A method for improving the signal-to-noise ratio of a multi-core optical fiber dual-channel distributed acoustic sensing system

Abstract

The present invention discloses a method for improving the signal-to-noise ratio based on a multi-core fiber dual-channel distributed acoustic sensing system. Light emitted by a narrow-linewidth laser is split by a first coupler C1. One beam of light passes through an acousto-optic modulator (AOM) to generate a pulse signal. The light is then split by a second coupler C2 into two beams of transmission light, which are respectively injected into connected sensing fibers through two groups of erbium-doped fiber amplifiers (EDFAs), two groups of filters, two groups of circulators, and a fan-in coupler. The pulsed light undergoes back Rayleigh scattering in the sensing fibers. The present invention is based on a dual-channel DAS system, uses a multi-core fiber as the sensing fiber, and connects three and four cores in series to form two sensing channels. The seven-core space division signal is multiplexed, and its equivalent self-noise pressure suppression effect is analyzed. After removing the noise correlation using a time-domain stagger-frequency-domain phase shift method, the signal-to-noise ratio gain can be increased to 8.65dB, thereby improving the system's detection capability for low-frequency weak signals.

Description

Technical Field

[0001] The present invention belongs to the technical field of underwater acoustic detection using a distributed acoustic sensing system, and in particular relates to a method for improving the 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 works by injecting a probe pulse laser from the transmitter into a sensing fiber. Influenced by impurities in the sensing fiber, the probe light undergoes coherent Rayleigh backscattering and returns to the transmitter. Phase demodulation of the probe light can be used to obtain strain information at any section of the sensing fiber, thereby restoring the acoustic signal. Therefore, the sensing fiber acts as a tightly packed acoustic sensor. DAS offers a streamlined system structure, high spatial resolution, strong resistance to electromagnetic interference, and flexible array structure. Therefore, it can provide a novel technical approach for ocean underwater acoustic sensing and detection. Common DAS systems typically use single-mode optical fiber as the sensing fiber. The Rayleigh backscattered light is extremely weak during long-distance transmission, and the system's equivalent self-noise pressure spectrum is high. Due to this limitation, traditional DAS systems have poor minimum detectable sound pressure performance and present difficulties in detecting weak acoustic signals.

[0003] To improve the signal-to-noise ratio (SNR) of single-mode fiber DAS systems, researchers have introduced specialized optical fibers to improve system performance, such as continuous scattering-enhanced fiber, discrete scattering-enhanced fiber with inscribed ultraweak fiber Bragg gratings (UWFBGs) or scattering enhancement points (SEPs), and fiber sensitization. Continuous scattering enhancement schemes primarily include inscribed continuous gratings and modified fiber doping. However, while increasing the amplitude of Rayleigh backscattering, they also exponentially increase optical power loss, significantly reducing detection range. UWFBGs have a narrow bandwidth. In extreme environments such as high pressure and low temperature, the wavelength of the probe light and the reflection wavelength of the UWFBG can mismatch, resulting in a sensing blind spot, making them impractical for underwater applications. Sensitization-enhancing fiber primarily utilizes helical winding and secondary coating to enhance the signal phase sensitivity to changes in sound pressure. Helically wound sensitizing fiber can significantly reduce the system's equivalent self-noise pressure. However, the diameter of the reported helical sensitization structures is at least 12.5 mm, making them difficult to use in applications requiring small 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 acoustic wave detection performance. Its sensitivity is lower than that of the spiral-sensitized optical fiber and is difficult to meet the needs.

[0004] Researchers have studied the in-phase superposition of the output signals of multiple detection arrays, but they primarily focused on sensitivity analysis of high-frequency signals. In reality, increasing sensitivity also increases noise. If the noise of each array element is correlated, the system's signal-to-noise ratio (SNR) will not improve, and the acquired signal quality will remain unreliable. Furthermore, since low-frequency noise decays slowly during propagation, it tends to be mixed with low-frequency signals, making detection of low-frequency signals difficult. Therefore, improving the SNR is crucial for achieving high-precision detection of weak signals. Summary of the Invention

[0005] In response to the problems of high equivalent self-noise pressure and insufficient detection capability of weak signals in existing DAS systems, the present invention provides a method for improving the signal-to-noise ratio based on a multi-core fiber dual-channel distributed acoustic sensing system. Based on the dual-channel DAS detection and demodulation system, the method ensures that the same detection pulse light is emitted simultaneously by the two channels, which is a prerequisite for the superposition of subsequent signals. Multi-core fiber (MCF) is used as the sensing fiber, and three fiber cores and four fiber cores are connected in series to form two sensing channels, which are then connected to the DAS module. Each fiber core has a consistent response to external disturbances but inconsistent noise. By superimposing the signals of the seven fiber cores in the same position, the signal-to-noise ratio is improved, which can improve the system's detection capability for low-frequency weak signals, and is of great significance for the detection of low-frequency weak signals.

[0006] To solve the above problems, the present invention provides the following technical solutions:

[0007] An 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] In step 1, light from a narrow-linewidth laser is split by the first coupler C1. One beam passes through an acousto-optic modulator (AOM) to generate a pulse signal. The second coupler C2 then splits the light into two transmission beams, which are then injected into the connected sensing fiber via two erbium-doped fiber amplifiers (EDFAs), two filters, two circulators, and a fan-in coupler. The pulsed light undergoes Rayleigh backscattering in the sensing fiber. 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 fiber is subjected to acoustic pressure, the backscattered light phase changes, interfering with the two 1% local oscillator lights split by the laser at the third coupler C3 and the fourth coupler C4, respectively. After photoelectric conversion is completed by the first balanced detector BPD1 and the second balanced detector BPD2, the backscattered light is acquired by the data acquisition card DAQ. The phase distribution of the scattered light in the entire sensing fiber is obtained through heterodyne phase detection, and the phase difference of the scattered light at different positions can be used to obtain the acoustic pressure fluctuation information. The splitting ratio of the third coupler C3 and the fourth coupler C4 is 1:1.

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

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

[0013] The first circulator Circulator1 in the two circulator groups injects the incident light of the first channel CH1 into the fiber core Core1. 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. The head end of the fiber core Core2 is then connected in series with the head end of the fiber core Core3. The tail end of the fiber core Core3 is then 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, fiber core Core2, fiber core Core3, and fiber core Core4 in sequence.

[0014] The second circulator Circulator2 in the two sets of circulators injects the incident light of the second channel CH2 into the fiber core Core5. 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, fiber core Core6 and fiber core Core7 in sequence.

[0015] A preferred embodiment of the present invention further includes: Step 4, assuming m array element signals s1(t), s2(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 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 = 10logm. When the signal is completely correlated but the noise is partially correlated,

[0028] Fiber cores Core1, Core2, Core3, Core4, Core5, Core6, and Core7 are spatially multiplexed, that is, there are seven array elements. When a certain position of the sensing fiber is vibrated, the seven fiber cores of the sensing fiber all 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.

[0029] A preferred embodiment of the present invention further includes: Step 5, calculating the cross-correlation coefficient matrix of the seven core noises using formula (5):

[0030]

[0031] 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 fiber cores are added is lower than the theoretical value AG=10lg7=8.45dB.

[0032] A preferred embodiment of the present invention further 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 fiber 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 fiber 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 fiber 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 program stored in the memory is executed, the processor is configured to execute a method for improving the signal-to-noise ratio of 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 storing 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 according to 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: Based on a dual-channel DAS system, the present invention uses multiple optical fiber cores as sensing optical fibers, and connects three or four cores in series to form two sensing channels and seven sensing array elements. The seven-core spatial 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 multiplexing the signals of the seven independent optical fiber cores, the system equivalent self-noise pressure can be reduced by 6.33dB in the frequency band of 100 to 1000Hz. After removing the correlation by the time domain dislocation-frequency domain phase shifting method, 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 the 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 following briefly introduces the drawings required for use in the embodiments or the description of the prior art. 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 3A schematic diagram of a signal-to-noise ratio improvement method based on time domain staggering and frequency domain phase shifting provided in an embodiment of the present application.

[0040] Figure 4 This is a schematic diagram of the 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 This is 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 following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the embodiments described are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without making creative efforts are within the scope of protection of this 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] In step 1, light from a narrow-linewidth laser is split by the first coupler C1. One beam passes through an acousto-optic modulator (AOM) to generate a pulse signal. The second coupler C2 then splits the light into two transmission beams, each injected into a connected sensing fiber via two erbium-doped fiber amplifiers (EDFAs), two filters, two circulators, and a fan-in coupler. The pulsed light undergoes Rayleigh backscattering in the sensing fiber. The splitting ratio of the first coupler C1 is 98:1:1, while the splitting ratio of the second coupler C2 is 1:1. In this embodiment, the fan-in coupler is optically connected to the multi-core fiber (MCF).

[0045] Step 2: When the sensing fiber is subjected to acoustic pressure, the backscattered light phase changes, interfering with the two 1% local oscillator lights split by the laser at the third coupler C3 and the fourth coupler C4, respectively. After photoelectric conversion is completed by the first balanced detector BPD1 and the second balanced detector BPD2, the backscattered light is acquired by the data acquisition card DAQ. The phase distribution of the scattered light in the entire sensing fiber is obtained through heterodyne phase detection, and the phase difference of the scattered light at different positions can be used to obtain the acoustic pressure fluctuation information. The splitting ratio of the third coupler C3 and the fourth coupler C4 is 1:1.

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

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

[0048] refer to Figure 2 Fiber core 7 in step 3 is located at the center of the seven fiber cores, and the other six fiber cores are evenly distributed around Core 7. The first circulator in the two circulator groups, Circulator 1, injects light from the first channel CH1 into Core 1. The tail end of Core 1 is connected in series with the tail end of Core 2. At this time, the incident light is transmitted back to the head end of Core 2. The head end of Core 2 is then connected in series with the head end of Core 3. The tail end of Core 3 is then connected in series with the tail end of Core 4. The incident light from the first channel CH1 is transmitted in a serpentine pattern through Core 1, Core 2, Core 3, and Core 4 in sequence. The second circulator Circulator2 in the two sets of circulators injects the incident light of the second channel CH2 into the fiber core Core5. 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, fiber core Core6 and fiber core Core7 in sequence.

[0049] Theoretically, when the phase difference signals demodulated from the same perturbation position on seven fiber cores are superimposed, the equivalent self-noise pressure of the system can be reduced by 10log7 = 8.45dB if the noise of the seven fiber cores is completely uncorrelated. However, in reality, the noise sources of both channels originate partially from the same optical device, such as the laser and the acousto-optic frequency shifter. The phase noise generated by these two devices causes a certain degree of correlation in the noise characteristics of the seven cores. Furthermore, the data from each fiber core in the same channel originates from the same pulse and also has a certain degree of correlation. Therefore, the cross-correlation coefficient of the noise of the seven fiber cores is first calculated. Based on this, the actual equivalent self-noise pressure suppression effect of the superimposed signals of the seven fiber cores is calculated. To further improve the signal-to-noise ratio of the system, a signal-to-noise ratio improvement method based on time-domain staggering and frequency-domain phase shifting is proposed.

[0050] The method further includes: 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:

[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 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 = 10logm. When the signal is completely correlated but the noise is partially correlated,

[0063] Fiber cores Core1, Core2, Core3, Core4, Core5, Core6, and Core7 are spatially multiplexed, that is, there are seven array elements. When a certain position of the sensing fiber is vibrated, the seven fiber cores of the sensing fiber all 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.

[0064] The method further includes: Step 5, calculating the cross-correlation coefficient matrix of the seven core noises using formula (5):

[0065]

[0066] 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 fiber cores are added together 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 TimeDomainSignal ( Figure 4 ) and PSD curves ( Figure 5 ), the PSD of the seven fiber cores at 500Hz is -33.68dB, the average noise is -80.92dB, and the signal-to-noise ratio is 47.24dB; after the seven fiber core signals are accumulated, the PSD at 500Hz is -16.77dB, the average noise 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 equivalent self-noise pressure of the system is reduced by 6.28dB; the PSD data before and after the accumulation of the seven fiber cores at four frequencies (Table 1) show that the average PSD gain after accumulation reaches 6.33dB, that is, the equivalent self-noise pressure of the system is reduced by 6.33dB, which is close to the calculated value.

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

[0069]

[0070] The improvement in 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 (dB / Hz) at each frequency point before and after decorrelation of the seven-core signal noise

[0072]

[0073] The present invention is based on a dual-channel DAS system, using multi-core optical fibers as sensing fibers. Three or four cores are connected in series to form two sensing channels and seven sensing array elements. The seven-core spatial 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-core optical fibers is 11.28. After multiplexing the seven-core signals, the system equivalent self-noise pressure can be reduced by 6.33dB in the 100-1000Hz frequency band. After removing the correlation by the time domain dislocation-frequency domain phase shifting method, 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 the 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 program stored in the memory is executed, the processor is configured to execute a method for improving the signal-to-noise ratio of 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 storing 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 according to the above embodiment.

[0076] Although the present invention has been disclosed above in terms of preferred embodiments, the above preferred embodiments are not intended to limit the present invention. Those 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: In step 1, light from a narrow-linewidth laser is split by the first coupler C1. One beam passes through an acousto-optic modulator (AOM) to generate a pulse signal. The second coupler C2 then splits the light into two transmission beams, which are then injected into the connected sensing fiber via two erbium-doped fiber amplifiers (EDFAs), two filters, two circulators, and a fan-in coupler. The pulsed light undergoes Rayleigh backscattering in the sensing fiber. 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 fiber is subjected to acoustic pressure, the backscattered light phase changes, interfering with the two 1% local oscillator lights split by the laser at the third coupler C3 and the fourth coupler C4, respectively. After photoelectric conversion is completed by the first balanced detector BPD1 and the second balanced detector BPD2, the backscattered light is acquired by the data acquisition card DAQ. The phase distribution of the scattered light in the entire sensing fiber is obtained through heterodyne phase detection, and the phase difference of the scattered light at different positions can be used to obtain the acoustic pressure fluctuation information. The splitting ratio of the third coupler C3 and the fourth coupler C4 is 1:

1. Step 3: The sensing fiber is configured with seven cores, and the seven cores are spatially multiplexed, i.e., there are seven array elements. When a certain position of the sensing fiber is vibrated, the seven cores of the sensing fiber demodulate the vibration signals and superimpose them. First, the system noise correlation is analyzed, and then the improvement in the signal-to-noise ratio of the system before and after superposition is analyzed under the premise of noise correlation. The noise is further decorrelated, and the improvement in the signal-to-noise ratio is analyzed again. Extract the 1st, 2nd, 3rd, 4th, 5th, 6th, 7th, 8th... pulses from Core1 of the seven fiber cores, extract the 2nd, 3rd, 4th, 5th, 6th, 7th, 8th... pulses from Core2 of the seven fiber cores, and so on, extract the 7th, 8th... pulses from Core7 of the seven fiber cores. Since the noise is random, the correlation of noise at different times is weak. The correlation of noise can be reduced by demodulating the pulses in the seven fiber cores respectively. At this time, the demodulated signals are not in phase. If they are directly accumulated, the signal gain will not be good. To increase and decrease, in order to ensure the in-phase superposition of signals, the time domain signal is transformed into the frequency domain by Fourier transform, and phase shift is performed in the frequency domain. Taking the phase of the signal in fiber core Core1 as the benchmark, fiber core Core2 needs to move the phase forward by 1 pulse point, fiber core Core3 needs to move the phase forward by 2 pulse points, and so on. The phases of the signals demodulated by all fiber cores are aligned with the phase of fiber core Core1, and then transformed into the time domain by inverse Fourier transform. 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 circulator groups injects the incident light of the first channel CH1 into the fiber core Core1. 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. The head end of the fiber core Core2 is then connected in series with the head end of the fiber core Core3. The tail end of the fiber core Core3 is then 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, fiber core Core2, fiber core Core3, and fiber core Core4 in sequence. The second circulator Circulator2 in the two sets of circulators injects the incident light of the second channel CH2 into the fiber core Core5. 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, fiber core Core6 and 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 includes: 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 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 = 10logm. When the signal is completely correlated but the noise is partially correlated, Fiber cores Core1, Core2, Core3, Core4, Core5, Core6, and Core7 are spatially multiplexed, that is, there are seven array elements. When a certain position of the sensing fiber is vibrated, the seven fiber cores of the sensing fiber all 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.

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 includes: Step 5, calculating the cross-correlation coefficient matrix of the seven core noises using formula (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 fiber cores are added together 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: The method further includes: step 6, assuming that the vibration frequency is 500 Hz, and accumulating the time domain signal and PSD curve of the demodulated phase signal before and after, the PSD of the seven fiber 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 accumulating the signals of the seven fiber cores, 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 accumulating the PSD data before and after the accumulation of the seven fiber 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.

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 according to 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 caused to execute the method for improving the signal-to-noise ratio based on a multi-core optical fiber dual-channel distributed acoustic sensing system according to any one of claims 1 to 5.

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