DAS-based ultra-weak fiber bragg grating vibration optical cable characteristic analysis method

Abstract

The invention discloses a DAS-based ultra-weak fiber bragg grating vibration optical cable characteristic analysis method, which comprises the following steps of: 1, constructing an optical cable experiment system: determining working sections of different types of ultra-weak fiber bragg grating vibration optical cables, carrying out serial welding on the optical cables, arranging the optical cables in a test area in different modes, and then accessing a DAS vibration demodulator; 2, arranging an electronic vibration system: arranging an electronic vibration sensor near a working section of a vibration optical cable, and synchronously connecting the electronic vibration sensor and an impact hammer into an electronic acquisition instrument; step 3, vibration source excitation and signal acquisition; step 4, matching time domain signals; step 5, frequency domain signal analysis; and 6, contrastive analysis: taking the impact hammer as an excitation source, taking the time-frequency signal of the electronic sensor as reference output, taking the time-frequency signal detected by the optical cable as target output, and analyzing the vibration characteristics of the optical cable in different layout modes.

Description

Technical Field

[0001] The present invention relates to the application field of distributed optical fiber sensing, and particularly to a method for analyzing characteristics of ultra-weak optical fiber grating vibration optical cables based on DAS. Background Art

[0002] Each grid point on the ultra-weak fiber Bragg grating vibration cable used in the distributed acoustic sensing system (DAS) has the ability to sense. Compared with traditional ordinary fiber optic sensors and electronic vibration sensors, its spatial resolution can reach several times the advantage, and it has broad application prospects in the field of complex environmental monitoring. In actual engineering needs, the sensitivity requirements of optical cables are different, and at this time, the calibration and analysis of optical cable sensitivity becomes a necessary step.

[0003] In 2002, Li Ranshan (Li Ranshan. Construction method of optical cable tensile test system [J]. Modern Wired Transmission, 2002, (01): 7-18.) proposed a variety of optical cable tensile test systems composed of computers, test instruments and test equipment to calibrate the strain coefficient of optical cables; in 2013, Gou Wuhou et al. (Gou Wuhou, Chang Yang, Li Gang. Impact calibration device for buried distributed optical fiber vibration monitoring system [P]. Beijing: CN202853737U, 2013-04-03.) proposed a method for calibrating the strain coefficient of optical cables. After the impact excitation composed of a hand-pushed flatbed cart, test bench, catheter and impact ball, the signal processing system collects the data in real time and records the voltage value converted into disturbance to calibrate the buried distributed optical fiber vibration monitoring system; in the same year, Gou Wuhou et al. (Gou Wuhou, Chang Yang, Zhao Hainan et al. Calibration device for long-distance distributed optical fiber vibration monitoring system [P]. Beijing: CN203053560U, 2013-07-10.) improved the test device by adding an odometer and other methods, and calibrated the long-distance distributed optical fiber vibration monitoring system.

[0004] However, there are few existing methods for calibrating and comparing the sensitivity of heterogeneous vibration optical cables. In addition, the environmental factors and vibration sources that actually separate and define the sensitivity of heterogeneous vibration optical cables are not fixed. Therefore, it is necessary to design a vibration characteristic of heterogeneous vibration optical cables under the same vibration source environment, such as a sensitivity analysis method, to establish a standard environment to assist in the calibration of vibration optical cables. Summary of the invention

[0005] In view of the shortcomings of the prior art, the present invention proposes a DAS-based ultra-weak fiber grating vibration cable characteristics analysis method, which aims to effectively solve the uncertain influence on the vibration cable characteristics in practical application scenarios, and provides a valuable solution for the DAS system to accurately calibrate and analyze the cable characteristics.

[0006] More specifically, the present invention proposes a DAS-based ultra-weak fiber Bragg grating vibration cable characteristic analysis method, comprising the following steps:

[0007] Step 1: Build the optical cable experimental system: determine the working sections of different types of ultra-weak fiber Bragg grating vibration optical cables, perform serial fusion splicing on the optical cables, arrange the optical cables in the test area in different ways, and then connect them to the DAS vibration demodulator;

[0008] Step 2: Electronic vibration system layout: The electronic vibration sensor is laid out near the working section of the vibration optical cable, and is connected to the electronic collector synchronously with the impact hammer;

[0009] Step 3, vibration source excitation and signal acquisition: hammer the medium in a direction perpendicular to the vibration optical cable and the electronic vibration sensor, the electronic collector collects and saves the time-frequency signal of the electronic vibration sensor; the DAS vibration demodulator collects and saves the time-frequency signal of the optical fiber sensor (vibration optical cable);

[0010] Step 4, time domain signal matching: convert the time domain signals of the two sensors into data files, match the maximum peak amplitudes, and establish the relationship between the voltage data of the electronic sensor and the phase of the optical fiber sensor;

[0011] Step 5: Frequency domain signal analysis: convert the frequency domain signals of the two sensors into data files, and establish the relationship between the spectrum of the electronic sensor and the spectrum of the optical fiber sensor;

[0012] Step 6: Comparative analysis: Using the impact hammer as the excitation source, the time-frequency signal of the electronic sensor as the reference output, and the time-frequency signal of the optical cable detection as the target output, analyze the vibration characteristics of the optical cable under different layout methods.

[0013] According to an embodiment of the present invention, in step 1, the different methods include surface pasting, slotting and embedding, and snap-on fixing.

[0014] According to an embodiment of the present invention, the vibration characteristics include effective value, amplitude, frequency, optical cable sensing time domain relationship and optical cable sensing frequency domain relationship.

[0015] According to an embodiment of the present invention, an electronic vibration sensor is arranged near the middle section of the vibrating optical cable, the vibration source excitation is divided into multiple intervals, multiple hammering is performed in the middle section of each interval and interval vibration data is collected.

[0016] According to the implementation scheme of the present invention, a vibration demodulator is used to test the signals of the optical cable and the electronic vibration sensor under the same vibration excitation in different layout modes, and the time domain and frequency domain signals at the event moment are split and demodulated.

[0017] According to the implementation scheme of the present invention, the time domain signal collected by the electronic vibration sensor is used as a standard, and the similarity analysis is performed using data of different fixed optical cables to analyze the time domain relationship of optical cable sensing under different layout modes.

[0018] According to the implementation scheme of the present invention, the frequency domain signal collected by the electronic vibration sensor is used as a standard, and linear fitting is performed using data of different fixed optical cables to analyze the frequency domain relationship of optical cable sensing under different layout modes.

[0019] According to an embodiment of the present invention, the test area is a track slab.

[0020] The present invention provides a DAS-based method for analyzing ultra-weak fiber grating vibration optical cable characteristics, which improves the shortcomings of traditional optical cable characteristics such as uncertainty that may exist in sensitivity analysis and the inability to ensure the consistency of excitation source conditions. It can simultaneously perform equal vibration excitation when the sensitivity of the optical cable is unknown and achieve good comparison effects.

[0021] The experimental device of the present invention is simple to set up. The experimental device includes two parts, namely, an optical cable experimental device and an electronic vibration system. On the basis of traditional vibration optical cable vibration characteristics such as sensitivity calibration analysis, the comparison of the electronic vibration system is added, thereby improving the reliability of the vibration optical cable sensitivity calibration analysis.

[0022] The present invention performs the same vibration source sensitivity calibration of heterogeneous optical cables by serial welding for the first time, thereby avoiding the inability to ensure the same vibration environment conditions when heterogeneous optical cables are calibrated separately, and improving the accuracy of sensitivity calibration. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 A schematic diagram of a DAS demodulation module used in a DAS-based ultra-weak fiber Bragg grating vibration optical cable characteristic analysis method according to an embodiment of the present invention;

[0024] Figure 2 A flow chart of a method for analyzing characteristics of ultra-weak fiber Bragg grating vibration optical cables based on DAS according to an embodiment of the present invention;

[0025] Figure 3 A schematic diagram of the structure of an experimental device for analyzing the characteristics of ultra-weak fiber Bragg grating vibration optical cables based on DAS according to an embodiment of the present invention;

[0026] Figure 4 A time domain evaluation relationship diagram of a method for analyzing ultra-weak fiber Bragg grating vibration optical cable characteristics based on DAS according to an embodiment of the present invention; and

[0027] Figure 5 It is a frequency domain evaluation relationship diagram of the DAS-based ultra-weak fiber Bragg grating vibration optical cable characteristic analysis method according to an embodiment of the present invention. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical scheme and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention, that is, the embodiments described are only part of the embodiments of the present invention, rather than all of the embodiments. The components of the embodiments of the present invention described and shown in the drawings herein can be arranged and designed in various different configurations.

[0029] The features and performance of the present invention are further described in detail below in conjunction with the embodiments:

[0030] Figure 2 This is a flow chart of a method for analyzing characteristics of ultra-weak fiber Bragg grating vibration optical cables based on DAS according to an embodiment of the present invention. As shown in the figure, the method for analyzing characteristics of ultra-weak fiber Bragg grating vibration optical cables based on DAS may include the following steps:

[0031] First, the optical cable experimental system is built, including determining the working sections of different types of ultra-weak fiber Bragg grating vibration optical cables, and then serially welding the determined heterogeneous optical cables, arranging the optical cables by different methods such as surface pasting, snap-on, and slotted burial, and then connecting them to the DAS vibration demodulator.

[0032] In the implementation manner of the present invention, a distributed fiber optic acoustic / vibration sensing system (DAS) hardware and a vibrating optical fiber (insensitive to sound signals) are used to collect vibration signals released by hammering excitation of a vibration source, and the sampling rate of the collected signal is 10 kHz. Figure 1 Schematic diagram of a DAS demodulation module used in a DAS-based ultra-weak fiber Bragg grating vibration optical cable characteristic analysis method according to an embodiment of the present invention.

[0033] like Figure 1As shown in the figure, the hardware for collecting data is a distributed fiber acoustic wave sensing system based on phase demodulated optical time domain reflectometry (Φ-OTDR) technology. The system hardware consists of three parts: ultra-weak fiber grating array, DAS demodulation module, and signal processing host. The ultra-weak fiber grating array consists of multiple ultra-weak fiber grating sensors with a grating interval of 2m. The DAS demodulation module is the core device for collecting and demodulating signals. Its internal components mainly include optical devices and electrical devices. The continuous light emitted by the narrow-band laser is modulated by a semiconductor amplifier (Semiconductor Optical Amplifiers, SOA), and the pulse power is increased by an erbium-doped optical fiber amplifier (Erbium-doped Optical Fiber Amplifier, EDFA), and then coupled into the UW-FBGs (Ultra Weak-Fiber Bragg Gratings) array. The light pulses reflected by the UW-FBG at different positions arrive at the photodetector at different times. The position of each grating can be determined by time-intensity analysis. After the reflected pulses of two adjacent gratings pass through the unbalanced Michelson interferometer (the arm length difference is L, which is equal to the spacing between the two adjacent gratings), they interfere in the 3×3 coupler to form three interference optical signals with a phase difference of 120°. After photoelectric conversion, they are synchronously collected by 3 high-speed A / D. At the same time, the embedded circuit completes the inverse tangent, digital filtering, and phase rewinding calculations in real time, and then outputs and displays through Ethernet. The two Faraday rotators in the unbalanced Michelson interferometer are used to compensate for polarization fading. The embedded circuit is built with Xilinx Zynq7035 chip as the core, integrating processor (Advanced RISC Machines, ARM) and field programmable gate array (Field Programmable Gate Array, FPGA), and performs timing control and algorithm analysis on SOA, EDFA, and acquisition circuits in the system. The signal processing host is an ordinary commercial laptop computer, which is used for the DAS demodulation module to collect signals and convert them into audio files and signal preprocessing.

[0034] Then the electronic vibration system is deployed, including the electronic vibration sensor being deployed near the working section of the vibration optical cable and connected to the electronic collector synchronously with the impact hammer. There is no special restriction on the electronic collector and the existing instrument can be used.

[0035] Figure 3The schematic diagram of the experimental device structure of the DAS-based ultra-weak fiber grating vibration cable characteristic analysis method according to the implementation scheme of the present invention. Taking the track plate as an example, the serially connected optical fibers are laid on the track plate in a basically parallel manner and in three fixing methods (including surface pasting, slotted embedding and snap-on). The numbers 1-9 in the optical fiber represent different grating points. For example, the data of the i-th grating point represents the average level of vibration between the i-th and i+1-th grating intervals. 10 and 11 represent electronic vibration sensors. The vibration is divided into two intervals, interval 1 and interval 2, and interval 2 spans different track plates. Multiple hammerings are performed at points A and B in the middle of the two intervals, focusing on the vibration data in these two intervals. Interval 1 can compare the differences between the three grating arrangements and sensor No. 10; since the interval 2 grating crosses the track, the vibration data may theoretically be quite different from that of sensor No. 11, so the comparison between the interval 2 grating data and the sensor data is only for reference.

[0036] After the device is laid out, vibration source excitation and signal collection are carried out, which specifically includes hammering the medium in a direction perpendicular to the vibration optical cable and the electronic vibration sensor, the electronic collector collecting and saving the electronic sensor signal, and the vibration demodulator collecting and saving the time-frequency signal of the optical fiber sensor. More specifically, hammering is carried out in the middle section of intervals A and B respectively and interval vibration data is collected; the vibration demodulator is used to test the signals of the optical fiber and the electronic vibration sensor under the same vibration excitation in three types of fixed modes, and the time domain and frequency domain signals at the time of the event are split and demodulated; the vibration excitation position is adjusted to continue to collect and save the time domain and frequency domain signals.

[0037] Then, time domain signal matching and frequency domain signal analysis are performed based on the collected data, and comparative analysis is carried out.

[0038] Time domain signal matching includes converting the time domain signals of the two sensors (optical fiber sensor and electronic vibration sensor) into data files, matching the maximum peak amplitude, and establishing the relationship between the electronic sensor voltage data and the optical fiber sensor phase. Frequency domain signal analysis includes converting the frequency domain signals of the two sensors into data files, and establishing the relationship between the electronic sensor spectrum and the optical fiber sensor spectrum. Comparative analysis includes using an impact hammer as an excitation source, the time-frequency signal of the electronic sensor as a reference output, and the time-frequency signal of the optical cable detection as a target output to analyze the vibration characteristics and response characteristics of the optical cable under different layout methods, including amplitude discreteness, frequency distortion, sensitivity, optical cable sensing time domain relationship, and optical cable sensing frequency domain relationship. It should be understood that the above-mentioned vibration characteristics, time domain and frequency domain matching and analysis are well known in the art and are not elaborated in detail here.

[0039] Figure 4 A time domain evaluation structure diagram of a DAS-based ultra-weak fiber Bragg grating vibration optical cable characteristic analysis method according to an embodiment of the present invention; Figure 5Graph showing the frequency domain evaluation results of the DAS-based ultra-weak fiber Bragg grating vibration cable characteristic analysis method according to an embodiment of the present invention.

[0040] The results show that from the comparison between the grating (optical cable) and the electronic vibration sensor, the embedded time course is similar, and the spectrum is also reflected at some peak frequencies. Since the grating in interval 2 crosses the track, the contrast of interval 2 is not as good as that of interval 1. The sensor measurement data, embedded, pasted, and snap-on, the data of the three cases are all consistent with each other, and the signal contrast of interval 1 is worse than that of interval 2.

[0041] The above description of the embodiments is to facilitate the understanding and application of the present invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the embodiments herein, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims

1. A method for analyzing the characteristics of ultra-weak fiber Bragg grating vibration optical cables based on DAS, characterized in that: The following steps are involved: Step 1: Construction of the optical cable experimental system: determine the working sections of different types of ultra-weak fiber Bragg grating vibration optical cables, perform serial fusion splicing on the optical cables, arrange the optical cables in the test area in different ways, and then connect them to the DAS vibration demodulator; Step 2: Electronic vibration system layout: The electronic vibration sensor is laid out near the working section of the vibration optical cable, and is connected to the electronic collector synchronously with the impact hammer; Step 3, vibration source excitation and signal acquisition: hammer the medium in a direction perpendicular to the vibration optical cable and the electronic vibration sensor, the electronic collector collects and saves the time-frequency signal of the electronic vibration sensor; the DAS vibration demodulator collects and saves the time-frequency signal of the optical fiber sensor (vibration optical cable); Step 4, time domain signal matching: convert the time domain signals of the two sensors into data files, match the maximum peak amplitudes, and establish the relationship between the electronic sensor data and the optical fiber sensor phase; Step 5: Frequency domain signal analysis: convert the frequency domain signals of the two sensors into data files, and establish the relationship between the spectrum of the electronic sensor and the spectrum of the optical fiber sensor; Step 6: Comparative analysis: Using the impact hammer as the excitation source, the time-frequency signal of the electronic sensor as the reference output, and the time-frequency signal of the optical cable detection as the target output, analyze the vibration characteristics of the optical cable under different layout methods.

2. The method according to claim 1, characterized in that In step 1, the different methods include surface pasting, slotting and embedding, and snap-on fixing.

3. The method according to claim 1 or 2, characterized in that: The vibration characteristics include effective value, amplitude, frequency, optical cable sensing time domain relationship and optical cable sensing frequency domain relationship.

4. The method according to claim 3, characterized in that The electronic vibration sensor is arranged near the middle section of the vibrating optical cable, and the vibration source excitation is divided into multiple sections. Multiple hammer strikes are performed in the middle section of each section and the vibration data of the section is collected.

5. The method according to claim 4, characterized in that The signals of the optical cable and the electronic vibration sensor under the same vibration excitation in different layout modes are obtained, and the time domain and frequency domain signals at the event moment are split and demodulated.

6. The method according to claim 5, characterized in that Taking the time domain signal collected by the electronic vibration sensor as the standard, the similarity analysis is performed using the data of different fixed optical cables to analyze the time domain relationship of optical cable sensing under different layout methods.

7. The method according to claim 5, characterized in that Taking the frequency domain signal collected by the electronic vibration sensor as the standard, the similarity analysis was performed using the data of different fixed optical cables to analyze the frequency domain relationship of optical cable sensing under different layout methods.

8. The method according to claim 1, characterized in that The test area is the track slab.

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