Signal-to-noise ratio improvement method based on multi-core optical fiber dual-channel distributed acoustic sensing system

Abstract

The invention discloses a signal-to-noise ratio improvement method based on a multi-core optical fiber dual-channel distributed acoustic sensing system, and the method comprises the steps: carrying out the light splitting of light emitted by a narrow linewidth laser through a first coupler C1, enabling one path of light to generate a pulse signal through an acoustic optical modulator (AOM), enabling the other path of light to pass through a second coupler C2, and enabling the other path of light to pass through a second coupler C1; the pulse light is divided into two beams of transmission light, the two beams of transmission light are injected into a connected sensing optical fiber through two groups of erbium-doped optical fiber amplifiers (EDFA), two groups of filters, two groups of circulators and a fan-in coupler Fan-in Coupler, and the pulse light is subjected to backward Rayleigh scattering in the sensing optical fiber; based on a dual-channel DAS system, a multi-core optical fiber is used as a sensing optical fiber, three cores and four cores of the multi-core optical fiber are connected in series to form two sensing channels, seven-core space division signals are multiplexed, the equivalent self-noise pressure suppression effect of the seven-core space division signals is analyzed, after noise correlation is removed through a time domain dislocation-frequency domain phase shift method, the signal-to-noise ratio gain can be increased to 8.65 dB, and the signal-to-noise ratio gain can be increased to 8.65 dB. And the detection capability of the system on low-frequency weak signals can be improved.

Description

Technical Field

[0001] The invention belongs to the technical field of underwater acoustic detection of distributed acoustic sensing systems, and in particular relates to a method for improving a signal-to-noise ratio based on a multi-core optical fiber dual-channel distributed acoustic sensing system. Background Art

[0002] Fiber-optic distributed acoustic sensing (DAS) technology injects the detection pulse laser emitted by the transmitter into the sensing fiber. Affected by impurities in the sensing fiber, the detection light undergoes coherent back-Rayleigh scattering and returns to the transmitter. Phase demodulation can obtain the strain information of any section of the sensing fiber, and then restore the acoustic wave signal. Therefore, the sensing fiber is a closely arranged acoustic wave sensor. DAS has the characteristics of simple system structure, high spatial resolution, strong anti-electromagnetic interference, and flexible array structure. Therefore, it can provide a new technical means for ocean underwater acoustic sensing and detection. Common DAS systems usually use single-mode optical fiber as the sensing fiber. The back-Rayleigh scattered light is extremely weak during long-distance transmission, and the system equivalent self-noise pressure spectrum level is high. Due to this limitation, the minimum detectable sound pressure performance of the traditional DAS system is poor, and there is a certain difficulty in detecting weak acoustic wave signals.

[0003] In order to improve the signal-to-noise ratio of single-mode fiber DAS systems, researchers have introduced special optical fibers to improve system performance, such as continuous scattering enhancement fiber, discrete scattering enhancement fiber with ultra-weak fiber Bragg grating (UWFBG) or scattering enhancement points (SEP), and fiber optic sensitivity enhancement. The continuous scattering enhancement scheme mainly includes writing continuous gratings and changing fiber doping. However, while enhancing the amplitude of backward Rayleigh scattering, it will also cause the optical power loss to increase exponentially, greatly reducing the detection distance. The bandwidth of UWFBG is narrow. In special environments such as high pressure and low temperature, the wavelength of the detection light will mismatch with the reflection wavelength of UWFBG, resulting in a sensing blind area. Therefore, it cannot be used in special environments such as underwater. Sensitization fiber mainly improves the sensitivity of the signal phase to the change of sound pressure by spiral winding and secondary coating. Spiral winding sensitivity enhancement fiber can significantly reduce the equivalent self-noise pressure of the system, but the diameter of the publicly reported spiral sensitivity enhancement structure is not less than 12.5mm, which is difficult to apply to some occasions that require small-sized hydrophones. Secondary coating sensitivity enhancement is to coat a layer of sound pressure sensitive material on the surface of the optical fiber to improve the sound wave detection performance. Its sensitivity is lower than that of the spirally enhanced sensitized optical fiber and it is difficult to meet the needs.

[0004] Researchers have conducted research on the in-phase superposition of output signals from multiple detection arrays, but they mostly conducted sensitivity analysis on high-frequency signals. In fact, as the sensitivity increases, the noise also increases. If the noise of each array element is correlated, the signal-to-noise ratio of the system cannot be improved, and the quality of the acquired signal cannot be guaranteed. In addition, since the energy of low-frequency noise decays slowly during propagation, it is easy to be mixed with low-frequency signals, making it difficult to detect low-frequency signals. Therefore, improving the signal-to-noise ratio of the signal is of great significance to achieve high-precision detection of weak signals. Summary of the invention

[0005] Aiming at the problem that the equivalent self-noise pressure of the existing DAS system is high and the detection capability of weak signals is insufficient, the present invention provides a method for improving the signal-to-noise ratio based on a multi-core optical fiber dual-channel distributed acoustic sensing system. Based on the dual-channel DAS detection and demodulation system, the same detection pulse light emitted by the dual channels is ensured to be simultaneously emitted, which is the premise for the subsequent signal to be superimposed. Multi-core fiber (MCF) is used as the sensing optical fiber, and three cores and four cores are connected in series to form two sensing channels, and connected to the DAS module. The response of each core to external disturbance is consistent but the noise is inconsistent. The signals of the seven cores at the same position are superimposed to achieve the improvement of the signal-to-noise ratio, which can improve the detection capability of the system to low-frequency weak signals, and is of great significance for the detection of low-frequency weak signals.

[0006] To solve the above problems, the technical solution provided by the present invention is as follows:

[0007] The embodiment of the present invention provides a method for improving the signal-to-noise ratio based on a multi-core optical fiber dual-channel distributed acoustic sensing system, comprising the following steps:

[0008] Step 1, the light emitted by the narrow linewidth laser Laser is split by the first coupler C1, one beam of light passes through the acousto-optic modulator AOM to generate a pulse signal, and then is split into two beams of transmission light by the second coupler C2, which are respectively injected into the connected sensing optical fiber through two groups of erbium-doped fiber amplifiers EDFA, two groups of filters Filter, two groups of circulators Circulator, and fan-in coupler Fan-in Coupler. The pulse light is back-scattered by Rayleigh in the sensing optical fiber; wherein the splitting ratio of the first coupler C1 is 98:1:1, and the splitting ratio of the second coupler C2 is 1:1;

[0009] Step 2, when the sensing optical fiber is subjected to the effect of sound pressure, the phase of the backscattered light changes, and interferes with the two 1% local oscillator lights separated by the laser in the third coupler C3 and the fourth coupler C4 respectively. After the photoelectric conversion is completed by the first balanced detector BPD1 and the second balanced detector BPD2, it is acquired by the data acquisition card DAQ, and the phase distribution of the scattered light in the entire sensing optical fiber is obtained by heterodyne phase detection, and then the sound pressure fluctuation information can be obtained from the phase difference of the scattered light between different positions; wherein, the splitting ratios of the third coupler C3 and the fourth coupler C4 are both 1:1;

[0010] Step 3: The sensing optical fiber is provided with seven cores, and the seven cores are spatially multiplexed, that is, there are seven array elements. When a certain position of the sensing optical fiber is vibrated, the seven cores of the sensing optical fiber demodulate the vibration signals and superimpose them. First, the noise correlation of the system is analyzed, and then the improvement of the signal-to-noise ratio of the system before and after superposition is analyzed under the premise that the noise is correlated; further, the noise is decorrelated, and then the improvement of the signal-to-noise ratio is analyzed;

[0011] Extract the 1st, 2nd, 3rd, 4th, 5th, 6th, 7th, 8th... pulses from the core Core1 of the seven cores, extract the 2nd, 3rd, 4th, 5th, 6th, 7th, 8th... pulses from the core Core2 of the seven cores, and so on, extract the 7th, 8th... pulses from the core Core7 of the seven cores. Since the noise is random, the correlation of noise at different times is weak, and the correlation of noise can be reduced. The pulses in the seven cores are demodulated respectively. At this time, the demodulated signals are not in phase. If they are directly accumulated, the signal gain will not be Increase, reverse and decrease. To ensure the in-phase superposition of signals, the time domain signal is transformed from the time domain to the frequency domain through Fourier transform, and phase shifting is performed in the frequency domain. Taking the phase of the signal in the fiber core Core1 as the benchmark, the fiber core Core2 needs to move the phase forward by 1 pulse point, and the fiber core Core3 needs to move the phase forward by 2 pulse points, and so on. The phases of the signals demodulated from all fiber cores are aligned with the phase of the fiber core Core1, and then inverse Fourier transform is performed to the time domain. In this way, noise decorrelation is achieved while the in-phase superposition of signals is achieved, which can further improve the signal-to-noise ratio.

[0012] In a preferred embodiment of the present invention, the fiber core Core7 in step 3 is located at the center of the seven fiber cores, and the other six fiber cores are evenly distributed around the fiber core Core7;

[0013] The first circulator Circulator1 in the two groups of circulators injects the incident light of the first channel CH1 into the fiber core Core1, and the tail end of the fiber core Core1 is connected in series with the tail end of the fiber core Core2. At this time, the incident light is transmitted back to the head end of the fiber core Core2, and the head end of the fiber core Core2 is connected in series with the head end of the fiber core Core3, and the tail end of the fiber core Core3 is connected in series with the tail end of the fiber core Core4. The incident light of the first channel CH1 is transmitted in a serpentine shape through the fiber core Core1, the fiber core Core2, the fiber core Core3, and the fiber core Core4 in sequence;

[0014] The second circulator Circulator2 in the two groups of circulators injects the incident light of the second channel CH2 into the fiber core Core5, and the tail end of the fiber core Core5 is connected in series with the tail end of the fiber core Core6. At this time, the incident light is transmitted back to the head end of the fiber core Core6, and the head end of the fiber core Core6 is connected in series with the head end of the fiber core Core7. The incident light of the second channel CH2 is transmitted in a serpentine shape through the fiber core Core5, the fiber core Core6 and the fiber core Core7 in sequence.

[0015] A preferred embodiment of the present invention further includes: Step 4, assuming m array element signals s with consistent sensitivity 1 (t), s 2 (t),…,s m (t) linear addition, the average power of the output signal is:

[0016]

[0017] In formula (1), a is the proportional coefficient and the average noise power is:

[0018]

[0019] make The average signal-to-noise ratio of the array can be expressed as:

[0020]

[0021] Among them, (ρ s ) ij ,(ρ n ) ij represent the mutual correlation coefficient between the signal and noise between the i-th array element and the j-th array element respectively;

[0022]

[0023] The logarithm of the ratio of the accumulated signal-to-noise ratio to the signal-to-noise ratio of a single array element is the array gain:

[0024]

[0025] The gain after array accumulation depends on the mutual correlation coefficient between the signal and the noise between the array elements. When the signal is completely correlated but the noise is completely uncorrelated,

[0026]

[0027] The array gain after the accumulation of m array elements is AG = 10lgm. When the signal is completely correlated but the noise is partially correlated,

[0028] Fiber core Core1, fiber core Core2, fiber core Core3, fiber core Core4, fiber core Core5, fiber core Core6 and fiber core Core7 are space-division multiplexed, that is, there are seven array elements. When a certain position of the sensor fiber is vibrated, the seven cores of the sensor fiber all demodulate the vibration signal and superimpose it. First, the noise correlation of the system is analyzed, and then the signal-to-noise ratio improvement of the system before and after superposition is analyzed under the premise that the noise is correlated; further, the noise is decorrelated, and then the signal-to-noise ratio improvement is analyzed.

[0029] A preferred embodiment of the present invention further includes: step 5, calculating the correlation coefficient matrix of the seven core noises by equation (5):

[0030]

[0031] Among them, i=1,2,3,4,5,6,7, j=1,2,3,4,5,6,7, then When the same signal acts on seven fiber cores, that is, the signals are completely correlated, (ρ s ) ij =1, after seven fiber cores are added together, the system equivalent self-noise pressure can be reduced Due to the existence of noise correlation, the equivalent self-noise pressure suppression effect after the seven cores are added is lower than the theoretical value AG=10lg 7=8.45dB.

[0032] A preferred embodiment of the present invention also includes: step 6, assuming that the vibration frequency is 500 Hz, the time domain signal Time Domain Signal and PSD curve before and after the demodulated phase signal is accumulated, the PSD of the seven cores at 500 Hz is -33.68 dB, the noise average is -80.92 dB, and the signal-to-noise ratio is 47.24 dB; after the seven core signals are accumulated, the PSD at 500 Hz is -16.77 dB, the noise average is -70.29 dB, and the signal-to-noise ratio is 53.52 dB; therefore, the signal-to-noise ratio is improved by 6.28 dB, that is, the system equivalent self-noise pressure is reduced by 6.28 dB; the average PSD gain after accumulation of the PSD data before and after the accumulation of the seven cores at four frequency points reaches 6.33 dB, that is, the system equivalent self-noise pressure is reduced by 6.33 dB, which is close to the calculated value.

[0033] An embodiment of the present invention provides an electronic device, comprising: at least one memory for storing computer programs; and at least one processor for executing the programs stored in the memory. When the programs stored in the memory are executed, the processor is used to execute a method for improving the signal-to-noise ratio based on a multi-core optical fiber dual-channel distributed acoustic sensing system according to the above embodiment.

[0034] An embodiment of the present invention provides a computer-readable storage medium, which stores a computer program. When the computer program runs on a processor, the processor executes a method for improving the signal-to-noise ratio of a multi-core optical fiber dual-channel distributed acoustic sensing system as described in the above embodiment.

[0035] Compared with the prior art, the embodiment of the present invention provides a method for improving the signal-to-noise ratio based on a multi-core optical fiber dual-channel distributed acoustic sensing system, which has the following beneficial effects: the present invention is based on a dual-channel DAS system, with multiple optical cores as sensing optical fibers, and three or four of them are connected in series to form two sensing channels and seven sensing array elements, and the seven-core space division signals are multiplexed, and the equivalent self-noise pressure suppression effect is analyzed; the results show that the sum of the noise correlation coefficients of the seven optical fiber cores is 11.28, and after the seven independent optical fiber core signals are multiplexed, the system equivalent self-noise pressure can be reduced by 6.33dB in the frequency band of 100 to 1000Hz. After the time domain dislocation-frequency domain phase shift method is used to remove the correlation, the signal-to-noise ratio gain can be increased to 8.65dB, which can improve the system's detection capability for low-frequency weak signals. The suppression of equivalent self-noise pressure is of great significance for improving the accuracy of the system's restoration of vibration signals and enhancing the system's detection capability for low-frequency weak signals. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the technical solutions in the embodiments or the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0037] Figure 1 A schematic diagram of a framework of a method for improving the signal-to-noise ratio based on a multi-core optical fiber dual-channel distributed acoustic sensing system provided in an embodiment of the present application.

[0038] Figure 2 A schematic diagram of a multi-core sensing optical fiber connection based on a fan-in / fan-out coupler provided in an embodiment of the present application.

[0039] Figure 3 A schematic diagram of a signal-to-noise ratio improvement method based on time domain misalignment and frequency domain phase shifting provided in an embodiment of the present application.

[0040] Figure 4 A schematic diagram of a time domain signal Time Domain Signal before and after the accumulation of the demodulated phase signal when the vibration frequency is 500 Hz provided in an embodiment of the present application.

[0041] Figure 5 A schematic diagram of the PSD curves of the demodulated phase signals before and after accumulation when the vibration frequency is 500 Hz provided in an embodiment of the present application. DETAILED DESCRIPTION

[0042] The technical solutions in the embodiments of the present application will be described clearly and completely below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present application.

[0043] refer to Figure 1 The embodiment of the present invention provides a method for improving the signal-to-noise ratio based on a multi-core optical fiber dual-channel distributed acoustic sensing system, comprising the following steps:

[0044] Step 1, the light emitted by the narrow linewidth laser Laser is split by the first coupler C1, one beam of light passes through the acousto-optic modulator AOM to generate a pulse signal, and then is divided into two beams of transmission light by the second coupler C2, which are respectively injected into the connected sensing optical fiber through two groups of erbium-doped fiber amplifiers EDFA, two groups of filters Filter, two groups of circulators Circulator, and fan-in coupler Fan-in Coupler. The pulse light is back-scattered Rayleigh in the sensing optical fiber; wherein the splitting ratio of the first coupler C1 is 98:1:1, and the splitting ratio of the second coupler C2 is 1:1. The fan-in coupler Fan-in Coupler of this embodiment is optically connected to the multi-core optical fiber MCF.

[0045] Step 2, when the sensing optical fiber is subjected to the effect of sound pressure, the phase of the backscattered light changes, and interferes with the two 1% local oscillator lights separated by the laser in the third coupler C3 and the fourth coupler C4 respectively. After the photoelectric conversion is completed by the first balanced detector BPD1 and the second balanced detector BPD2, it is acquired by the data acquisition card DAQ, and the phase distribution of the scattered light in the entire sensing optical fiber is obtained by heterodyne phase detection, and then the sound pressure fluctuation information can be obtained from the phase difference of the scattered light between different positions; wherein, the splitting ratios of the third coupler C3 and the fourth coupler C4 are both 1:1;

[0046] Step 3: The sensing optical fiber is provided with seven cores, and the seven cores are spatially multiplexed, that is, there are seven array elements. When a certain position of the sensing optical fiber is vibrated, the seven cores of the sensing optical fiber demodulate the vibration signals and superimpose them. First, the noise correlation of the system is analyzed, and then the improvement of the signal-to-noise ratio of the system before and after superposition is analyzed under the premise that the noise is correlated; further, the noise is decorrelated, and then the improvement of the signal-to-noise ratio is analyzed;

[0047] refer to Figure 3, extract the 1st, 2nd, 3rd, 4th, 5th, 6th, 7th, 8th... pulses from the core Core1 of the seven cores, extract the 2nd, 3rd, 4th, 5th, 6th, 7th, 8th... pulses from the core Core2 of the seven cores, and so on, extract the 7th, 8th... pulses from the core Core7 of the seven cores. Since the noise is random, the correlation of noise at different times is weak, and the correlation of noise can be reduced. The pulses in the seven cores are demodulated respectively. At this time, the demodulated signals are not in phase. If they are directly accumulated, the signal gain Instead of increasing, it decreases. To ensure the in-phase superposition of signals, the time domain signal is transformed from the time domain to the frequency domain through Fourier transform, and the phase is shifted in the frequency domain. Taking the phase of the signal in the fiber core Core1 as the benchmark, the fiber core Core2 needs to move the phase forward by 1 pulse point, and the fiber core Core3 needs to move the phase forward by 2 pulse points, and so on. The phases of the signals demodulated from all the fiber cores are aligned with the phase of the fiber core Core1, and then inverse Fourier transform is performed to the time domain. In this way, noise decorrelation is achieved while the in-phase superposition of signals is achieved, which can further improve the signal-to-noise ratio.

[0048] refer to Figure 2 , the fiber core Core7 in step 3 is located at the center of the seven fiber cores, and the other 6 fiber cores are evenly distributed around the fiber core Core7. The first circulator Circulator1 in the two groups of circulators injects the incident light of the first channel CH1 into the fiber core Core1, and the tail end of the fiber core Core1 is connected in series with the tail end of the fiber core Core2. At this time, the incident light is transmitted back to the head end of the fiber core Core2, and the head end of the fiber core Core2 is connected in series with the head end of the fiber core Core3, and the tail end of the fiber core Core3 is connected in series with the tail end of the fiber core Core4. The incident light of the first channel CH1 is transmitted in a serpentine shape through the fiber core Core1, the fiber core Core2, the fiber core Core3, and the fiber core Core4 in sequence. The second circulator Circulator2 in the two groups of circulators injects the incident light of the second channel CH2 into the fiber core Core5, and the tail end of the fiber core Core5 is connected in series with the tail end of the fiber core Core6. At this time, the incident light is transmitted back to the head end of the fiber core Core6, and the head end of the fiber core Core6 is connected in series with the head end of the fiber core Core7. The incident light of the second channel CH2 is transmitted in a serpentine shape through the fiber core Core5, the fiber core Core6 and the fiber core Core7 in sequence.

[0049] The phase difference signals demodulated from the same disturbance position of the seven cores are superimposed. Theoretically, when the noises of the seven cores are completely uncorrelated, the equivalent self-noise pressure of the system can be reduced by 10lg7=8.45dB after superposition. However, in fact, part of the noise sources of the two channels come from the same optical device, such as lasers and acousto-optic frequency shifters. The phase noise generated by the two makes the seven cores have a certain correlation in noise characteristics; on the other hand, the data of each core in the same channel comes from the same pulse and also has a certain correlation. Therefore, the mutual correlation coefficient of the noise of the seven cores is calculated first, and on this basis, the actual equivalent self-noise pressure suppression effect after the seven core signals are superimposed is calculated; in order to further improve the signal-to-noise ratio of the system, a signal-to-noise ratio improvement method based on time domain dislocation-frequency domain phase shift is proposed.

[0050] The method further comprises: step 4, assuming that m array element signals s have the same sensitivity. 1 (t), s 2 (t),…,s m (t) linear addition, the average power of the output signal is:

[0051]

[0052] In formula (1), a is the proportional coefficient and the average noise power is:

[0053]

[0054] make The average signal-to-noise ratio of the array can be expressed as:

[0055]

[0056] Among them, (ρ s ) ij ,(ρ n ) ij represent the mutual correlation coefficient between the signal and noise between the i-th array element and the j-th array element respectively;

[0057]

[0058] The logarithm of the ratio of the accumulated signal-to-noise ratio to the signal-to-noise ratio of a single array element is the array gain:

[0059]

[0060] The gain after array accumulation depends on the mutual correlation coefficient between the signal and the noise between the array elements. When the signal is completely correlated but the noise is completely uncorrelated,

[0061]

[0062] The array gain after the accumulation of m array elements is AG = 10lgm. When the signal is completely correlated but the noise is partially correlated,

[0063] Fiber core Core1, fiber core Core2, fiber core Core3, fiber core Core4, fiber core Core5, fiber core Core6 and fiber core Core7 are space-division multiplexed, that is, there are seven array elements. When a certain position of the sensor fiber is vibrated, the seven cores of the sensor fiber all demodulate the vibration signal and superimpose it. First, the noise correlation of the system is analyzed, and then the signal-to-noise ratio improvement of the system before and after superposition is analyzed under the premise that the noise is correlated; further, the noise is decorrelated, and then the signal-to-noise ratio improvement is analyzed.

[0064] The method further comprises: step 5, calculating the correlation coefficient matrix of the seven core noises by equation (5):

[0065]

[0066] Among them, i=1,2,3,4,5,6,7, j=1,2,3,4,5,6,7, then When the same signal acts on seven fiber cores, that is, the signals are completely correlated, (ρ s ) ij =1, after seven fiber cores are added together, the system equivalent self-noise pressure can be reduced Due to the existence of noise correlation, the equivalent self-noise pressure suppression effect after the seven cores are added is lower than the theoretical value AG=10lg7=8.45dB.

[0067] The method also includes: Step 6, assuming that the vibration frequency is 500 Hz, and accumulating the demodulated phase signal before and after the time domain signal TimeDomain Signal ( Figure 4 ) and PSD curve ( Figure 5 ), the PSD of the seven cores at 500Hz is -33.68dB, the noise average is -80.92dB, and the signal-to-noise ratio is 47.24dB; after the seven core signals are accumulated, the PSD at 500Hz is -16.77dB, the noise average is -70.29dB, and the signal-to-noise ratio is 53.52dB; therefore, the signal-to-noise ratio is improved by 6.28dB, that is, the system equivalent self-noise pressure is reduced by 6.28dB; the PSD data before and after the accumulation of the seven cores at four frequencies (Table 1) show that the average PSD gain after accumulation reaches 6.33dB, that is, the system equivalent self-noise pressure is reduced by 6.33dB, which is close to the calculated value.

[0068] Table 1 PSD data of each frequency point before and after the accumulation of seven core signals (dB / Hz)

[0069]

[0070] The improvement of the signal-to-noise ratio after removing the noise correlation by the time domain shift-frequency domain phase shift method is shown in Table 2. Taking 5 shifted pulses as an example, it can be seen that the average signal-to-noise ratio gain can reach 8.65dB.

[0071] Table 2 PSD data of each frequency point before and after decorrelation of seven-core signal noise (dB / Hz)

[0072]

[0073] The present invention is based on a dual-channel DAS system, with multi-core optical fiber as the sensing optical fiber, and three or four cores thereof are connected in series to form two sensing channels and seven sensing array elements, and the seven-core space division signal is multiplexed, and its equivalent self-noise pressure suppression effect is analyzed. The results show that the sum of the seven-core optical fiber noise correlation coefficients is 11.28, and after the seven-core signal is multiplexed, the system equivalent self-noise pressure can be reduced by 6.33dB in the 100-1000Hz frequency band. After the time domain dislocation-frequency domain phase shift method is used to remove the correlation of noise, the signal-to-noise ratio gain can be increased to 8.65dB, which can improve the system's detection capability for low-frequency weak signals. The suppression of equivalent self-noise pressure is of great significance for improving the accuracy of the system's restoration of vibration signals and enhancing the system's detection capability for low-frequency weak signals.

[0074] An embodiment of the present invention provides an electronic device, comprising: at least one memory for storing computer programs; and at least one processor for executing the programs stored in the memory. When the programs stored in the memory are executed, the processor is used to execute a method for improving the signal-to-noise ratio based on a multi-core optical fiber dual-channel distributed acoustic sensing system according to the above embodiment.

[0075] An embodiment of the present invention provides a computer-readable storage medium, which stores a computer program. When the computer program runs on a processor, the processor executes a method for improving the signal-to-noise ratio of a multi-core optical fiber dual-channel distributed acoustic sensing system as described in the above embodiment.

[0076] Although the present invention has been disclosed as above in terms of preferred embodiments, the above preferred embodiments are not intended to limit the present invention. A person skilled in the art may make various modifications and alterations without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention shall be based on the scope defined in the claims.

Claims

1. A method for improving the signal-to-noise ratio of a multi-core optical fiber dual-channel distributed acoustic sensing system, characterized in that: The following steps are involved: Step 1, the light emitted by the narrow linewidth laser Laser is split by the first coupler C1, one beam of light passes through the acousto-optic modulator AOM to generate a pulse signal, and then is split into two beams of transmission light by the second coupler C2, which are respectively injected into the connected sensing optical fiber through two groups of erbium-doped fiber amplifiers EDFA, two groups of filters Filter, two groups of circulators Circulator, and fan-in coupler Fan-in Coupler. The pulse light is back-scattered by Rayleigh in the sensing optical fiber; wherein the splitting ratio of the first coupler C1 is 98:1:1, and the splitting ratio of the second coupler C2 is 1:1; Step 2, when the sensing optical fiber is subjected to the effect of sound pressure, the phase of the backscattered light changes, and interferes with the two 1% local oscillator lights separated by the laser in the third coupler C3 and the fourth coupler C4 respectively. After the photoelectric conversion is completed by the first balanced detector BPD1 and the second balanced detector BPD2, it is acquired by the data acquisition card DAQ, and the phase distribution of the scattered light in the entire sensing optical fiber is obtained by heterodyne phase detection, and then the sound pressure fluctuation information can be obtained from the phase difference of the scattered light between different positions; wherein, the splitting ratios of the third coupler C3 and the fourth coupler C4 are both 1:1; Step 3: The sensing optical fiber is provided with seven cores, and the seven cores are spatially multiplexed, that is, there are seven array elements. When a certain position of the sensing optical fiber is vibrated, the seven cores of the sensing optical fiber demodulate the vibration signals and superimpose them. First, the noise correlation of the system is analyzed, and then the improvement of the signal-to-noise ratio of the system before and after superposition is analyzed under the premise that the noise is correlated; further, the noise is decorrelated, and then the improvement of the signal-to-noise ratio is analyzed; Extract the 1st, 2nd, 3rd, 4th, 5th, 6th, 7th, 8th... pulses from the core Core1 of the seven cores, extract the 2nd, 3rd, 4th, 5th, 6th, 7th, 8th... pulses from the core Core2 of the seven cores, and so on, extract the 7th, 8th... pulses from the core Core7 of the seven cores. Since the noise is random, the correlation of noise at different times is weak, and the correlation of noise can be reduced. The pulses in the seven cores are demodulated respectively. At this time, the demodulated signals are not in phase. If they are directly accumulated, the signal gain will not be Increase, reverse and decrease. To ensure the in-phase superposition of signals, the time domain signal is transformed from the time domain to the frequency domain through Fourier transform, and phase shifting is performed in the frequency domain. Taking the phase of the signal in the fiber core Core1 as the benchmark, the fiber core Core2 needs to move the phase forward by 1 pulse point, and the fiber core Core3 needs to move the phase forward by 2 pulse points, and so on. The phases of the signals demodulated from all fiber cores are aligned with the phase of the fiber core Core1, and then inverse Fourier transform is performed to the time domain. In this way, noise decorrelation is achieved while the in-phase superposition of signals is achieved, which can further improve the signal-to-noise ratio.

2. The method for improving the signal-to-noise ratio of a multi-core optical fiber dual-channel distributed acoustic sensing system according to claim 1, characterized in that: The fiber core Core7 in step 3 is located at the center of the seven fiber cores, and the other six fiber cores are evenly distributed around the fiber core Core7; The first circulator Circulator1 in the two groups of circulators injects the incident light of the first channel CH1 into the fiber core Core1, and the tail end of the fiber core Core1 is connected in series with the tail end of the fiber core Core2. At this time, the incident light is transmitted back to the head end of the fiber core Core2, and the head end of the fiber core Core2 is connected in series with the head end of the fiber core Core3, and the tail end of the fiber core Core3 is connected in series with the tail end of the fiber core Core4. The incident light of the first channel CH1 is transmitted in a serpentine shape through the fiber core Core1, the fiber core Core2, the fiber core Core3, and the fiber core Core4 in sequence; The second circulator Circulator2 in the two groups of circulators injects the incident light of the second channel CH2 into the fiber core Core5, and the tail end of the fiber core Core5 is connected in series with the tail end of the fiber core Core6. At this time, the incident light is transmitted back to the head end of the fiber core Core6, and the head end of the fiber core Core6 is connected in series with the head end of the fiber core Core7. The incident light of the second channel CH2 is transmitted in a serpentine shape through the fiber core Core5, the fiber core Core6 and the fiber core Core7 in sequence.

3. The method for improving the signal-to-noise ratio of a multi-core optical fiber dual-channel distributed acoustic sensing system according to claim 2, characterized in that: The method further comprises: step 4, assuming m array element signals s1(t), s2(t), ..., s with the same sensitivity. m (t) linear addition, the average power of the output signal is: In formula (1), a is the proportional coefficient and the average noise power is: make The average signal-to-noise ratio of the array can be expressed as: Among them, (ρ s ) ij ,(ρ n ) ij represent the mutual correlation coefficient between the signal and noise between the i-th array element and the j-th array element respectively; The logarithm of the ratio of the accumulated signal-to-noise ratio to the signal-to-noise ratio of a single array element is the array gain: The gain after array accumulation depends on the mutual correlation coefficient between the signal and the noise between the array elements. When the signal is completely correlated but the noise is completely uncorrelated, The array gain after the accumulation of m array elements is AG = 10lgm. When the signal is completely correlated but the noise is partially correlated, Fiber core Core1, fiber core Core2, fiber core Core3, fiber core Core4, fiber core Core5, fiber core Core6 and fiber core Core7 are space-division multiplexed, that is, there are seven array elements. When a certain position of the sensor fiber is vibrated, the seven cores of the sensor fiber all demodulate the vibration signal and superimpose it. First, the noise correlation of the system is analyzed, and then the signal-to-noise ratio improvement of the system before and after superposition is analyzed under the premise that the noise is correlated; further, the noise is decorrelated, and then the signal-to-noise ratio improvement is analyzed.

4. The method for improving the signal-to-noise ratio of a multi-core optical fiber dual-channel distributed acoustic sensing system according to claim 3, characterized in that: The method further comprises: step 5, calculating the correlation coefficient matrix of the seven core noises by equation (5): Where i = 1, 2, 3, 4, 5, 6, 7 and j = 1, 2, 3, 4, 5, 6, 7, then When the same signal acts on seven fiber cores, that is, the signals are completely correlated, (ρ s ) ij =1, after seven fiber cores are added together, the system equivalent self-noise pressure can be reduced Due to the existence of noise correlation, the equivalent self-noise pressure suppression effect after the seven cores are added is lower than the theoretical value AG=10lg7=8.45dB.

5. The method for improving the signal-to-noise ratio of a multi-core optical fiber dual-channel distributed acoustic sensing system according to claim 4, characterized in that: It also includes: Step 6, assuming that the vibration frequency is 500Hz, the time domain signal Time Domain Signal and PSD curve before and after the demodulated phase signal is accumulated, the PSD of the seven cores at 500Hz is -33.68dB, the noise average is -80.92dB, and the signal-to-noise ratio is 47.24dB; after the seven core signals are accumulated, the PSD at 500Hz is -16.77dB, the noise average is -70.29dB, and the signal-to-noise ratio is 53.52dB; therefore, the signal-to-noise ratio is improved by 6.28dB, that is, the system equivalent self-noise pressure is reduced by 6.28dB; the average PSD gain after accumulation of the PSD data before and after the accumulation of the seven cores at four frequency points reaches 6.33dB, that is, the system equivalent self-noise pressure is reduced by 6.33dB, which is close to the calculated value.

6. An electronic device, characterized in that: include: at least one memory for storing a computer program; At least one processor is used to execute the program stored in the memory. When the program stored in the memory is executed, the processor is used to execute the signal-to-noise ratio improvement method based on a multi-core optical fiber dual-channel distributed acoustic sensing system as described in any one of claims 1 to 5.

7. A computer-readable storage medium storing a computer program, characterized in that: When the computer program runs on a processor, the processor is enabled to execute a method for improving the signal-to-noise ratio based on a multi-core optical fiber dual-channel distributed acoustic sensing system as claimed in any one of claims 1 to 5.

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