A method for identifying the position of weak frequency shift signals in STFT-BOTDR system

Abstract

The present invention proposes a method for identifying the position of a weak frequency shift signal in a STFT-BOTDR system, which relates to the field of distributed optical fiber sensing. The method mainly involves subtracting the Brillouin gain spectrum lines of two adjacent points along the optical fiber to obtain a difference spectrum line. If the difference spectrum line is found to have obvious fluctuations, the position corresponding to the difference spectrum line is used as the position where the optical fiber frequency shift occurs. The present invention identifies the position of a weak frequency shift signal without increasing expensive devices and algorithm complexity by calculating the root mean square error of the BGS difference spectrum line of two adjacent points along the optical fiber, thereby detecting an area with a slight temperature or stress change.

Description

Technical Field

[0001] The invention relates to the field of distributed optical fiber sensing and proposes a method for identifying the position of a weak frequency shift signal in a STFT-BOTDR system. Background Art

[0002] With the continuous development of my country's social economy, the development speed of the electric power industry, civil engineering, petrochemical industry, communication engineering, environmental monitoring and other fields has been accelerating, the number and scale of large-scale infrastructure have been increasing, and the total construction volume has exceeded the total of other countries in the world. In terms of economic development and people's livelihood, the safe and stable operation of these facilities is of great significance. It is very necessary to monitor the health status of these facilities and carry out timely maintenance.

[0003] In recent years, fiber optic sensing technology has been increasingly used in structural health monitoring. By utilizing the nonlinear scattering characteristics of optical fiber materials to light waves, distributed fiber optic sensing technology can accurately measure the temperature and stress information at each location along the optical fiber. Distributed fiber optic sensing technology also has the advantages of small footprint of sensing optical fiber, long laying distance, high temperature resistance of special optical fiber, and good electromagnetic resistance. Therefore, distributed fiber optic sensing technology has received extensive attention and research from researchers. Among them, distributed Brillouin fiber optic sensing technology can measure temperature and strain over long distances with high detection accuracy, which is very suitable for structural safety monitoring of large infrastructure projects.

[0004] The Brillouin optical time-domain reflectometry (BOTDR) sensing system is a typical distributed optical fiber sensing system. Based on the linear relationship between the frequency shift of spontaneous Brillouin scattered light and temperature change and strain, the BOTDR sensing system uses a convenient structure of single-ended incident light detection to measure the temperature and stress information along the optical fiber over long distances. This reduces the complexity of the measurement and the application cost of the system. Moreover, when the optical fiber breaks at the construction site, the BOTDR system can still perform effective detection, avoiding the disadvantage of using the Brillouin optical time-domain analysis (BOTDA) sensing system with a double-ended ring structure, which cannot be used once the optical fiber breaks, making the BOTDR system more convenient in actual engineering applications.

[0005] The principle of the BOTDR sensing system is: pulsed light is injected into the optical fiber. Due to the effect of sound waves, the pulsed light will generate Brillouin Stokes light and Brillouin anti-Stokes light in the optical fiber. Changes in the temperature or stress of the optical fiber will change the optical fiber characteristics, thereby changing the frequency of the Stokes and anti-Stokes light. By demodulating the frequency shift of the Stokes or anti-Stokes light before and after the temperature or stress changes, the magnitude of the change in external temperature or stress can be obtained. However, due to limitations in pulse power and demodulation algorithms, the detection accuracy of the BOTDR sensing system is limited, and it is difficult to detect tiny changes in temperature or stress. Summary of the invention

[0006] In view of the deficiencies in the prior art, the present invention aims to provide a method for identifying the position of a weak frequency shift signal in a STFT-BOTDR system, which is used for identifying the position of a weak frequency shift signal in a BOTDR system.

[0007] The purpose of this disclosure can be achieved through the following technical solutions:

[0008] A method for identifying a position of a weak frequency shift signal in a STFT-BOTDR system, comprising:

[0009] The scattered light time domain signal collected by the BOTDR system along the optical fiber is processed by STFT to obtain the Brillouin gain spectrum along the optical fiber;

[0010] Subtract the Brillouin gain spectra of two adjacent points along the optical fiber to obtain the difference spectrum line;

[0011] Identify the fluctuation of the difference spectrum line. If it is found that the difference spectrum line has obvious fluctuations, the position corresponding to the difference spectrum line at this time is used as the position where the frequency shift of the optical fiber occurs.

[0012] Furthermore, the scattered light time domain signal collected by the BOTDR system is subjected to STFT processing to obtain the Brillouin gain spectrum along the optical fiber, including: selecting a sliding window spacing smaller than the window length, and after performing sliding window processing on the entire scattered light time domain signal, all the Brillouin gain spectrum signals obtained constitute the Brillouin gain spectrum along the optical fiber.

[0013] Furthermore, the difference spectrum line has obvious fluctuation phenomenon, including: calculating a root mean square error of the difference spectrum line, if the root mean square error is greater than a threshold, it is considered that the difference spectrum line has obvious fluctuation phenomenon.

[0014] Further, the threshold is 0.05dBm.

[0015] Furthermore, the stimulus source of the weak frequency shift signal includes temperature change or strain.

[0016] Furthermore, the weak frequency shift of the weak frequency shift signal is a frequency shift having a magnitude close to the measurement accuracy of the STFT-BOTDR, and the system measurement accuracy is determined by the system parameters and the average times.

[0017] Beneficial effects of the present disclosure:

[0018] In the present invention, by calculating the root mean square error of the difference spectrum of the Brillouin gain spectrum of two consecutive points along the optical fiber, it is determined whether there is temperature change or strain along the optical fiber, and it is not limited by the detection accuracy of the BOTDR system, and the area where weak temperature change or strain occurs along the optical fiber can be identified;

[0019] The present invention uses a method of calculating the root mean square error of the BGS difference spectrum lines of two adjacent points along the optical fiber to identify the position of the weak frequency shift signal without increasing expensive devices and algorithm complexity, thereby detecting areas with slight temperature or stress changes. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0021] Figure 1 This is a flowchart of a case in this application;

[0022] Figure 2 This is a schematic diagram of the STFS sliding process in a case of this application;

[0023] Figure 3 A schematic diagram of the Brillouin gain spectrum along an optical fiber in a case of this application;

[0024] Figure 4 The two windows in one case of this application contain the Brillouin gain spectra when there is no information of temperature change or strain;

[0025] Figure 5 In one case of this application, the two windows contain the Brillouin gain spectra when the temperature change or strain information occurs;

[0026] Figure 6 A Brillouin gain spectrum when a window in a case of the present application contains a portion of the information of temperature change or strain;

[0027] Figure 7 Two different difference spectra in one case for this application;

[0028] Figure 8 This is a Brillouin frequency shift distribution diagram in a case of this application;

[0029] Fig. 9 This is a root mean square error distribution diagram in a case of this application;

[0030] Fig.10 The Brillouin frequency shift distribution diagram for increasing temperature change in a case of this application;

[0031] Fig.11 This is the RMS error distribution diagram for increasing temperature variation in a case study of this application. DETAILED DESCRIPTION

[0032] The following will be combined with the drawings in the embodiments of the present disclosure to clearly and completely describe the technical solutions in the embodiments of the present disclosure. Obviously, the described embodiments are only part of the embodiments of the present disclosure, rather than all the embodiments. Based on the embodiments in the present disclosure, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present disclosure.

[0033] Step 1: Perform STFT processing on the scattered light time domain signal collected by BOTDR to obtain the Brillouin gain spectrum along the optical fiber.

[0034] The scattered light time domain signal collected by the BOTDR system, such as the current BOTDR system, when it is working, the continuous light emitted by the laser is divided into two paths through the coupler, one path is used as the reference light, and the other path is modulated to obtain a pulse and injected into the optical fiber, causing the Brillouin scattering effect. The obtained Brillouin scattered light and the reference light are mixed and beat in the coupler. The high-frequency signal obtained by the beat includes both the sum frequency signal of the scattered light and the reference light, and the difference frequency signal of the two. The difference frequency signal is the Brillouin frequency shift signal to be detected, and its frequency is 10.8 GHz. The mixed signal is converted into a 10.8 GHz radio frequency signal through a photodetector, and then converted into an intermediate frequency signal of about 400 MHz through a signal down-conversion processing module (-10.4 GHz). The output intermediate frequency signal is collected by a data acquisition card, and the output intermediate frequency signal is used as the scattered light time domain signal to complete the collection of the scattered light time domain signal;

[0035] The scattered light time domain signal collected by BOTDR is subjected to STFT processing. For example, a sliding window spacing smaller than the window length is selected, and the Brillouin gain spectrum signal is obtained after sliding window processing of the entire scattered light time domain signal. All Brillouin gain spectrum signals overlap to form the Brillouin gain spectrum along the optical fiber.

[0036] STFT is a method for analyzing non-stationary signals, mainly by introducing a window length of h The specific window function is used to perform window segmentation processing on the scattered light time domain signal collected by the data acquisition card, and then Fourier transform processing is performed on it to obtain a Brillouin gain spectrum signal, which can be equivalent to a collection of signals at different positions within a window length. h Divide into m The length of each copy is , then the Brillouin gain spectrum at z0 is m Molecular Brillouin signal The overlap result of the Brillouin gain spectrum at position z0 is As shown in formula (1):

[0037] (1)

[0038] in represents the Brillouin frequency shift of the optical fiber at room temperature. Represents the weight coefficient.

[0039] If the window length corresponding to position z1 is within m The molecular Brillouin signal has n The part contains temperature change or strain information. represents the Brillouin frequency shift of the temperature-dependent or strained fiber. The Brillouin gain spectrum at position z1 is It can be expressed as:

[0040] (2)

[0041] Then the entire signal is covered by sliding the window function, and the sliding window spacing is Δ t ,like Figure 2 As shown, at this time, the signal in each window can be approximated as a stationary signal. Each time the window function slides, a Brillouin gain spectrum signal is obtained. After sliding the window on the entire scattered light time domain signal, all the Brillouin gain spectrum signals obtained constitute the Brillouin gain spectrum along the optical fiber, as shown in Figure 3 As shown, this local frequency domain analysis is used to obtain the joint distribution of the entire signal in the time domain and frequency domain, and finally the frequency shift information along the optical fiber is obtained by analyzing the peak intensity of the Brillouin gain spectrum, thereby solving the temperature or stress information along the optical fiber.

[0042] In the STFT process, we often choose a value smaller than the window length. h The sliding window spacing Δ t , the Brillouin gain spectrum along the fiber will contain more information, and the obtained temperature or stress changes will be more accurate, but the system solution time will also increase with the sliding window spacing Δ t The decrease of the sliding window spacing Δ t Less than window length h , so part of the information contained in two adjacent window functions overlaps, and if the two windows contain information that has not undergone temperature change or strain, or information that has undergone temperature change or strain, then the Brillouin gain spectra calculated by the two windows will be very close, such as Figure 4 and Figure 5 As shown; if one window contains some temperature change or strain information and the other does not, then the Brillouin gain spectra calculated from these two windows will be different, such as Figure 6 shown.

[0043] Step 2: Subtract the Brillouin gain spectrum lines of two adjacent points to obtain the difference spectrum line.

[0044] Then, the Brillouin gain spectrum lines of two adjacent windows are subtracted to obtain the difference spectrum line. It can be understood that, for the same frequency, the corresponding intensities of two adjacent windows are subtracted, and the corresponding intensity subtraction values ​​of continuous frequencies are connected to obtain the difference spectrum line; if the two windows contain information that no temperature change or strain has occurred, or both contain information that temperature change or strain has occurred, then the difference spectrum line is as follows Figure 7 As shown by the middle dotted line, at this time, the dotted line shows or is in a relatively stable state;

[0045] If one window contains some information about temperature change or strain, but the other does not, such as Figure 7 As shown by the solid line in the middle, at this time, the solid line shows or is in an undulating state;

[0046] At this time, the fluctuation of the difference spectrum line can be identified. If it is found that the difference spectrum line has obvious fluctuations, the position corresponding to the difference spectrum line at this time is used as the position where the frequency shift of the optical fiber occurs.

[0047] In some cases, the following steps may also be performed:

[0048] Step 3: Calculate the root mean square error for all difference spectral lines.

[0049] Step 4: Determine whether temperature change or strain occurs at a certain location based on the size of the root mean square error.

[0050] Of course, in order to more conveniently process the status information of the difference spectrum line, the root mean square error of all the difference spectrum lines can be calculated, such as according to Figure 7 As shown in the middle solid line, at this time the solid line shows or is in an obvious undulating state; at this time, the root mean square error of the difference spectrum line at the starting position and the end position of the frequency shift segment is much larger than the root mean square error of the difference spectrum line at other positions. This method can identify frequency shifts below the frequency resolution.

[0051] Since there are many difference spectrum lines obtained by BGS along the optical fiber, and the specific number is determined by the sampling rate of the data acquisition card and the parameters of the STFT algorithm, judging by the difference spectrum lines themselves will increase the difficulty of the algorithm and the calculation time. Therefore, the root mean square error of all difference spectrum lines is used for judgment. This method can reduce the difficulty of the algorithm and the calculation time, and the results obtained can intuitively reflect whether there is a slight temperature change or strain along the optical fiber.

[0052] In practice, if the RMS error is greater than a threshold, it is considered that the difference spectrum line has obvious fluctuations; the threshold can be selected as 0.05dBm; if the threshold is selected to be less than 0.05dBm, the inherent noise of the system will be mistaken for the frequency shift caused by temperature change or strain; if the threshold is selected to be greater than 0.05dBm, possible temperature change or strain will be ignored.

[0053] The root mean square error is calculated for all data points on the difference spectrum line, and the size of all root mean square errors is compared to determine whether there is an abnormality along the optical fiber. The size of the root mean square error can be used to determine whether the optical fiber has a frequency shift at a certain position, and identify slight temperature changes or strains.

[0054] The description of weak temperature change or strain refers to the temperature change or strain corresponding to the frequency shift whose size is close to the STFT-BOTDR measurement accuracy. Taking temperature change as an example, when the temperature measurement accuracy of the STFT-BOTDR system is 1°C, then 1°C-1.5°C can be considered as a weak temperature change.

[0055] Explanation of weak frequency shift: frequency shift with a size close to the STFT-BOTDR measurement accuracy. Taking temperature change as an example, when the temperature measurement accuracy of the STFT-BOTDR system is 1°C, the corresponding frequency shift is 1.3MHz, then 1.3MHz-2MHz can be considered a weak frequency shift.

[0056] Test case:

[0057] like Figure 8 The Brillouin frequency shift distribution diagram obtained by water bath heating at three locations on a section of optical fiber is shown, and the root mean square error mentioned in the present invention is calculated, and the result is as follows Fig. 9 As shown. Figure 8 and Fig. 9 It can be seen that the RMS error distribution is basically consistent with the Brillouin frequency shift distribution.

[0058] At the same time, the present invention can identify the location of slight temperature change or strain along the optical fiber, and we have verified this. We added a slight temperature change at the front end of the above optical fiber, and the new Brillouin frequency shift distribution is as follows: Fig.10 As shown in FIG. 1 , we cannot see the frequency shift caused by the newly added temperature change from this figure. The root mean square error mentioned in the present invention is calculated, and the result is as follows: Fig.11 As shown, it can be seen that Fig. 9 compared to, Fig.11 A new mutation has appeared at the front end. When the temperature change or strain is very weak, the Brillouin frequency shift will also be very small, and the measured data has a certain width. At this time, it will be impossible to observe obvious frequency shifts from the Brillouin frequency shift distribution diagram, which will affect the judgment of the state along the optical fiber. Ignoring weak temperature changes and strains, although the detection accuracy of the system can be improved by adding high-precision optical devices or changing system parameters, this will increase the detection cost of the system or the operation time of the system, reducing the engineering practicality. By calculating the root mean square error and taking advantage of the fact that there must be a large difference between the Brillouin gain spectrum before and after the frequency shift, it is possible to identify weak temperature changes or strains without increasing the complexity of system devices and algorithms, prevent them from developing into uncontrollable factors, and enhance the engineering practicality of the system.

[0059] In the description of this specification, the description with reference to the terms "one embodiment", "example", "specific example", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present disclosure. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0060] The above shows and describes the basic principles, main features and advantages of the present disclosure. Those skilled in the art should understand that the present disclosure is not limited by the above embodiments, and the above embodiments and descriptions are only for explaining the principles of the present disclosure. Without departing from the spirit and scope of the present disclosure, the present disclosure may have various changes and improvements, and these changes and improvements fall within the scope of the present disclosure to be protected.

Claims

1. A method for identifying the position of a weak frequency shift signal in a STFT-BOTDR system, characterized in that: include: The scattered light time domain signal collected by the BOTDR system along the optical fiber is processed by STFT to obtain the Brillouin gain spectrum along the optical fiber; Subtract the Brillouin gain spectrum lines of two adjacent points along the optical fiber to obtain the difference spectrum line; Identify the fluctuation of the difference spectrum line. If the difference spectrum line is found to have obvious fluctuations, the position corresponding to the difference spectrum line at this time is taken as the position where the optical fiber frequency shift occurs. The scattered light time domain signal collected by the BOTDR system is subjected to STFT processing to obtain the Brillouin gain spectrum along the optical fiber, including: selecting a sliding window spacing smaller than the window length, performing sliding window processing on the entire scattered light time domain signal, and obtaining a Brillouin gain spectrum signal, wherein all Brillouin gain spectrum signals are overlapped to form a Brillouin gain spectrum along the optical fiber; The difference spectrum line has obvious fluctuation phenomenon, including: calculating the root mean square error of the difference spectrum line, if the root mean square error is greater than a threshold, it is considered that the difference spectrum line has obvious fluctuation phenomenon.

2. The method for identifying the position of a weak frequency shift signal in a STFT-BOTDR system according to claim 1, characterized in that: The threshold is 0.05dBm.

3. The method for identifying the position of a weak frequency shift signal in a STFT-BOTDR system according to claim 1, characterized in that: The stimulus sources of weak frequency shift signals include temperature change or strain.

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