: 1. Tuned laser; 2. Coupler I; 3. Auxiliary interferometer; 4. Main interferometer; 5. 50:50 coupler I; 6. Delay fiber; 7. 50:50 coupler II; 8. Balanced photodetector I; 9. Coupler II; 10. Circulator; 11. Polarization beam splitter; 12. 50:50 coupler III; 13. 50:50 coupler IV; 14. Balanced photodetector II; 15. Balanced photodetector III; 16. Data acquisition equipment; 17. Fiber under test; 18. Sensing fiber; 19. Displacement stage. Detailed Implementation

[0021] To more clearly illustrate the present invention, the following description, in conjunction with preferred embodiments and accompanying drawings, further clarifies the invention. Those skilled in the art should understand that the specific description below is illustrative rather than restrictive and should not be construed as limiting the scope of protection of the present invention.

[0022] Example 1: Since the differential phase obtained by the demodulation scheme using the phase method is generally a wrapped phase, denoted as... That is, the phase range is between -π and π, thus greatly limiting the measurement range of the phase method. To achieve a measurement range similar to that of the spectroscopic method, this invention proposes a combined scheme of the spectroscopic and phase methods based on the vernier effect, further processing the demodulation results of the phase method. Figure 2 The demodulation steps of the combined scheme are demonstrated.

[0023] The distributed optical fiber strain / temperature detection method of the present invention specifically includes the following steps: S1. Acquire the time-domain signal before the environmental parameter to be measured is applied to the OFDR-based distributed fiber optic sensor. This set of signals does not contain strain / temperature information and serves as a reference signal. Acquire the signal after the environmental parameter to be measured is applied to the distributed fiber optic sensor. This signal contains strain / temperature information and serves as a measurement signal. S2. Resample the reference signal and the measurement signal to obtain the resampled reference and measurement signals.

[0024] Due to the nonlinearity of the sweep frequency of the tunable laser, the acquired time-domain signal needs to be corrected for nonlinearity. The specific operation is described as follows: the continuous wave laser output from the tunable laser is split into a beam and fed into the clock signal or wavelength correction signal in the output system of the auxiliary interferometer. The wavelength correction signal measured each time is used to perform equal-frequency resampling on the time-domain signal output by the main interferometer to obtain the corrected time-domain signal.

[0025] S3. By mapping the reference signal and the measurement signal onto the range domain through a fast Fourier transform, the range domain distribution of the FBG (Fiber Bragg Grating, which is a grating with a periodic spatial phase distribution formed in the fiber core) can be obtained. S4. First, use a spectroscopic method, specifically the spectrum of the FBG, to demodulate the strain / temperature at the FBG location. The specific steps are as follows: Divide the signal into N equal parts by setting the window size C to the length of FBG; Zero-padding and fast inverse Fourier transform are performed on the range domain information of the corresponding grating positions of the reference signal and the measurement signal to obtain the grating reference spectrum and measurement spectrum at each position respectively; One-dimensional cross-correlation results are obtained by cross-correlation calculation of the reference and measured spectra after fast inverse Fourier transform at each location; Since changes in external strain / temperature can cause changes in the grating pitch and effective refractive index of the fiber core of the fiber grating, resulting in a corresponding shift in the center wavelength of the fiber grating, the strain / temperature value at each FBG location can be obtained by finding the maximum value of the cross-correlation curve and multiplying it by the corresponding strain / temperature conversion coefficient.

[0026] Simultaneously, the strain / temperature of the optical fibers at the intermediate sections of adjacent FBGs is detected using the classical phase method, and the phase difference of the optical fibers at the intermediate sections of all adjacent FBGs is obtained. The steps for obtaining the phase difference of the optical fibers at the intermediate sections of adjacent FBGs using the phase method are as follows: For the range domain signal obtained in step S3, extract the phase difference of the corresponding gratings in the range domain of the reference signal and the measurement signal, and subtract the phases to obtain the time difference phase. Next, the phases of the FBG points before and after adjacent positions on the sensing fiber are differentially divided; following the differential method described above, the differential phase at a certain position is obtained. By performing phase difference analysis on all FBG mid-section positions of the fiber under test in sequence according to the above method, the phase difference of the fiber at the mid-section positions of all adjacent FBGs can be obtained, which is the phase change caused by strain / temperature at the mid-section positions of adjacent FBGs.

[0027] Since the differential phase value obtained in the above phase method is generally a wrapped phase, denoted as... That is, the phase range is between -π and π, thus greatly limiting the measurement range of the phase method. In order to achieve the same measurement range as the spectroscopic method described in step S4 of Example 1, this embodiment further proposes a combination scheme of the spectroscopic method and the phase method based on the vernier effect, and further processes the demodulation results obtained by the classical phase method.

[0028] S5. For the strain / temperature signals at each FBG location obtained in step S4, calculate the average strain of adjacent FBGs as a coarse estimate of the strain / temperature of the fiber optic segment at the middle of adjacent FBG locations. Combined with the strain / temperature sensitivity coefficient and the length of the middle section of adjacent FBGs, a coarse estimate of the phase of the fiber optic segment at the middle can be obtained, denoted as [missing information]. φ rough .

[0029] S6. Utilize the coarsely estimated phase obtained in step S5 φ rough The precise winding phase obtained in step S4 Perform phase unwrapping; the specific steps are as follows: First, according to the formula judge The phase difference between the actual phase and the phase value is an integer multiple of 2π, and then the formula is used. The phase value after unwinding can then be calculated. By combining the corresponding strain / temperature sensitivity coefficient and the length of the middle section of adjacent FBGs, it is possible to demodulate the strain / temperature at the middle position of adjacent FBGs.

[0030] Example 2: Based on the same inventive concept, this invention proposes a distributed optical fiber strain / temperature detection device, comprising: The signal acquisition module is configured to acquire the time-domain signals before and after the environmental parameters under test are applied to the OFDR-based distributed optical fiber sensor, which are denoted as the reference signal and the measurement signal, respectively (the signal acquisition method can be referred to in Example 5). The first processing module is configured to resample the reference signal and the measurement signal to obtain the resampled reference signal and the measurement signal. The second processing module is configured to map the reference signal and the measurement signal onto the range domain through a fast Fourier transform to obtain the range domain distribution of the FBG. The third processing module is configured to use a spectral method to demodulate the strain / temperature at the FBG location using the spectrum of the FBG; at the same time, it uses a phase method to detect the strain / temperature of the optical fiber at the middle section of adjacent FBGs and obtain the phase difference of the optical fiber at the middle section of all adjacent FBGs. The fourth processing module is configured to calculate the average strain / temperature of adjacent FBGs as a coarse estimate of the strain / temperature of the fiber in the middle section of adjacent FBGs, based on the strain / temperature sensitivity coefficient and the length of the middle section of adjacent FBGs. The module then combines this with the length of the middle section of adjacent FBGs to obtain a coarse estimate of the phase of the fiber in the middle section. The fifth processing module is configured to use the coarsely estimated phase to unwrap the phase difference obtained by the phase method, and obtain the unwrapped phase value. By combining the corresponding strain / temperature sensitivity coefficient and the length of the middle section of adjacent FBGs, it is possible to demodulate the strain / temperature of the optical fiber at the middle section of adjacent FBGs. The output module is configured to use the strain / temperature at the FBG location obtained by demodulation by spectroscopy, combined with the strain / temperature at the middle position of the FBG obtained by unwinding by the fifth processing module, to monitor the strain / temperature of the fiber at the middle section of adjacent FBGs in the sensing fiber.

[0031] The first processing module resamples the reference signal and the measurement signal as follows: a beam of continuous wave laser output from the tunable laser is split and fed into the clock signal or wavelength correction signal in the output system of the auxiliary interferometer. The wavelength correction signal measured each time is used to perform equal-frequency resampling on the time domain signal output by the main interferometer to obtain the corrected time domain signal.

[0032] The third processing module uses a spectroscopic method to demodulate the strain / temperature at the FBG location using the spectrum of the FBG. This includes: dividing the signal into N equal parts with a window size C equal to the FBG length; performing zero-padding and fast inverse Fourier transform on the distance domain information of the corresponding grating positions of the reference and measurement signals to obtain the grating reference spectrum and measurement spectrum at each position; calculating the one-dimensional cross-correlation result by cross-correlation of the reference and measurement spectra after fast inverse Fourier transform at each position; finding the maximum value of the cross-correlation curve and multiplying it by the corresponding strain / temperature conversion coefficient to obtain the strain / temperature value at each FBG position.

[0033] Preferably, the third processing module uses the phase method to detect the strain / temperature of the optical fiber at the midpoint of adjacent FBGs, including: extracting the time difference phase by subtracting the phases of the corresponding gratings in the range domain from the phases of the reference signal and the measurement signal. Next, the phases of adjacent weak FBG points on the sensing fiber are differentially analyzed. Following the differential method described above, the differential phase at a certain location is obtained. The phase difference of all FBG middle sections of the fiber under test is performed sequentially according to the above method to obtain the phase difference of all fiber positions in the middle section of adjacent FBGs, which is the phase change caused by strain / temperature at the middle position of adjacent FBGs.

[0034] When the fifth processing module uses the coarsely estimated phase to unwrap the phase difference obtained by the phase method, it first uses the formula... judge The phase difference between the actual phase and the phase value is an integer multiple of 2π; where, φ rough This provides a coarse estimate of the phase for the middle section of the optical fiber. The phase difference of the fiber at the midpoint of adjacent FBGs; then using the formula Calculate and obtain the phase value after unwinding. Then multiply it by the corresponding strain / temperature sensitivity coefficient to demodulate the strain / temperature.

[0035] The functions of the above-mentioned modules correspond to the steps in Example 1. Any parts not described in detail will not be repeated.

[0036] Example 3: This embodiment provides a distributed optical fiber strain / temperature detection device, including: Memory, used to store computer programs; A processor is used to execute the computer program to implement the steps in the distributed optical fiber strain / temperature detection method as described in Embodiment 1 above.

[0037] Preferably, the computer program can be divided into one or more modules / units, which are stored in the memory and executed by the processor to complete the present invention. The one or more modules / units can be a series of computer program instruction segments capable of performing a specific function, which describe the execution process of the computer program in the device.

[0038] The processor can be a central processing unit, or other general-purpose processors, digital signal processors, application-specific integrated circuits, off-the-shelf programmable gate arrays or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The general-purpose processor can be a microprocessor, or the processor can be any conventional processor. The processor is the control center of the device and connects the various parts of the device using various interfaces and lines.

[0039] The memory mainly includes a program storage area and a data storage area. The program storage area can store the operating system, applications required for at least one function, etc., while the data storage area can store related data, etc. In addition, the memory can be a high-speed random access memory, or a non-volatile memory, such as a plug-in hard disk, a smart memory card, a secure digital card, and a flash memory card, or the memory can be other volatile solid-state storage devices.

[0040] Example 4 This invention also provides a computer-readable storage medium storing a computer program, which, when executed by a processor, implements the steps of the distributed optical fiber strain / temperature detection method described in Embodiment 1 above.

[0041] Computer storage media can be tangible media that may contain or store programs for use by or in conjunction with an instruction execution system, apparatus or device.

[0042] The embodiments of the fiber optic strain / temperature detection device and computer-readable storage medium in this application include all the technical features of the above-described embodiments of the fiber optic strain / temperature detection method. The descriptions and explanations are basically the same as those of the above-described embodiments of the fiber optic strain / temperature detection method, and will not be repeated here.

[0043] Example 5 Based on the content of any of the above embodiments, this embodiment uses... Figure 1 The present invention is illustrated by taking the actual unwound strain of the optical fiber in the middle section of FBG2 and FBG3 when strain is applied at the position from FBG2 to FBG3 as an example. The demodulation principle of the dead zone strain at other positions is the same as this step.

[0044] For the resampled reference and measurement signals obtained in step S2, the strain at the positions of FBG2 and FBG3 is first obtained by demodulation using the spectral method described in S4. and Then calculate the average of the two values. Multiplying this value by the strain conversion factor and the length of the intermediate blind zone gives a rough estimate of the phase change caused by strain within the blind zone, denoted as . Simultaneously, demodulation using the phase method described by S4 can obtain the precise phase change caused by external strain within the blind zone. This phase is the wrapped phase, denoted as... .

[0045] For the two types of phase values ​​obtained, a coarse phase estimation is used. For precise winding phase To perform phase unwinding, the steps are as follows: First, according to the formula... judge The phase difference between the actual phase and the phase value is an integer multiple of 2π, and then the formula is used. The phase value after unwinding can then be calculated. Then, by multiplying it by the corresponding strain sensitivity coefficient, the strain / temperature can be demodulated.

[0046] This approach can monitor the strain of the fiber at the midpoint of adjacent FBGs using the demodulation range of the spectral method, increasing the spatial sampling rate of the system by one time, which greatly helps to improve the shape reconstruction accuracy in the field of shape sensing.

[0047] Figure 3 The OFDR sensing system used in the experiment is demonstrated. Tuned laser 1 outputs a highly coherent continuous-wave laser with wavelength scanning. Its output is connected to coupler I2, which splits the laser beam into a first beam and a second beam. The first beam is input into auxiliary interferometer 3, and the second beam is input into main interferometer 4. The structures of the two interferometers are shown below. Figure 3 As shown in the two dashed boxes, the auxiliary interferometer 3 mainly performs wavelength scanning based on the first beam, serving as the internal clock for the subsequent acquisition system, or as a wavelength correction signal if an internal clock is already available.

[0048] In the real-time solution provided by this invention, the auxiliary interferometer 3 structure includes a 50:50 coupler I5, a delay fiber 6, a 50:50 coupler II7, and a photodetector 8. The first beam entering the auxiliary interferometer 3 first passes through the 50:50 coupler I5 into the non-equidistant Mach-Zehnder interferometer structure. The arm length difference in this system uses a 5m long delay fiber. Subsequently, it passes through the 50:50 coupler II7 into the balanced photodetector I8 for photoelectric conversion and is then acquired by the system. The acquired signal is mainly used as a wavelength correction signal to correct the signal of the main interferometer.

[0049] For the main interferometer part 4, coupler II9 can split the beam into a high-energy second beam and a low-energy first beam at a ratio of 90:10. The low-energy first beam directly enters the polarization beam splitter 11. The high-energy second beam enters from the first port of circulator 10 and is sent to sensing fiber 18 from the second port. Sensing fiber 18 serves as an external detection end, and its surface is engraved with a weak FBG array. The scattered light reflected back from sensing fiber 18 continues to enter the second port of circulator 10 and is output from the third port of circulator 10 into polarization beam splitter 11. Polarization beam splitter 11 splits the incoming beam into P-polarized light and S-polarized light. The P-polarized light of the scattered light and the local light is beat-frequencyd by coupler III12 at a ratio of 50:50 and then photoelectrically converted and detected by balanced photodetector II14. The S-polarized light of the scattered light and the local light is beat-frequencyd by coupler IV13 at a ratio of 50:50 and then photoelectrically converted and detected by balanced photodetector III15. The received electrical signals from the three balanced photodetectors 8, 14, and 15 are collected and received by data acquisition device 16.

[0050] in addition, Figure 3 The invention also demonstrates a device for applying strain to the optical fiber under test in an example of this invention. In this example, the length of the ultra-weak FBG array grating to be tested is approximately 5 mm, and the grating spacing is approximately 5 mm. A displacement stage 19 is used as a tensioning device to precisely induce strain on the optical fiber. One end of the test section of the optical fiber 17 is fixed to a fixed stage, and the other end is fixed to a piezoelectric nanostage (approximately 100 μm of strain needs to be applied to the optical fiber under test). (Pre-straining is applied to eliminate the influence of gravity on the horizontal direction of the fiber under test). Strain is applied to a segment of the fiber under test by moving a piezoelectric nanoplatform over a certain length.

[0051] The received signal can then be demodulated. The strain in the dead zone of the FBG middle section is demodulated using spectral, phase, and joint demodulation algorithms. Finally, the coarse phase, winding phase, and strain curves after bonding are obtained. The demodulation results show that the final demodulated bonded strain matches the actual applied strain location and magnitude, thus proving the effectiveness of the proposed method.

[0052] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications or equivalent changes made to the above embodiments based on the technical essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A distributed optical fiber strain / temperature detection method, characterized in that, Includes the following steps: S1. Obtain the time-domain signals before and after the environmental parameters to be measured are applied to the OFDR-based distributed optical fiber sensor, and denote them as the reference signal and the measurement signal, respectively. S2. Resample the reference signal and the measurement signal to obtain the resampled reference signal and measurement signal; S3. Map the reference signal and the measurement signal onto the range domain using a fast Fourier transform to obtain the range domain distribution of the FBG; S4. The strain / temperature at the FBG location is demodulated using the spectrum of the FBG using the spectral method; at the same time, the strain / temperature of the fiber at the middle section of the adjacent FBG is detected using the phase method to obtain the phase difference of the fiber at the middle section of all adjacent FBGs. S5. For the strain / temperature signals at each FBG location obtained by the spectral method, calculate the average strain / temperature of adjacent FBGs as a coarse estimate of the strain / temperature of the fiber in the middle section of adjacent FBGs. Combine the strain / temperature sensitivity coefficient and the length of the middle section of adjacent FBGs to obtain a coarse estimate of the phase of the fiber in the middle section. S6. Using the coarsely estimated phase, the phase difference of the fiber at the middle section is unwound to obtain the unwound phase value at the middle position of the FBG. Then, combined with the corresponding strain / temperature sensitivity coefficient and the length of the middle section of the adjacent FBG, the strain / temperature at the middle position of the adjacent FBG can be demodulated.

2. The distributed optical fiber strain / temperature detection method as described in claim 1, characterized in that, Resampling the reference signal and the measurement signal includes: splitting the continuous wave laser output from the tunable laser into a clock signal or wavelength correction signal in the output system of the auxiliary interferometer, and using the wavelength correction signal from each measurement to perform equal-frequency resampling on the time domain signal output by the main interferometer to obtain the corrected time domain signal.

3. The distributed optical fiber strain / temperature detection method as described in claim 1, characterized in that, The aforementioned method of using the spectrum of the FBG to demodulate the strain / temperature at the FBG location includes: Divide the signal into N equal parts by setting the window size C to the length of FBG; Zero-padding and fast inverse Fourier transform are performed on the range domain information of the corresponding grating positions of the reference signal and the measurement signal to obtain the grating reference spectrum and measurement spectrum at each position respectively; One-dimensional cross-correlation results are obtained by cross-correlation calculation of the reference and measured spectra after fast inverse Fourier transform at each location; When the external strain / temperature changes, the center wavelength of the FBG shifts accordingly. The maximum value of the cross-correlation curve is found, and multiplied by the corresponding strain / temperature conversion coefficient to obtain the strain / temperature value at each FBG location.

4. The distributed optical fiber strain / temperature detection method as described in claim 1, characterized in that, The method of obtaining the phase difference of the optical fiber at the intermediate segment of adjacent FBGs using the phase method is called the entanglement phase, which includes: For the range domain signal obtained from S3, the time difference phase is obtained by subtracting the phases of the corresponding gratings in the range domain of the reference signal and the measurement signal. ; Next, the phases of the FBG points before and after adjacent positions on the sensing fiber are differentially divided, and the differential phase at a certain position is obtained according to the differential method described above. ; The phase difference is performed sequentially at the middle section of all FBGs of the fiber under test using the above method to obtain the phase difference of the fiber at the middle section of all adjacent FBGs, which is the phase change caused by strain / temperature at the middle position of adjacent FBGs.

5. The distributed optical fiber strain / temperature detection method as described in claim 1, characterized in that, The method of obtaining a coarse estimate of the phase of the fiber optic segment between adjacent FBGs using the strain / temperature signals at each FBG location obtained by the spectral method in S4 includes: calculating the average strain / temperature of adjacent FBGs as a coarse estimate of the strain / temperature of the fiber optic segment between adjacent FBGs based on the strain / temperature signals at each FBG location obtained by the spectral method in S4; combining the strain / temperature sensitivity coefficient and the length of the segment between adjacent FBGs to obtain the coarse estimate of the phase of the fiber optic segment between adjacent FBGs, denoted as... φ rough。 6. The distributed optical fiber strain / temperature detection method as described in claim 1, characterized in that, The step of using the coarsely estimated phase to unwrap the phase difference obtained by the phase method in S4 includes: First, according to the formula judge The phase difference between the actual phase and the phase value is an integer multiple of 2π; where, φ rough This provides a coarse estimate of the phase for the middle section of the optical fiber. The phase difference of the optical fiber at the midpoint of adjacent FBGs; Reuse formula Calculate and obtain the phase value after unwinding. By combining the corresponding strain / temperature sensitivity coefficient and the length of the middle section of adjacent FBGs, the strain / temperature of the optical fiber at the middle section of adjacent FBGs can be demodulated.

7. A distributed optical fiber strain / temperature detection device, characterized in that, include: The signal acquisition module is configured to acquire the time-domain signals before and after the environmental parameters under test are applied to the OFDR-based distributed optical fiber sensor, which are denoted as the reference signal and the measurement signal, respectively. The first processing module is configured to resample the reference signal and the measurement signal to obtain the resampled reference signal and the measurement signal. The second processing module is configured to map the reference signal and the measurement signal onto the range domain through a fast Fourier transform to obtain the range domain distribution of the FBG. The third processing module is configured to use a spectral method to demodulate the strain / temperature at the FBG location using the spectrum of the FBG; at the same time, it uses a phase method to detect the strain / temperature of the optical fiber at the middle section of adjacent FBGs and obtain the phase difference of the optical fiber at the middle section of all adjacent FBGs. The fourth processing module is configured to calculate the average strain / temperature of adjacent FBGs as a coarse estimate of the strain / temperature of the fiber in the middle section of adjacent FBGs, based on the strain / temperature sensitivity coefficient and the length of the middle section of adjacent FBGs. The module then combines this with the length of the middle section of adjacent FBGs to obtain a coarse estimate of the phase of the fiber in the middle section. The fifth processing module is configured to use the coarsely estimated phase to unwrap the phase difference obtained by the phase method, and obtain the unwrapped phase value. By combining the corresponding strain / temperature sensitivity coefficient and the length of the middle section of adjacent FBGs, it is possible to demodulate the strain / temperature of the optical fiber at the middle section of adjacent FBGs. The output module is configured to use the strain / temperature at the FBG location obtained by demodulation by spectroscopy, combined with the strain / temperature at the middle position of the FBG obtained by unwinding by the fifth processing module, to monitor the strain / temperature of the fiber at the middle section of adjacent FBGs in the sensing fiber.

8. The distributed optical fiber strain / temperature detection device as described in claim 7, characterized in that, When the fifth processing module uses the coarsely estimated phase to unwrap the phase difference obtained by the phase method, it first uses the formula... judge The phase difference between the actual phase and the phase value is an integer multiple of 2π; where, φ rough This provides a coarse estimate of the phase for the middle section of the optical fiber. The phase difference of the optical fiber at the midpoint of adjacent FBGs; Reuse formula Calculate the phase value after unwinding. By combining the corresponding strain / temperature sensitivity coefficient and the length of the middle section of adjacent FBGs, the strain / temperature of the optical fiber at the middle section of adjacent FBGs can be demodulated.

9. A distributed optical fiber strain / temperature detection device, characterized in that, It includes a memory, a processor, and a strain / temperature detection program stored in the memory and executable on the processor, wherein the strain / temperature detection program, when executed by the processor, implements the steps of the method as described in any one of claims 1-6.

10. A medium, said medium being a computer-readable storage medium, characterized in that, The computer-readable storage medium stores a strain / temperature detection program, which, when executed by a processor, implements the steps of the method as described in any one of claims 1-6.

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