Method for extending the distributed sensing distance of an optical frequency domain reflectometer

Abstract

This invention provides a method for extending the measurement distance of distributed sensing based on an optical frequency domain reflectometer. The method addresses the problem in distributed sensing technology based on an optical frequency domain reflectometer where the measurement distance is affected by tests of the reference state before signal generation and the measurement state after signal generation. The slight difference in the scanning range of the light source wavelength during these two tests causes a slight difference in spatial resolution. This difference gradually accumulates over long distances, leading to a mismatch between the positions of the sensing fiber in the reference and measurement states, or a change in the size of the sensing fiber after signal generation, resulting in a mismatch in the sensing fiber's position and causing measurement failure or decreased sensing resolution. This invention utilizes the invariance of local Rayleigh backscattering in the sensing fiber to calculate the mismatch between the corresponding positions of the sensing fiber in the reference and measurement states, and then performs correction, thereby overcoming the problems of measurement failure or decreased sensing resolution caused by mismatched sensing fiber positions.

Description

Technical Field

[0001] This invention belongs to the field of distributed optical fiber sensing technology, and particularly relates to a method for extending the distributed sensing measurement distance of an optical frequency domain reflectometer. Background Technology

[0002] Optical frequency domain reflectometry (OFDR) is a preferred technology in the field of distributed fiber optic sensing. Compared with traditional optical time domain reflectometry (OTDR), OFDR has advantages such as high spatial resolution and large dynamic range, making it one of the research hotspots in this field in recent years.

[0003] The distributed sensing technology based on optical frequency domain reflectometers operates as follows: First, the sensing fiber is tested once, called the reference state test. After a signal is generated (such as temperature or strain), the sensing fiber is tested again, called the measurement state test. The measurement state test results are then compared position by position with the reference state test results, thereby achieving distributed sensing of the physical parameters to be measured within the coverage area of ​​the entire sensing fiber. The spatial resolution of the distributed sensing technology based on optical frequency domain reflectometers is determined by the wavelength scanning range of the probe light; the larger the wavelength scanning range, the higher the spatial resolution performance. For current optical frequency domain reflectometer systems, the wavelength scanning range of the probe light is generally tens of nanometers. However, the wavelength scanning ranges cannot be strictly equal in reference and measurement state tests. This leads to a slight difference in the system's spatial resolution between the two tests. Especially in long-distance, high spatial resolution applications, this slight difference gradually accumulates with the length of the sensing fiber, causing the reference state position and the measurement state position to not correspond accurately at the far end of the sensing fiber, thus causing test failure or a decrease in sensing resolution. In addition, when a signal is generated, especially for signals with relatively large amplitude, it will cause a significant change in the size of the sensing fiber (mainly along the fiber length direction). Therefore, after the location where the signal occurs, the reference state position and the measurement state position cannot correspond accurately, which will also lead to test failure or a decrease in sensing resolution.

[0004] Existing research has employed several methods. One approach involves a stepwise compensation method, which recursively calculates the mismatch at the far end based on the signal magnitude demodulated at the near end position, thus correcting the mismatch. However, this method cannot address the positional mismatch caused by differences in the scanning range of the light source wavelength. Another approach uses marking gratings to calibrate the position of the sensing fiber. However, this method requires marking a large number of gratings on the sensing fiber, altering its structure and incurring significant costs. Yet another approach involves adding an additional interferometer to finely calibrate the scanning wavelength of the light source based on the interferometer's interference spectrum. However, this approach cannot resolve the positional mismatch caused by signal-induced changes in the sensing fiber's dimensions, and its calibration results are not ideal for long-distance measurements. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for extending the measurement distance of distributed sensing in optical frequency domain reflectometers. This method solves the problem of sensor position mismatch caused by changes in the scanning range of the light source wavelength or changes in the size of the sensing fiber after signal generation in long-distance, high spatial resolution applications of distributed sensing in optical frequency domain reflectometers, thereby extending the measurement distance of this technology.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] In a first aspect, the present invention provides a method for extending the distributed sensing measurement distance of an optical frequency domain reflectometer, comprising the following steps:

[0008] S1. Acquire Rayleigh backscattered light from the sensing fiber using an optical frequency domain reflectometer to obtain two sets of spectral domain data before and after the signal is generated;

[0009] S2. Perform Fourier transform on the two sets of spectral domain data respectively to convert the spectral domain to the distance domain, and obtain two curves of Rayleigh backscattering intensity as a function of distance before and after the signal is generated.

[0010] S3. Select a sliding window length N, and on the two Rayleigh backscattering intensity distribution curves with distance, cut local windows of length N from the starting point at each position to obtain the reference local intensity sequence and the measured local intensity sequence.

[0011] S4. Perform cross-correlation calculation on the reference local intensity sequence and the measured local intensity sequence taken at the same location to obtain the cross-correlation curve, and confirm the corresponding location M based on the peak value of the cross-correlation curve;

[0012] S5. Based on the peak value, confirm the corresponding position M and the sliding window length N, calculate the current local mismatch P, and perform mismatch correction on the measured local intensity sequence according to the local mismatch P, so that it is aligned with the reference local intensity sequence in the distance domain.

[0013] S6. Repeat steps S2 to S5, and slide the window along the distance direction of the sensing fiber until the distance domain mismatch at all positions of the entire sensing fiber is corrected.

[0014] S7. Demodulate the signal of the calibrated sensing fiber to obtain the measurement results.

[0015] Furthermore, step S2 also includes refining the curve of Rayleigh backscattering intensity as a function of distance.

[0016] Furthermore, the refinement can be achieved by padding the spectral domain data with zeros before the Fourier transform, or by interpolating the curve after the Fourier transform.

[0017] Furthermore, in step S3, the sliding window length N is dynamically determined based on the local mismatch at different positions of the sensing fiber. A smaller window length is used at the near end of the sensing fiber where the mismatch is small, and a larger window length is used at the far end of the sensing fiber where the mismatch is large.

[0018] Further, in step S5, the formula for calculating the local mismatch amount P is:

[0019] P = M - N + 1;

[0020] In the formula, M is the location corresponding to the peak of the cross-correlation curve, and N is the length of the sliding window.

[0021] Furthermore, in step S7, the signal demodulation includes phase demodulation or spectral demodulation, etc.

[0022] In a second aspect, the present invention provides an optical frequency domain reflectometer device, comprising:

[0023] The light source module is used to provide the frequency-sweeping laser;

[0024] An interference module, connected to the light source module, is used to incident the swept laser onto the sensing fiber and generate an interference signal.

[0025] A detection module, connected to the interference module, is used to acquire the interference signal and convert it into an electrical signal;

[0026] The signal processing module is connected to the detection module and is used to execute the above-described method for distributed sensing measurement distance of the extended optical frequency domain reflectometer.

[0027] Thirdly, the present invention provides a computer program product, including a computer program / instruction, which, when executed by a processor, implements the above-described method for distributed sensing measurement distance of an extended optical frequency domain reflectometer.

[0028] The beneficial effects of this invention are as follows: This invention can effectively solve the problems of measurement failure or decreased sensing resolution caused by sensing position mismatch due to changes in the scanning range of the probe wavelength or changes in the size of the sensing fiber after signal generation in distributed sensing applications of optical frequency domain reflectometers. Compared with the prior art, the method described in this invention is low in cost, simple to operate, and simultaneously solves the sensing position mismatch problem caused by multiple factors, resulting in superior performance. In the subsequent signal demodulation process, whether based on spectral demodulation or phase demodulation, better results can be achieved compared to before correction, including but not limited to restoring the effectiveness of sensing data and improving sensing resolution. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 This is a flowchart illustrating a distributed sensing distance measurement method for an extended optical frequency domain reflectometer, as provided in an embodiment of the present invention.

[0031] Figure 2 This is a schematic diagram comparing Rayleigh backscattering before and after sensor position mismatch as described in this invention; wherein, Figure 2 (a) shows the waveforms of the Rayleigh backscattering intensity distribution with distance in the reference state and the measurement state when the fiber is located near the sensing fiber and no obvious sensing position mismatch occurs; Figure 2 (b) in the figure represents the autocorrelation curve of the reference state waveform in (a) and the cross-correlation curve between the reference state and the measurement state; Figure 2 (c) in the figure represents the waveforms of the Rayleigh backscattering intensity distribution with distance in the reference state and the measurement state after a sensing position mismatch has occurred at the far end of the sensing fiber. Figure 2 In (d), the autocorrelation curve of the reference state signal and the cross-correlation curve between the reference state and the measurement state are shown in (c).

[0032] Figure 3 The figure shows the results of calculating the mismatch of Rayleigh backscattering along the entire sensing fiber based on the Rayleigh backscattering invariance described in this invention.

[0033] Figure 4 The figure shows experimental results of the method for extending the distributed sensing distance of an optical frequency domain reflectometer based on the present invention; taking the phase demodulation result as an example, it compares the phase demodulation results before and after correction when there is no significant mismatch in the sensing position at the near end of the sensing fiber. Figure 4 (a) and Figure 4(b) Comparison: At the far end of the sensing fiber, a sensing position mismatch occurred. A comparison of the phase demodulation results before and after correction is shown. Figure 4 (c) and Figure 4 (d) in the comparison.

[0034] Figure 5 To compare the test results of whether the extended optical frequency domain reflectometer distributed sensing distance measurement method of the present invention is applied after applying a strain signal of 150 microstrain at a position of about 20 cm at the far end of the optical fiber, based on the method of the extended optical frequency domain reflectometer distributed sensing distance measurement described in the present invention. Detailed Implementation

[0035] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0036] It should be noted that, unless otherwise specified, the features in the following embodiments and implementation methods can be combined with each other.

[0037] The distributed sensing of the optical frequency domain reflectometer described in this invention operates based on Rayleigh backscattering in the sensing fiber. Rayleigh backscattering is an intrinsic property of optical fibers, caused by random fluctuations in the refractive index of the medium. After the optical fiber is drawn, its refractive index distribution, i.e., the random fluctuations, is determined, and therefore the resulting local Rayleigh backscattering waveform is also determined. Thus, when this optical fiber is used as the sensing fiber of the optical frequency domain reflectometer, the changes in the corresponding position of the sensing fiber can be determined based on the local Rayleigh backscattering waveform.

[0038] Figure 1 This is a flowchart illustrating a method for extending the distributed sensing measurement distance of an optical frequency domain reflectometer, as provided by the present invention. Figure 1 As shown, it includes the following steps:

[0039] Step S1: The OFDR system detects and collects Rayleigh backscattered light from the sensing fiber to obtain spectral domain data before and after the signal is generated;

[0040] Step S2: Perform Fourier transform on the two sets of spectral domain data before and after the signal generation, respectively, to convert the spectral domain data to the range domain, and obtain two curves of Rayleigh backscattering intensity distribution with distance before and after the signal generation; in this process, since the position mismatch of local Rayleigh backscattering before and after the signal generation may be less than one sampling unit, the Rayleigh backscattering intensity curve can be interpolated and refined. For example, a common method is to pad the spectral domain data with zeros before the Fourier transform;

[0041] Step S3: Using a sliding window, the intensity distribution curve as a function of distance is segmented. A sliding window length N is selected, and the segmentation begins at the starting point of each of the two Rayleigh backscattering intensity distribution curves before and after the signal generation, segmenting N points at each position. The resulting local Rayleigh backscattering intensity signals are the reference local intensity sequence I_r(d) and the measured local intensity sequence I_m(d), where d represents the position of the sensing fiber. Step S4: Cross-correlation is performed on I_r(d) and I_m(d) to obtain the cross-correlation curve I_c(d). The cross-correlation formula is as follows:

[0042] I_c(d) = I_r(d) I_m(d)

[0043] In the formula, For cross-correlation operations.

[0044] Take the position M corresponding to the maximum value of the cross-correlation curve I_c(d);

[0045] Step S5: Calculate the local Rayleigh backscattering mismatch P before and after signal generation, i.e., the displacement between the two. This mismatch P characterizes the spatial resolution difference caused by the difference in the scanning range of the light source wavelength in the reference state and the measurement state, which gradually accumulates with the length of the sensing fiber, or the spatial mismatch caused by changes in the size of the sensing fiber after signal generation, resulting in local sensing positions in the reference state and the measurement state. The formula for calculating the mismatch P is as follows:

[0046] P = M - N + 1;

[0047] In the formula, M is the position corresponding to the cross-correlation peak, and N is the sliding window length.

[0048] The Rayleigh backscattered signal I_m(d) after the signal is generated is shifted by P refined sampling points, that is, the mismatch correction is performed between the measured local intensity sequence I_m(d) and I_r(d) in step S3;

[0049] Step S6: Repeat steps S3 to S5, traversing all local windows of the sensing fiber until the positional mismatch of the entire sensing fiber is completely corrected.

[0050] Step S7: Based on phase demodulation or spectral demodulation methods, perform signal demodulation on the calibrated sensing fiber to obtain high-performance demodulation results.

[0051] In the above method, steps S2 to S6 are the main processes that significantly distinguish this invention from traditional demodulation algorithms, as shown in the flowchart. Figure 1 The location marked in red.

[0052] In step S3, the selection of the sliding window length N should be determined based on factors such as the actual experimental parameters, the local sensor position mismatch, and the expected mismatch calculation effect. For example, for the near end of the sensing fiber with insignificant mismatch, N can be selected as a smaller value, sufficient to cover the mismatch; while for the far end of the sensing fiber with severe mismatch, N needs to be selected as a larger value to cover the mismatch. Furthermore, considering that cross-correlation calculation requires certain waveform characteristics, the value of N cannot be too small, otherwise the characteristics of the Rayleigh scattering waveform cannot be effectively characterized, and the mismatch is difficult to calculate accurately; however, if the value of N is too large, it can only reflect the overall trend of the mismatch and cannot reflect its local position characteristics, affecting the effect of subsequent correction and distributed measurement. Therefore, the selection of N needs to comprehensively consider the above factors and achieve a balance among them.

[0053] In an embodiment of the present invention, the sliding window length N is selected as 100. This value allows for a better calculation result of the local Rayleigh scattering waveform mismatch, which can support the subsequent implementation of distributed sensing.

[0054] Figure 2 (a) shows a comparison of the Rayleigh backscatter intensity distribution waveforms of the reference state and the measurement state with distance at the near end of the sensing fiber, i.e., when no position mismatch occurs; the two waveforms completely overlap. Figure 2 (b) in the middle shows Figure 2 In (a) of the comparison between the autocorrelation curve of the reference state waveform and the cross-correlation curve of the reference state and the measurement state waveform, the positions corresponding to the maximum values ​​of the two are the same. At this time, the calculated result of the mismatch P is 0, which proves that there is no position mismatch in the Rayleigh backscattering of the reference state and the measurement state. Figure 2 (c) shows a comparison of the Rayleigh backscatter intensity distribution waveforms with distance in the reference state and the measurement state at the far end of the sensing fiber, where a positional mismatch has occurred. There is a significant positional shift between the two. Figure 2 (d) in the middle shows Figure 2In section (c), a comparison of the autocorrelation curve of the reference state waveform and the cross-correlation curves of the reference and measurement states shows a significant shift in the position corresponding to their maximum values. This shift is the calculated position mismatch. It should be noted that in this embodiment, the Rayleigh backscattering curve as a function of distance was first refined (achieved by zero-padding the spectral domain data). Therefore, the mismatch can be directly obtained by observing and comparing the Rayleigh scattering intensity waveforms before and after the signal generation, or by calculating using a cross-correlation algorithm. Without this refinement process, this method cannot be implemented.

[0055] Figure 3 This paper demonstrates the positional mismatch correction performed segment by segment along a sensing fiber approximately 150 meters long, and the calculated mismatch at each location. As shown in the figure, the mismatch increases with the fiber length, confirming the phenomenon of mismatch accumulation with fiber length. A strain signal of 150 microstrain was applied to the far end of the sensing fiber. Through the cross-correlation operation in step S4, the strain signal caused a shift in the local similarity peak position between the reference curve and the measured curve, resulting in a sharp increase in the calculated mismatch P. This indicates that the microscopic deformation of the fiber caused by strain has been accurately quantified as data point mismatch in the distance domain.

[0056] Figure 4 Taking the phase demodulation method as an example, the phase demodulation results were compared with and without the application of the sensing position correction algorithm described in this invention. The entire sensing fiber is approximately 150 meters long. At the beginning of the sensing fiber, where no significant position mismatch occurs, the phase demodulation results are essentially the same regardless of whether the sensing position correction algorithm described in this invention is applied. Figure 4 (a) shows the uncorrected result and Figure 4 The corrected results shown in (b) all yield the correct phase distribution with distance; however, at the far end of the sensing fiber, where a significant positional mismatch occurs, Figure 4 The uncorrected result shown in (c) is a phase demodulation failure, which manifests as irregular noise in the demodulation result. Figure 4 The result shown in (d) is the corrected result, and the phase demodulation result is restored to normal, thus proving the effectiveness of the correction algorithm described in this invention.

[0057] Figure 5 Taking the phase demodulation method as an example, the results of demodulating a strain signal of 150 microstrain applied to the far end (approximately 20 cm) of a sensing fiber approximately 150 meters long were compared with and without the application of the sensing position correction algorithm described in this invention. Before applying the correction algorithm, the applied signal could not be correctly demodulated, and the demodulation result appeared as random noise; however, after applying the correction algorithm, the correct 150 microstrain signal was demodulated, thus proving the effectiveness of the correction algorithm described in this invention.

[0058] The present invention also provides an optical frequency domain reflectometer device, the device comprising:

[0059] The light source module is used to provide the frequency-sweeping laser;

[0060] An interference module, connected to the light source module, is used to incident the swept laser onto the sensing fiber and generate an interference signal.

[0061] A detection module, connected to the interference module, is used to acquire the interference signal and convert it into an electrical signal;

[0062] The signal processing module is connected to the detection module and is used to execute the method for distributed sensing measurement distance of the extended optical frequency domain reflectometer described in this invention.

[0063] The present invention also provides a computer program product, including a computer program / instruction, which, when executed by a processor, implements the method for distributed sensing distance measurement of an extended optical frequency domain reflectometer as described in the present invention.

[0064] The above description is merely an embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principle of the present invention should be included within the scope of the claims of the present invention.

Claims

1. A method for extending the distributed sensing measurement distance of an optical frequency domain reflectometer, characterized in that, Includes the following steps: S1. Acquire Rayleigh backscattered light from the sensing fiber using an optical frequency domain reflectometer to obtain two sets of spectral domain data before and after the signal is generated; S2. Perform Fourier transform on the two sets of spectral domain data respectively to convert the spectral domain to the distance domain, and obtain two curves of Rayleigh backscattering intensity as a function of distance before and after the signal is generated. S3. Select a sliding window length N, and on the two Rayleigh backscattering intensity distribution curves with distance, cut local windows of length N from the starting point at each position to obtain the reference local intensity sequence and the measured local intensity sequence. S4. Perform cross-correlation calculation on the reference local intensity sequence and the measured local intensity sequence taken at the same location to obtain the cross-correlation curve, and confirm the corresponding location M based on the peak value of the cross-correlation curve; S5. Based on the peak value, confirm the corresponding position M and the sliding window length N, calculate the current local mismatch P, and perform mismatch correction on the measured local intensity sequence according to the local mismatch P, so that it is aligned with the reference local intensity sequence in the distance domain. S6. Repeat steps S2 to S5, and slide the window along the distance direction of the sensing fiber until the distance domain mismatch at all positions of the entire sensing fiber is corrected. S7. Demodulate the signal of the calibrated sensing fiber to obtain the measurement results.

2. The method for extending the distributed sensing measurement distance of an optical frequency domain reflectometer according to claim 1, characterized in that, Step S2 further includes refining the curve of Rayleigh backscattering intensity as a function of distance.

3. The method for extending the distributed sensing measurement distance of an optical frequency domain reflectometer according to claim 2, characterized in that, The refinement can be achieved by padding the spectral domain data with zeros before the Fourier transform, or by interpolating the curve after the Fourier transform.

4. The method for extending the distributed sensing measurement distance of an optical frequency domain reflectometer according to claim 1, characterized in that, In step S3, the sliding window length N is dynamically determined based on the local mismatch at different positions of the sensing fiber. A smaller window length is used at the near end of the sensing fiber where the mismatch is small, and a larger window length is used at the far end of the sensing fiber where the mismatch is large.

5. The method for extending the distributed sensing measurement distance of an optical frequency domain reflectometer according to claim 1, characterized in that, In step S5, the formula for calculating the local mismatch P is: P = M - N + 1; In the formula, M is the position corresponding to the peak of the cross-correlation curve, and N is the length of the sliding window.

6. The method for extending the distributed sensing measurement distance of an optical frequency domain reflectometer according to claim 1, characterized in that, In step S7, the signal demodulation includes phase demodulation or spectral demodulation.

7. An optical frequency domain reflectometer device, characterized in that, include: The light source module is used to provide the frequency-sweeping laser; An interference module, connected to the light source module, is used to incident the swept laser onto the sensing fiber and generate an interference signal. A detection module, connected to the interference module, is used to acquire the interference signal and convert it into an electrical signal; The signal processing module is connected to the detection module and is used to execute the method for distributed sensing distance measurement of the extended optical frequency domain reflectometer as described in any one of claims 1-6.

8. A computer program product comprising a computer program / instructions, characterized in that, When executed by a processor, the computer program / instruction implements the method for distributed sensing distance measurement of an extended optical frequency domain reflectometer as described in any one of claims 1-6.

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