Temperature strain separable dynamic and static combined measurement distributed optical fiber sensing system

Abstract

A temperature strain separable dynamic and static combined measurement distributed optical fiber sensing system, comprising: laser, optical modulator, radio frequency signal generator, optical filter, DFB laser, polarization controller, first acoustooptic modulator, depolarizer, erbium-doped fiber amplifier, two optical circulators, optical wavelength division multiplexer, receiving module, signal acquisition and processing module are arranged in sequence, the application carries out temperature sensing to the back Raman scattering light of the pulse while receiving the Rayleigh scattering light of the detection pulse, and controls the time interval of adjacent pump pulse and detection pulse to carry out three-bit pulse coding and decoding, so that the back Raman scattering signals of individual pulse in pulse pair are separated in time, and finally the simultaneous measurement of dynamic variable (vibration) and static variable (temperature, static strain) is realized and the spatial resolution of measurement is ensured not to be lost.

Description

Technical Field

[0001] This invention relates to a technology in the field of fiber optic sensing, specifically a distributed fiber optic sensing system for joint dynamic and static measurement of temperature and strain that can be separated. Background Technology

[0002] Distributed fiber optic sensing systems are widely used in various fields such as building structural health monitoring, rail transit monitoring, and submarine security, enabling simultaneous measurement over long distances and at multiple points. There are various options for sensing fibers used in distributed fiber optic sensing systems (ordinary single-mode fiber, polarization-maintaining fiber, multimode fiber, few-mode fiber, multi-core fiber, etc.). To utilize existing fiber optic networks for sensing, the most economical solution is to use ordinary single-mode fiber. However, distributed fiber optic sensing systems using single-mode fiber typically only measure one physical quantity, or they can simultaneously measure temperature and strain but suffer from cross-sensitivity issues.

[0003] Existing distributed fiber optic sensing technology using two pulses, while simultaneously detecting temperature and static strain changes, still cannot overcome the cross-sensitivity problem of temperature and strain, meaning it cannot distinguish between changes caused by temperature and static strain. This leads to measurement errors or limitations in measurement scenarios. Furthermore, when using two pulses for sensing, the backscattered Raman light frequency bands overlap significantly, making it impossible to distinguish the backscattered light from the two pulses in the frequency domain using conventional methods. This degrades the spatial resolution of temperature measurements, affecting the measurement results. Summary of the Invention

[0004] This invention addresses the issues of temperature and strain cross-sensitivity and simultaneous measurement of dynamic and static strain in existing distributed fiber optic sensing systems. It proposes a distributed fiber optic sensing system for the joint measurement of temperature and strain, enabling separate dynamic and static measurements. By sensing the temperature of the backscattered Raman light from the probe pulse while simultaneously receiving the Rayleigh scattering light, and controlling the time interval between adjacent pump pulses and probe pulses for three-bit pulse encoding and decoding, the backscattered Raman signals of individual pulses in the pulse pair are separated in time. This ultimately achieves simultaneous measurement of dynamic variables (vibration) and static variables (temperature, static strain) while ensuring no loss of spatial resolution.

[0005] This invention is achieved through the following technical solution:

[0006] This invention relates to a distributed fiber optic sensing system for temperature and strain separable dynamic and static joint measurement, comprising: a laser, an optical modulator, a radio frequency signal generator, an optical filter, a DFB laser, a polarization controller, a first acousto-optic modulator, a polarization scrambler, an erbium-doped fiber amplifier, two optical circulators, an optical wavelength division multiplexer, a receiving module, and a signal acquisition and processing module, arranged sequentially. The measuring fiber, the erbium-doped fiber amplifier, and the optical wavelength division multiplexer are respectively located at three ports of the second optical circulator. A second acousto-optic modulator, with its signal terminal connected to the optical modulator, is further provided between the laser and the erbium-doped fiber amplifier. The receiving module is connected to the laser to receive local light. The optical modulator, optical filter, DFB laser, polarization controller, first optical circulator, first acousto-optic modulator, and polarization scrambler constitute a Brillouin optical time-domain analysis (BOTDA) based branch. The second acousto-optic modulator generates a probe pulse as a phase-sensitive light time-domain reflection. In the measurement fiber branch, the Rayleigh scattered light output from the measurement fiber interacts with the pump pulse output from the BOTDA branch through a stimulated Brillouin reaction. The backscattered Rayleigh and backscattered Raman light of the pulse pair enter the optical wavelength division multiplexer via an optical circulator, where they are separated according to wavelength before entering the receiving module. The backscattered Rayleigh light of the probe pulse enters the receiving module and interferes with the local light to obtain a current signal. The backscattered Raman light of the pulse pair enters the receiving module and is converted into a current signal to realize Raman time-domain reflectometry (ROTDR) based on Raman scattering. The signal generated by the receiving module is demodulated by the signal acquisition and processing module, and vibration measurement data, temperature and static strain information at different locations and temperature information are obtained simultaneously.

[0007] In the BOTDA branch, the output of the optical modulator is controlled by a DC source and a voltage-controlled oscillator. The output of the voltage-controlled oscillator is controlled by a first radio frequency signal generator, so that its output radio frequency signal is a swept frequency signal within a fixed frequency range.

[0008] The output of the first acousto-optic modulator in the BOTDA branch and The output of the second acousto-optic modulator in the branch is controlled by the second radio frequency signal generator. The first and second acousto-optic modulators generate pump pulses and probe pulses respectively, and the radio frequency signal generator controls their pulse width and interval to achieve three-bit pulse encoding.

[0009] The laser's output end is equipped with a first fiber optic coupler, the input end of the optical modulator is connected to the first fiber optic coupler, the second acousto-optic modulator and the receiving module are respectively connected to the first fiber optic coupler through the second fiber optic coupler, and the output ends of the second acousto-optic modulator and the polarization scrambler are respectively connected to the erbium-doped fiber amplifier through the third fiber optic coupler.

[0010] The splitting ratio of the fiber optic coupler is 50 / 50.

[0011] The receiving module includes: a polarization diversity photodetector for receiving backscattered Rayleigh light and two avalanche photodetectors for receiving backscattered Raman light, wherein: the backscattered Raman light consists of two wavelengths of light, and the two wavelengths of backscattered Raman light are received by the two avalanche photodetectors respectively.

[0012] The signal acquisition and processing module includes: a data acquisition card and a data processor that are interconnected.

[0013] The data processing refers to the following: For the backscattered Rayleigh light signal received by the polarization diversity photodetector and encoded with three-bit pulses, a Hilbert transform is performed, and the phase and amplitude are calculated separately. The phase is used to demodulate the vibration information on the measurement fiber, and the amplitude is used to demodulate the temperature and static strain information on the fiber. For the backscattered Raman light of two wavelengths from the pulse pair received by the two avalanche photodetectors, the same data processing is performed: first, the backscattered Raman light is decoded, and the backscattered Raman light of the two superimposed pulses is separated to obtain the intensity distribution of the backscattered Raman light along the fiber for each pulse, thereby demodulating the temperature change on the fiber. Since backscattered Raman light is not sensitive to strain, combining the temperature demodulation results of the backscattered Raman light with the temperature and strain demodulation results based on the amplitude of the backscattered Rayleigh light can achieve temperature-strain separation.

[0014] The aforementioned three-bit pulse encoding is achieved through pulse pairs composed of periodically generated pump pulses and probe pulses. It includes three different code patterns: "101", "110", and "011". Each code element consists of either a "1" or a "0". In each code pattern, from left to right, the first "1" represents the probe pulse, the second "1" represents the pump pulse, and the "0" represents the interval T between the pump pulse and the probe pulse. p =τ p Where: the pulse width of the pump pulse and the probe pulse, i.e., the symbol length, is τ. p .

[0015] Preferably, in order to ensure that the sampling time interval of the vibration signal remains consistent, the "011" code pattern is shifted forward by one code element when it is generated, so as to ensure that the time interval of the detection pulse in each adjacent code pattern is consistent.

[0016] The code pattern is: generated periodically. That is, the code is generated from the first row to the third row, and then generated again in the same order, with the interval between adjacent code patterns fixed at T. C ≥C' / 2L c Where: C' is the speed of light in the optical fiber, L c To measure the length of the optical fiber.

[0017] Preferably, the probe pulse precedes the pump pulse.

[0018] The demodulation mentioned includes simultaneous operations. Demodulation, Brillouin frequency shift demodulation, and Raman scattering intensity ratio demodulation, wherein:

[0019] Demodulation: After narrowband filtering, the Rayleigh scattering signals corresponding to the three code patterns are directly processed. Demodulation yields vibration measurement data.

[0020] b) Brillouin frequency shift demodulation: The amplitude of the Rayleigh scattering signal corresponding to the three code patterns after the first step of narrowband filtering is taken. Then, the Rayleigh scattering signal at each position is normalized to obtain the corresponding Brillouin gain. The Brillouin gain corresponding to the first code pattern, i.e. "101", is shifted forward by one symbol, i.e., one spatial resolution. The Brillouin gain spectrum can be obtained according to the Brillouin frequency sweep period. The Brillouin frequency shift is obtained through the Brillouin gain spectrum, and then the temperature and static strain information at different positions on the measurement fiber is demodulated.

[0021] c) Raman scattering intensity ratio demodulation: The Raman scattering signal corresponding to the third code pattern, i.e. "011", is shifted back by one code element. Combined with the Raman scattering signals corresponding to the first and second code patterns, i.e. "110", pulse decoding is performed to obtain the Raman scattering signals of the two pulses. The ratio of the Raman scattering anti-Stokes light intensity to the Stokes light intensity is calculated to obtain the temperature information at different locations on the measurement fiber.

[0022] Technical effect

[0023] This invention combines pulse coding technology with a distributed fiber optic sensing system that simultaneously measures the dynamic and static variables of two pulses. It receives and decodes the backscattered Raman light from each of the two pulses, integrating ROTDR with BOTDA. Simultaneous measurement of dynamic and static variables, including temperature and strain, can be achieved within the same system. Based on the existing system, by adding only a three-channel wavelength division multiplexer and two avalanche photodetectors, combined with pulse code modulation, a distributed fiber optic sensing system can be implemented to simultaneously measure dynamic and static variables, including temperature and strain, without affecting the performance of the existing system. Attached Figure Description

[0024] Figure 1 A schematic diagram of a three-bit pulse encoding;

[0025] Figure 2 This is a schematic diagram of the structure of the present invention;

[0026] Figure 3A schematic diagram of temperature and static strain measurement results for an example;

[0027] Figure 4 This is a schematic diagram of the dynamic signal measurement results obtained in an example.

[0028] In the diagram: 1. Main laser; 2. First 50 / 50 fiber coupler; 3. Optical modulator; 4. DC source; 5. First radio frequency signal generator; 6. Voltage-controlled oscillator; 7. First radio frequency signal amplifier; 8. Optical filter; 9. First optical circulator; 10. Polarization controller; 11. DFB laser; 12. First acousto-optic modulator; 13. Second radio frequency signal generator; 14. Second radio frequency signal amplifier; 15. Polarization scrambler; 16. Second 50 / 50 fiber coupler; 17. Second acousto-optic modulator; 18. Third radio frequency signal amplifier; 19. Third 50 / 50 fiber coupler; 20. Erbium-doped fiber amplifier; 21. Second optical circulator; 22. Measurement fiber; 23. Optical wavelength division multiplexer; 24. Polarization diversity photodetector; 25. First avalanche photodetector; 26. Second avalanche photodetector; 27. Data acquisition card; 28. Data processor; 1st port to 3rd port a to c. Detailed Implementation

[0029] like Figure 2As shown in the figure, this embodiment relates to a distributed fiber optic sensing system for temperature and strain separable dynamic and static joint measurement, comprising: a main laser 1, a first 50 / 50 fiber coupler 2, an optical modulator 3, a DC source 4, a first radio frequency signal generator 5, a voltage-controlled oscillator 6, a first radio frequency signal amplifier 7, an optical filter 8, a first optical circulator 9, a polarization controller 10, a DFB laser 11, a first acousto-optic modulator 12, a second radio frequency signal generator 13, a second radio frequency signal amplifier 14, a polarization scrambler 15, a second 50 / 50 fiber coupler 16, a second acousto-optic modulator 17, a third radio frequency signal amplifier 18, a third 50 / 50 fiber coupler 19, an erbium-doped fiber amplifier 20, a second optical circulator 21, a measurement fiber 22, an optical wavelength division multiplexer 23, a polarization diversity photodetector 24, a first avalanche photodetector 25, and a second avalanche photodetector 26. 6. Data acquisition card 27 and data processor 28, wherein: the main laser 1 is connected in sequence to the first 50 / 50 fiber coupler 2, optical modulator 3, optical filter 8, first optical circulator 9, first acousto-optic modulator 12, polarizer 15, third 50 / 50 fiber coupler 19, erbium-doped fiber amplifier 20, second optical circulator 21 and measurement fiber 22; the first radio frequency signal generator 5 is connected to the voltage-controlled oscillator 6, the first radio frequency amplifier 7 and the radio frequency signal input port of the optical modulator 3; the DC source 4 is connected to the DC input port of the optical modulator 3; the DFB laser 11 and polarization controller 10 are connected in sequence and connected to the second port b of the first optical circulator 9; the third port c of the first 50 / 50 fiber coupler 2 is connected in sequence to the second 50 / 50 fiber coupler 16, the second acousto-optic modulator 17 and the second port b of the third 50 / 50 fiber coupler 19. The output port I of CH1 of the second RF signal generator 13 is sequentially connected to the RF signal input ports of the second RF signal amplifier 14 and the first acousto-optic modulator 12. The output port Q of CH1 of the second RF signal generator 13 is sequentially connected to the RF signal input ports of the third RF signal amplifier 18 and the second acousto-optic modulator 17. The third port c of the second optical circulator 21 is connected to the optical wavelength division multiplexer 23. The third port c of the second 50 / 50 fiber coupler 16 and the 1550nm port of the optical wavelength division multiplexer 18 are respectively connected to the LO port and the S port of the polarization diversity photodetector 24. The 1663nm port of the optical wavelength division multiplexer 23 is connected to the first avalanche photodetector 25. The 1450nm port of the optical wavelength division multiplexer 23 is connected to the second avalanche photodetector 26. The polarization diversity photodetector 24, the first avalanche photodetector 25, and the second avalanche photodetector 26 are sequentially connected to the data acquisition card 27 and the data processor 28.The clock output port clockoutput of the first RF signal generator 5, the clock input port Refclock of the second RF signal generator 13, and the clock input port AUXclock of the data acquisition card 27 are connected. The trigger signal output port triggeroutput of the first RF signal generator 5 and the trigger signal input port AUX trigger of the data acquisition card 27 are connected.

[0030] The main laser 1 outputs a laser beam with a wavelength of 1550.28 nm and a constant power of 12 mW, which is then transmitted to the first 50 / 50 fiber coupler 2 and split into two paths: one path serves as the pump light for BOTDA, used to generate pump pulses; the other path serves as... The detection light and the local light.

[0031] In the BOTDA branch, the pump light enters the optical modulator 3, which has two electrical signal input ports: a DC input port and an RF signal input port. A DC source 4 is connected to the DC input port of the optical modulator 3 to control its operating point, causing it to operate in carrier suppression mode (in this embodiment, the optical modulator 3 operates in carrier suppression mode when the output voltage of the DC source 4 is 7.8V).

[0032] The voltage-controlled oscillator 6 outputs a sweep signal, which is then input to the optical modulator 3 after passing through the first radio frequency signal amplifier 7 to excite the sidebands of the frequency sweep. The frequency difference between the excited sideband light and the input carrier light is equal to the frequency of the radio frequency modulation signal. The voltage-controlled oscillator 6 receives a periodic triangular wave voltage signal from the first radio frequency signal generator 5, causing the frequency of the output radio frequency signal of the voltage-controlled oscillator 6 to increase or decrease periodically within a fixed frequency range, which is the frequency sweep process.

[0033] The frequency sweep sidebands are filtered out by optical filter 8 to obtain positive first-order sidebands. After being injected and locked by a DFB laser 9 with a linewidth of ~1MHz and a center wavelength of 1550.02nm, the light is modulated into stable linearly swept continuous light. The linearly swept continuous light enters the first acousto-optic modulator 12 and is modulated into pulsed light in the time domain. The generation of the pulsed light is controlled by the radio frequency signal generated by the second radio frequency signal generator 13. The radio frequency pulse signal generated by the second radio frequency signal generator 13 is amplified by the second radio frequency signal amplifier 14 and then input into the first acousto-optic modulator 12. After passing through the polarization scrambler 15 and the third 50 / 50 fiber coupler 19, the light enters the erbium-doped fiber amplifier 20 for power amplification. The light then enters the measurement fiber 22 through the second optical circulator 21. The backscattered Rayleigh light from the probe pulse undergoes stimulated Brillouin interaction.

[0034] The aforementioned In the process, the detection light is formed by the second acousto-optic modulator 17. The second acousto-optic modulator 17 receives the radio frequency signal output by the second radio frequency signal generator 13. The radio frequency frequency is equal to the operating frequency of the second acousto-optic modulator 17, which is 80MHz, and is amplified by the third radio frequency signal amplifier 18.

[0035] The probe pulse enters the erbium-doped fiber amplifier 20 via the third 50 / 50 fiber coupler 19 for power amplification, and then enters the measurement fiber 22 via the second optical circulator 21.

[0036] The local light and the light passing through the second optical circulator 21 and the optical wavelength division multiplexer 23 The backscattered Rayleigh light of the probe pulse enters the polarization diversity photodetector 24 and interferes. The back Raman-Stokes signal and the anti-Stokes signal of the two pulses are received by the first avalanche photodetector 25 and the second avalanche photodetector 26 respectively through the optical wavelength division multiplexer 23. The signal is photoelectrically converted into a current signal I(t). Finally, the current signal is acquired and processed by the data acquisition card 27 and the data processor 28.

[0037] In the measurement fiber 22, the backscattering of the pump pulse and the probe pulse includes Stokes light with a wavelength channel of 1663 nm, anti-Stokes light with a wavelength channel of 1450 nm, and backscattered Rayleigh light with a wavelength channel of 1550 nm from the probe pulse, which is amplified by a stimulated Brillouin interaction with the pump pulse. The backscattered Raman light of the two pulses and the backscattered Rayleigh light of the Rayleigh probe pulse are separated according to their different wavelengths by the optical wavelength division multiplexer 23 and received by the polarization diversity photodetector 24 and the avalanche photodetectors 25 and 26, respectively.

[0038] When the pulsed light power and the attenuation coefficient along the fiber are constant, the power of the backscattered Stokes light and anti-Stokes light is only related to the temperature. This system uses this temperature characteristic to measure the temperature in the fiber. Specifically, based on the measurement results of avalanche photodetectors 25 and 26, the power ratio of the Stokes light and anti-Stokes light generated by a pulse at the same temperature is first calculated to eliminate the influence of possible pulsed light power changes. Then, a reference temperature is selected, and the ratio of other temperatures to the reference temperature is calculated to eliminate the influence of fiber attenuation on the measurement. The calculated ratio and temperature can be regarded as linearly correlated, i.e., optical time-domain analysis (ROTDR) based on Raman scattering.

[0039] The gain of the Brillouin amplification is: Where: g0 is the peak gain coefficient, which is related to the fiber type. The typical value of the peak gain coefficient for ordinary single-mode fiber is g0 = 5.0 × 10⁻⁶. -11 m / W; Γ BΩ represents the Brillouin gain bandwidth, typically tens of megahertz in ordinary single-mode fiber; Ω is the frequency difference between the pump and probe light; Ω B For the Brillouin frequency shift of the optical fiber; when Ω = Ω B At this point, the gain of the probe light reaches its maximum value. In this embodiment, the Brillouin gain of the probe light under different frequency differences between the pump light and the probe light can be obtained through the frequency sweep process of the first RF signal generator 5 and the voltage-controlled oscillator 6. By plotting the frequency difference-amplitude diagram of the signal at each position on the optical fiber, the Brillouin frequency shift at that position can be obtained through the frequency difference corresponding to the peak position of the graph, and then the temperature and strain information at that position can be obtained through the temperature and strain coefficient.

[0040] The aforementioned Demodulation is as follows: Since vibration on the optical fiber will change the phase of the Rayleigh scattered light back of the optical fiber, the real signal received by the polarization diversity photodetector 24 can be converted into a complex signal after Hilbert transformation. The vibration signal on the optical fiber can be detected by taking the phase and phase difference of the signals of different pulse pairs within a certain period of time.

[0041] In the distributed optical fiber sensing described above, the time it takes for backscattered light from different positions on the measurement optical fiber 22 to reach the polarization diversity photodetector 24 and the avalanche photodetectors 25 and 26 satisfies the following condition: L = c'·2t, where: L is the distance between different positions on the measurement optical fiber 22 and the starting point of the optical fiber, c' is the speed of light in the measurement optical fiber 22, and t is the time it takes for backscattered light from different positions on the measurement optical fiber 22 to reach the polarization diversity photodetector 24 and the avalanche photodetectors 25 and 26.

[0042] By demodulating the signals received by the receiving modules at different times, the dynamic and static variable information corresponding to different positions L on the measurement fiber 22 can be obtained.

[0043] This embodiment relates to a distributed fiber optic sensing method for the joint dynamic and static measurement of temperature strain in the above-mentioned system, which includes the following steps:

[0044] Step 1: Clock synchronization: Synchronize the internal clocks of the first RF signal generator 5, the second RF signal generator 13, and the data acquisition card 27.

[0045] Step 2: Set the frequency sweep Brillouin pump light: Based on the desired Brillouin gain spectrum range Δf, step size δf, and interval T between two pulse pairs... c The amplitude and period of the triangular wave signal output by the first RF signal generator 5 are determined. The triangular wave signal is used to drive the voltage-controlled oscillator 6 to achieve the frequency sweep process. The desired Brillouin gain spectrum range needs to cover the Brillouin frequency shift Ω of the measuring fiber 22 under normal conditions (i.e., without temperature and additional strain changes). BThe Brillouin shift is a known quantity. The amplitude of the triangular wave signal is determined by the Brillouin shift and the gain spectrum range. Plotting the gain spectrum requires N = Δf / δf Rayleigh scattering curves; therefore, the period of the triangular wave signal is N*T. c Since this scheme uses three-bit pulse encoding, the pulse pairs of the three code types appear periodically. To facilitate demodulation and subsequent averaging, N is taken as a multiple of 3. To synchronize the trigger time of the data acquisition card 27 with the frequency sweep start time of the voltage-controlled oscillator 6, the first RF signal generator 5 operates in burst mode, and the time interval is set to 3*T. c The trigger signal output terminal of the first RF signal generator 5 is connected to the external trigger signal input terminal AUXTRIGGER of the data acquisition card 27. The RF signal generated by the voltage-controlled oscillator 6 is modulated by the first RF signal amplifier 7. The modulated output light is connected to the spectrum analyzer for observation, and the output voltage of the DC source 4 is adjusted to make the modulator work in carrier suppression state. The optical path is restored, and the parameters of the optical filter 8 are adjusted so that only the positive first-order sideband of the output light of the optical modulator 3 can pass through the optical filter 8. The output of the optical filter 8 is the pump light, which is used to generate pump pulses. In this embodiment, the frequency sweep range of the voltage-controlled oscillator 6 is 180MHz, 300 pulse pairs correspond to one frequency sweep process, and the frequency sweep step size is 0.6MHz. The Brillouin frequency shift of the optical fiber at room temperature and without strain is measured to be 10.75GHz. The amplitude of the triangular wave is adjusted so that the pump-probe pulse frequency difference corresponding to the midpoint of the frequency sweep is equal to the Brillouin frequency shift. The optical fiber used is approximately 10km long, and the triangular wave period of the radio frequency signal generator 5 is 30ms, with an interval of 300us.

[0046] Step 3: Modulation of the optical pulse pair: The control signals for the two acousto-optic modulators are output from the two output ports (I and Q) of the same channel of the second radio frequency signal generator. The only difference between the two is that they are inverted; all other parameters are the same. Since the two acousto-optic modulators operate at different frequencies, when one acousto-optic modulator is normally transmitting light, the other is not. This allows the same radio frequency signal to control the two acousto-optic modulators to generate pulses sequentially. This method facilitates control over the time interval between the two pulses. The pulse modulation method refers to the specific implementation process of the three-bit pulse encoding described in the invention description above.

[0047] Step 4: Polarization Scrambling and Pulse Amplification: Turn on the polarization scrambling device. The pump pulse and probe pulse after polarization are coupled into one path and amplified by the erbium-doped fiber amplifier 20. The peak power of the amplified pulse should not be too high to avoid nonlinear effects that could affect the sensing distance. First, observe the Rayleigh scattering signal acquired by the data acquisition card 28. Amplify the pulse by increasing the current parameter of the erbium-doped fiber amplifier 20, provided that there is no sudden drop in power at the fiber end.

[0048] Step 5: Data Acquisition: The signals from the polarization diversity photodetector 24 and the two avalanche photodetectors 25 and 26 are output to different channels of the data acquisition card 27, and the data from each channel is acquired simultaneously. To improve the signal-to-noise ratio of the measurement results, multiple measurements need to be acquired and averaged. For the polarization diversity photodetector 24, in order to receive only the target signal (backscattered Rayleigh light from the probe pulse) and filter out the backscattered Rayleigh light from the pump pulse, the operating bandwidth of the polarization diversity photodetector 24 needs to be greater than the beat frequency of the target signal and less than the beat frequency of the backscattered Rayleigh light from the pump pulse. In this embodiment, the operating bandwidth of the polarization diversity photodetector 24 is AC-1.6GHz. For the avalanche photodetectors 25 and 26, the bandwidth requirement is greater than the pulse frequency. In this embodiment, the operating bandwidth of the avalanche photodetectors 25 and 26 is 100MHz. For the data acquisition card 27, the sampling rate is required to conform to the Nyquist sampling theorem for the target signal. In this embodiment, the sampling rate is 250MSa / s.

[0049] Step 6: Data demodulation, specifically including:

[0050] 6.1) First, align the starting positions of each group of signals, that is, each pulse pair, as a complete scattering signal along the optical fiber, and ensure that the length of each group of signals is equal.

[0051] 6.2) The signals acquired by the polarization diversity photodetector 24 are subjected to ① vibration signal demodulation and ② temperature and strain demodulation.

[0052] 6.3) Avalanche photodetectors 25 and 26 receive backscattered Raman light signals at 1663nm and 1450nm respectively and perform ③ROTDR demodulation respectively;

[0053] The aforementioned vibration signal demodulation, i.e. In the demodulation process, 800 groups of signals are used for demodulation in this embodiment. After aligning the start time of each group of signals, the phase of each group of signals is extracted and differentially analyzed to demodulate the vibration signal.

[0054] The temperature and strain demodulation mentioned above refers to the synchronization of the trigger signal of the RF signal generator 5 and the frequency sweep start point of the voltage-controlled oscillator 6. The time of one complete frequency sweep process is the period of the triangular wave. Through continuous trigger signals, the back Rayleigh scattering signal under multiple frequency sweep processes can be obtained.

[0055] The vibration signal demodulation and temperature and strain demodulation preferably use the measurement results from multiple frequency sweep processes. First, the amplitude of each group of backscattered Rayleigh signals is calculated, processed according to the demodulation steps described above, and then the amplitudes of multiple measurements with the same frequency difference are summed. After summing, the amplitude without Brillouin gain is taken as a reference (the amplitude of the scattered light corresponding to the frequency sweep start point is taken) to calculate the Brillouin gain for each frequency difference, thus obtaining the temperature and strain information on the optical fiber.

[0056] The ROTDR demodulation method preferably adds up the signal strengths at the same location measured multiple times and takes the average to improve the signal-to-noise ratio. In this embodiment, the average number of measurements is 1.2 million.

[0057] Step 7: Calculation of temperature strain separation. Brillouin frequency shift Δν V The linear relationship between the temperature change ΔT and the strain change Δε is Δν B =c 11 Δε+c 12 ΔT, where c 11 Let c be the Brillouin strain coefficient. 12 Let R be the Brillouin temperature coefficient. The relationship between the Raman scattering ratio R(T) and the temperature change ΔT is given by ΔR(T) = c. 22 ΔT, where: c 22 Here, represents the Raman temperature coefficient. The Brillouin strain coefficient, temperature coefficient, and Raman temperature coefficient can be considered known constants given a fixed fiber and wavelength. Based on the above two formulas, the temperature and static strain information at different locations L on the measuring fiber 22 can be demodulated by combining the measured Brillouin frequency shift and the ratio of Raman scattering intensity.

[0058] Through specific experiments, a 120-meter temperature variation section, a 30-meter strain variation section, and a 10-meter vibration point were set at the rear end of an 8600-meter-long G.652 single-mode optical fiber. The fiber at all three sections was G.652 single-mode. The fiber at the temperature variation section was placed in a water bath, and its temperature was changed by altering the water temperature. The fiber at the strain variation section was fixed at both ends to horizontal displacement stages, and the fiber was stretched by changing the distance between the two stages, thus inducing strain. The fiber at the vibration point was wound and fixed onto a piezoelectric ceramic cylinder. Changing the voltage applied to the piezoelectric ceramic caused it to expand accordingly, thus applying dynamic strain, i.e., vibration, to the wound fiber. The static temperature and strain measurement results are as follows: Figure 3 As shown, Figure 3 (a) shows the temperature measurement results from ROTDR. The measured Raman scattering ratio and temperature exhibit a good linear relationship, as shown in Figure 1. Figure 3 As shown in (c) Figure 3(b) shows the temperature and strain measurement results from BOTDA. The measured Brillouin frequency shift, temperature, and strain exhibit a good linear relationship, as shown in the figure. Figure 3 As shown in (d), the temperature and strain measurement accuracies after temperature-strain separation are 0.5 degrees Celsius and 21.36 microstrains, respectively. The vibration measurement results are as follows... Figure 4 As shown, sinusoidal vibrations of 1 kHz can be detected and reproduced in the experiment, with a measurement accuracy of up to [percentage missing].

[0059] In this embodiment, the pulse widths of both the Brillouin pump pulse and the Rayleigh probe pulse are 100 ns, i.e., BOTDA, All three—along with ROTDR—have a spatial resolution of 10 meters. The peak power of the two pulses is the same, at 176mW.

[0060] Compared with existing technologies, this invention, based on a joint distributed fiber optic sensing system capable of simultaneously measuring dynamic and static variables, achieves temperature and strain separation measurement by incorporating Raman scattering in the optical fiber within the joint system for simultaneous dynamic and static measurements. Pulse coding is used to separate the Raman scattering of two pulses, reducing the number of measurements while maintaining system spatial resolution. Furthermore, existing pulse coding patterns are adjusted, and decoding processes are designed separately for the three different subsystems. This allows pulse coding technology to be seamlessly integrated with the system's demodulation steps without affecting the system's spatial resolution, frequency response, or measurement accuracy. This successfully realizes a distributed fiber optic sensing system for dynamic and static joint measurement of temperature and strain separation, achieving temperature measurement accuracy of 0.5℃ and strain measurement accuracy of 21.36 με after temperature and strain separation.

[0061] The above-described specific implementations can be partially adjusted by those skilled in the art in different ways without departing from the principles and purpose of the present invention. The scope of protection of the present invention is defined by the claims and is not limited to the above-described specific implementations. All implementation schemes within the scope of the claims are bound by the present invention.

Claims

1. A distributed optical fiber sensing system for joint dynamic and static measurement, characterized in that, include: The system sequentially comprises a laser, an optical modulator, a radio frequency signal generator, an optical filter, a DFB laser, a polarization controller, a first acousto-optic modulator, a polarization scrambler, an erbium-doped fiber amplifier, two optical circulators, an optical wavelength division multiplexer, a receiving module, and a signal acquisition and data processing module. The measurement fiber, the erbium-doped fiber amplifier, and the optical wavelength division multiplexer are respectively located at the three ports of the second optical circulator. A second acousto-optic modulator, with its signal terminal connected to the optical modulator, is further located between the laser and the erbium-doped fiber amplifier. The receiving module is connected to the laser to receive local light. The optical modulator, optical filter, DFB laser, polarization controller, first optical circulator, first acousto-optic modulator, and polarization scrambler constitute a Brillouin optical time-domain analysis (BOTDA) branch. The second acousto-optic modulator... The probe pulse serves as the phase-sensitive optical time-domain reflectometry (OTDR) branch, where the Rayleigh scattering light output from the measurement fiber interacts with the pump pulse output from the BOTDA branch via stimulated Brillouin interaction. The backscattered Rayleigh and backscattered Raman light from the pulse pair enter the optical wavelength division multiplexer via a second optical circulator, are separated according to wavelength, and then enter the receiving module. The backscattered Rayleigh light from the φ-OTDR probe pulse enters the receiving module and interferes with the local light to obtain a current signal. The backscattered Raman light from the pulse pair enters the receiving module and is converted into a current signal to achieve Raman time-domain reflectometry based on Raman scattering. The signal generated by the receiving module is demodulated by the signal acquisition and data processing module, which simultaneously obtains vibration measurement data, temperature and static strain information at different locations, and temperature information.

2. The distributed optical fiber sensing system for combined dynamic and static measurement according to claim 1, characterized in that, In the BOTDA branch, the output of the optical modulator is controlled by a DC source and a voltage-controlled oscillator. The output of the voltage-controlled oscillator is controlled by a first radio frequency signal generator, so that its output radio frequency signal is a swept frequency signal within a fixed frequency range.

3. The distributed optical fiber sensing system for combined dynamic and static measurement according to claim 1, characterized in that, The output of the first acousto-optic modulator in the BOTDA branch and the output of the second acousto-optic modulator in the φ-OTDR branch are both controlled by the second radio frequency signal generator. The first and second acousto-optic modulators generate pump pulses and probe pulses respectively, and the radio frequency signal generator controls their pulse width and interval to achieve three-bit pulse encoding. The aforementioned three-bit pulse encoding is achieved through pulse pairs composed of periodically generated pump pulses and probe pulses. It includes three different code patterns: "101", "110", and "011". Each code pattern consists of either a "1" or a "0". From left to right, the first "1" represents the probe pulse, the second "1" represents the pump pulse, and the "0" represents the interval T between the pump pulse and the probe pulse. p =τ p Where: the pulse width of the pump pulse and the probe pulse, i.e., the symbol length, is τ. p .

4. The distributed optical fiber sensing system for combined dynamic and static measurement according to claim 1, characterized in that, The laser's output end is equipped with a first fiber optic coupler, the input end of the optical modulator is connected to the first fiber optic coupler, the second acousto-optic modulator and the receiving module are respectively connected to the first fiber optic coupler through the second fiber optic coupler, and the output ends of the optical modulator and the polarization scrambler are respectively connected to the erbium-doped fiber amplifier through the third fiber optic coupler.

5. The distributed optical fiber sensing system for combined dynamic and static measurement according to claim 3, characterized in that, The receiving module includes: a polarization diversity photodetector for receiving backscattered Rayleigh light and two avalanche photodetectors for receiving backscattered Raman light, wherein: the backscattered Raman light consists of two wavelengths of light, and the two wavelengths of backscattered Raman light are received by the two avalanche photodetectors respectively. For the backscattered Rayleigh light signal of the pulsed light received by the polarization diversity photodetector and encoded with three pulses, the phase and amplitude are calculated after Hilbert transformation. The phase is used to demodulate the vibration information on the measurement fiber, and the amplitude is used to demodulate the temperature and static strain information on the fiber. For the backscattered Raman light of the two wavelengths of the pulse pair received by the two avalanche photodetectors, the same data processing is performed: first, the backscattered Raman light is decoded, and the backscattered Raman light of the two pulses superimposed is separated to obtain the intensity distribution of the backscattered Raman light of the two pulses along the fiber for each pulse. The temperature change on the fiber is then demodulated. Since the backscattered Raman light is not sensitive to strain, the temperature and strain separation effect can be achieved by combining the temperature demodulation result of the backscattered Raman light and the temperature and strain demodulation result based on the amplitude of the backscattered Rayleigh light.

6. The distributed optical fiber sensing system for combined dynamic and static measurement according to claim 3, characterized in that, The "011" code pattern was shifted forward by one code element to ensure that the time interval of the probe pulses in each adjacent code pattern is consistent. The code pattern is: generated periodically. That is, it starts generating from the first row to the third row, and then generates again in the same order, with a fixed interval between adjacent code patterns. ,in: To measure the speed of light in an optical fiber, L c To measure the length of the optical fiber.

7. The distributed fiber optic sensing system for combined dynamic and static measurement according to claim 1 or 5, characterized in that, The demodulation includes simultaneous φ-OTDR demodulation, Brillouin frequency shift demodulation, and Raman scattering intensity ratio demodulation, wherein: a) φ-OTDR demodulation: After narrowband filtering, the Rayleigh scattering signals corresponding to the three code patterns are directly demodulated by φ-OTDR to obtain the vibration measurement data; b) Brillouin frequency shift demodulation: The amplitude of the Rayleigh scattering signal corresponding to the three code patterns after the first narrowband filtering is taken. Then, the Rayleigh scattering signal at each position is normalized to obtain the corresponding Brillouin gain. The Brillouin gain corresponding to the first code pattern, i.e. "101", is shifted forward by one symbol distance, i.e., one spatial resolution distance. The Brillouin gain spectrum can be obtained according to the Brillouin frequency sweep period. The Brillouin frequency shift is obtained through the Brillouin gain spectrum, and then the temperature and static strain information at different positions on the measurement fiber is demodulated. c) Raman scattering intensity ratio demodulation: The Raman scattering signal corresponding to the third code pattern, i.e. "011", is shifted back by one code element. Combined with the Raman scattering signals corresponding to the first and second code patterns, i.e. "110", pulse decoding is performed to obtain the Raman scattering signals of the two pulses. The ratio of the Raman scattering anti-Stokes light intensity to the Stokes light intensity is calculated to obtain the temperature information at different locations on the measurement fiber.

8. A distributed fiber optic sensing method for joint dynamic and static measurement of temperature strain based on any one of the systems described in claims 1-7, characterized in that, Includes the following steps: Step 1: Clock synchronization; Step 2: Set the frequency sweep Brillouin pump light; Step 3: Modulation of optical pulse pairs; Step 4: Polarization interference and pulsed light amplification; Step 5: Data Acquisition; Step 6: Data demodulation; Step 7: Calculation of temperature strain separation: Brillouin frequency shift and temperature change and strain change The linear relationship between them is , where: c 11 Let c be the Brillouin strain coefficient. 12 Let R(T) be the Brillouin temperature coefficient, the Raman scattering ratio, and the temperature change. The relationship between them is , where: c 22 The temperature and static strain information at different locations L on the measuring fiber are obtained by combining the measured Brillouin frequency shift and the ratio of Raman scattering intensity with the Raman temperature coefficient.

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