A method and device for suppressing equivalent self-noise pressure based on multi-core optical fiber space division multiplexing signal

Abstract

The present invention discloses a method and device for suppressing equivalent self-noise pressure based on multi-core optical fiber space division multiplexing signals. 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 into two beams of transmission light by a second coupler C2. The two beams are respectively injected into connected sensing optical 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 backscattering Rayleigh in the sensing optical fibers. The present invention is based on a dual-channel DAS system, uses multi-core optical fibers as sensing optical fibers, connects three and four cores in series to form two sensing channels, multiplexes seven-core space division signals, and analyzes the equivalent self-noise pressure suppression effect. This method can improve 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 of distributed acoustic sensing systems, and in particular relates to a method and device for suppressing equivalent self-noise pressure based on multi-core optical fiber space division multiplexing signals. 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 address the challenges of suppressing the equivalent self-noise pressure and improving the sensitivity of single-mode fiber DAS systems, researchers have introduced specialized optical fibers to improve system performance, such as continuous scattering-enhanced fibers, discrete scattering-enhanced fibers with inscribed ultraweak fiber Bragg gratings (UWFBGs) or scattering enhancement points (SEPs), and fiber-optic sensitivity enhancement. Continuous scattering enhancement solutions primarily include inscribed continuous gratings and modified fiber doping. However, while enhancing 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 temperatures, the wavelength of the probe light and the reflection wavelength of the UWFBG can mismatch, resulting in a sensing blind spot and making them unsuitable for underwater applications. Enhanced sensitivity fibers primarily utilize helical winding and secondary coating to enhance the signal phase sensitivity to sound pressure changes. Helically wound enhanced sensitivity fibers can significantly reduce the system's equivalent self-noise pressure, but the diameter of publicly reported helical enhancement 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 output signals from 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 equivalent self-noise pressure cannot be suppressed, and the acquired signal quality remains unreliable. Furthermore, since low-frequency noise decays slowly during propagation, it easily blends with low-frequency signals, making detection difficult. Therefore, suppressing the system's low-frequency equivalent self-noise pressure is crucial for achieving high-precision detection of weak signals.

[0005] To suppress the DAS system's equivalent self-noise pressure spectrum, reduce the system's low-frequency equivalent self-noise pressure, and maintain the small diameter of the hydrophone unit, a dual-channel DAS detection and demodulation system was developed. This ensured that both channels emitted the same probe pulse light simultaneously, a prerequisite for subsequent signal superposition. Multi-core fiber (MCF) was used as the sensing fiber, with three and four cores connected in series, respectively, to form two sensing channels, which were then connected to the DAS module. Each core responds uniformly to external perturbations, but generates inconsistent noise. By superimposing the signals from the seven cores at the same location, the system's equivalent self-noise pressure is reduced, which is crucial for detecting weak low-frequency signals. Summary of the Invention

[0006] 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 and device for suppressing equivalent self-noise pressure based on multi-core optical fiber space division multiplexing signals, thereby improving the system's detection capability for low-frequency weak signals.

[0007] To solve the above problems, the present invention provides the following technical solutions:

[0008] The embodiment of the present invention provides a method for suppressing equivalent self-noise pressure based on a multi-core optical fiber space division multiplexing signal, comprising the following steps:

[0009] 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 through 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.

[0010] In step 2, when the sensing fiber is subjected to sound pressure, the phase of the backscattered light changes, and interferes with the two 1% local oscillator lights separated by the laser at the third coupler C3 and the fourth coupler C4, respectively. After the first balanced detector BPD1 and the second balanced detector BPD2 complete the photoelectric conversion, the 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 sound pressure fluctuation information can be obtained from the phase difference of the scattered light between different positions. Among them, the splitting ratio of the first coupler C1 is 98:1:1, and the splitting ratios of the second coupler C2, the third coupler C3, and the fourth coupler C4 are all 1:1.

[0011] In a preferred embodiment of the present invention, the sensing optical fiber in step 1 is provided with a first fiber core Core1, a second fiber core Core2, a third fiber core Core3, a fourth fiber core Core4, a fifth fiber core Core5, a sixth fiber core Core6 and a seventh fiber core Core7;

[0012] The first circulator Circulator1 in the two circulator groups injects the incident light of the first channel CH1 into the first fiber core Core1. The tail end of the first fiber core Core1 is connected in series with the tail end of the second fiber core Core2. At this time, the incident light is transmitted back to the head end of the second fiber core Core2. The head end of the second fiber core Core2 is then connected in series with the head end of the third fiber core Core3. The tail end of the third fiber core Core3 is then connected in series with the tail end of the fourth fiber core Core4. The incident light of the first channel CH1 is transmitted in a serpentine shape through the first fiber core Core1, the second fiber core Core2, the third fiber core Core3, and the fourth fiber core Core4 in sequence.

[0013] The second circulator 2 in the two circulator groups injects the incident light from the second channel CH2 into the fifth fiber core Core5. The tail end of the fifth fiber core Core5 is connected in series with the tail end of the sixth fiber core Core6. At this time, the incident light is transmitted back to the head end of the sixth fiber core Core6. The head end of the sixth fiber core Core6 is then connected in series with the head end of the seventh fiber core Core7. The incident light from the second channel CH2 is transmitted in a serpentine shape through the fifth fiber core Core5, the sixth fiber core Core6, and the seventh fiber core Core7 in sequence.

[0014] The first fiber core Core1, the second fiber core Core2, the third fiber core Core3, the fourth fiber core Core4, the fifth fiber core Core5, the sixth fiber core Core6 and the seventh fiber core Core7 are all in one optical fiber. The underwater acoustic pressure acts evenly on each fiber core, and the phase changes of the transmitted light caused by the strain changes of each fiber core are consistent. This is the prerequisite for signal accumulation.

[0015] A preferred embodiment of the present invention further includes: Step 3, 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] The first fiber core Core1, the second fiber core Core2, the third fiber core Core3, the fourth fiber core Core4, the fifth fiber core Core5, the sixth fiber core Core6 and the seventh fiber core Core7 are spatially multiplexed, that is, there are seven array elements. When a certain position of the sensing fiber is subjected to vibration, the seven cores of the sensing fiber all demodulate the vibration signal and superimpose it. First, the noise correlation of the system is analyzed, and then it is analyzed whether the equivalent self-noise pressure of the system before and after superposition is suppressed under the premise that the noise is correlated.

[0029] A preferred embodiment of the present invention further includes: Step 4, calculating the cross-correlation coefficient matrix of the seven independent optical fiber 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 the seven optical fiber cores, that is, the signals are completely correlated, (ρ s ) ij =1, after the seven independent optical 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 seven cores are accumulated is lower than the theoretical value AG=10lg7=8.45dB.

[0032] A preferred embodiment of the present invention further includes: step 5, 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 independent optical 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 signals of the seven independent optical fiber cores 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 independent optical 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] The embodiment of the present invention further provides an equivalent self-noise pressure suppression device based on a multi-core optical fiber space division multiplexing signal, comprising a narrow linewidth laser Laser, a first coupler C1, an acousto-optic modulator AOM, a second coupler C2, a first amplifier EDFA, two erbium-doped fiber amplifiers EDFA, two filters Filter, two circulators Circulator, a fan-in coupler Fan-in Coupler, a multi-core optical fiber Multi-core Fiber, a third coupler C3, a fourth coupler C4, a first balanced detector BPD1 and a second balanced detector BPD2;

[0034] The narrow-linewidth laser emits light that is split by the first coupler C1 and then generates a pulse signal through the acousto-optic modulator AOM. The light is then split into two beams by the second coupler C2 and respectively injected into the connected sensing fiber through two sets of erbium-doped fiber amplifiers EDFA, two sets of filters, two circulators, and a fan-in coupler. The fan-in coupler is then optically connected to the multi-core fiber.

[0035] The first circulator Circulator1 of the two circulators is connected to the third coupler C3, the first coupler C1 is connected to the third coupler C3, and the third coupler C3 is connected to the first balanced detector BPD1; the second circulator Circulator2 of the two circulators is connected to the fourth coupler C4, the first coupler C1 is connected to the fourth coupler C4, and the fourth coupler C4 is connected to the second balanced detector BPD2; wherein, the splitting ratio of the first coupler C1 is 98:1:1, and the splitting ratios of the second coupler C2, the third coupler C3, and the fourth coupler C4 are all 1:1.

[0036] In a preferred embodiment of the present invention, the sensing optical fiber is provided with a first fiber core Core1, a second fiber core Core2, a third fiber core Core3, a fourth fiber core Core4, a fifth fiber core Core5, a sixth fiber core Core6 and a seventh fiber core Core7; the first fiber core Core1, the second fiber core Core2, the third fiber core Core3, the fourth fiber core Core4, the fifth fiber core Core5 and the sixth fiber core Core6 are arranged in a ring, and the seventh fiber core Core7 is located at the center of the ring;

[0037] Among them, the first circulator Circulator1 injects the incident light of the first channel CH1 into the first fiber core Core1, and the tail end of the first fiber core Core1 is connected in series with the tail end of the second fiber core Core2. At this time, the incident light is transmitted back to the head end of the second fiber core Core2, the head end of the second fiber core Core2 is connected in series with the head end of the third fiber core Core3, and the tail end of the third fiber core Core3 is connected in series with the tail end of the fourth fiber core Core4. The incident light of the first channel CH1 passes through the first fiber core Core1, the second fiber core Core2, the third fiber core Core3, and the fourth fiber core Core4 in sequence in a serpentine transmission;

[0038] The second circulator Circulator2 injects the incident light of the second channel CH2 into the fifth fiber core Core5. The tail end of the fifth fiber core Core5 is connected in series with the tail end of the sixth fiber core Core6. At this time, the incident light is transmitted back to the head end of the sixth fiber core Core6. The head end of the sixth fiber core Core6 is then connected in series with the head end of the seventh fiber core Core7. The incident light of the second channel CH2 passes through the fifth fiber core Core5, the sixth fiber core Core6 and the seventh fiber core Core7 in sequence and is transmitted in a serpentine shape.

[0039] Compared with the prior art, the present invention provides a method for suppressing equivalent self-noise pressure based on multi-core fiber space-division multiplexing signals, which has the following beneficial effects: Based on a dual-channel DAS system, the present invention uses multiple independent optical fiber cores as sensing fibers, connects three or four cores in series to form two sensing channels, multiplexes the seven-core space-division signals, and analyzes the equivalent self-noise pressure suppression effect. The results show that the sum of the noise correlation coefficients of the seven independent optical fiber cores is 11.28, and after multiplexing the seven independent optical fiber core signals, the system equivalent self-noise pressure can be reduced by 6.33dB in the 100-1000Hz frequency band. Suppressing equivalent self-noise pressure is of great significance for improving the accuracy of the system's vibration signal restoration and enhancing the system's ability to detect low-frequency weak signals. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] 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.

[0041] Figure 1 A schematic diagram of the framework of an equivalent self-noise pressure suppression device based on multi-core optical fiber space division multiplexing signals provided in an embodiment of the present application.

[0042] 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.

[0043] Figure 3 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.

[0044] Figure 4 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

[0045] 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.

[0046] refer to Figure 1 The embodiment of the present invention provides a method for suppressing equivalent self-noise pressure based on a multi-core optical fiber space division multiplexing signal, comprising the following steps:

[0047] 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 through 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.

[0048] In step 2, when the sensing fiber is subjected to sound pressure, the phase of the backscattered light changes, and interferes with the two 1% local oscillator lights separated by the laser at the third coupler C3 and the fourth coupler C4, respectively. After the first balanced detector BPD1 and the second balanced detector BPD2 complete the photoelectric conversion, the 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 sound pressure fluctuation information can be obtained from the phase difference of the scattered light between different positions. Among them, the splitting ratio of the first coupler C1 is 98:1:1, and the splitting ratios of the second coupler C2, the third coupler C3, and the fourth coupler C4 are all 1:1.

[0049] refer to Figure 2 The sensing optical fiber of step 1 is provided with a first fiber core Core1, a second fiber core Core2, a third fiber core Core3, a fourth fiber core Core4, a fifth fiber core Core5, a sixth fiber core Core6 and a seventh fiber core Core7.

[0050] The first circulator Circulator1 in the two circulator groups injects the incident light of the first channel CH1 into the first fiber core Core1. The tail end of the first fiber core Core1 is connected in series with the tail end of the second fiber core Core2. At this time, the incident light is transmitted back to the head end of the second fiber core Core2. The head end of the second fiber core Core2 is then connected in series with the head end of the third fiber core Core3. The tail end of the third fiber core Core3 is then connected in series with the tail end of the fourth fiber core Core4. The incident light of the first channel CH1 passes through the first fiber core Core1, the second fiber core Core2, the third fiber core Core3, and the fourth fiber core Core4 in sequence in a serpentine transmission.

[0051] The second circulator Circulator2 in the two sets of circulators injects the incident light of the second channel CH2 into the fifth fiber core Core5. The tail end of the fifth fiber core Core5 is connected in series with the tail end of the sixth fiber core Core6. At this time, the incident light is transmitted back to the head end of the sixth fiber core Core6, and the head end of the sixth fiber core Core6 is connected in series with the head end of the seventh fiber core Core7. The incident light of the second channel CH2 passes through the fifth fiber core Core5, the sixth fiber core Core6 and the seventh fiber core Core7 in sequence and is transmitted in a serpentine shape.

[0052] The first fiber core Core1, the second fiber core Core2, the third fiber core Core3, the fourth fiber core Core4, the fifth fiber core Core5, the sixth fiber core Core6 and the seventh fiber core Core7 are all in one optical fiber. The underwater acoustic pressure acts evenly on each fiber core, and the phase changes of the transmitted light caused by the strain changes of each fiber core are consistent. This is the prerequisite for signal accumulation.

[0053] Theoretically, if the noise in the seven cores is completely uncorrelated, the phase difference signals demodulated from the same perturbation position in seven independent optical fibers are superimposed. Theoretically, the equivalent self-noise pressure of the system can be reduced by 10log7 = 8.45dB after superposition. 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 components causes a certain degree of correlation in the noise characteristics of the seven cores. Furthermore, the data from each core in the same channel originates from the same pulse and also exhibits a certain degree of correlation. Therefore, the cross-correlation coefficient of the noise in the seven cores is first calculated. Based on this, the actual equivalent self-noise pressure suppression effect of the superposition of the signals from the seven independent optical fibers is calculated.

[0054] A method for suppressing equivalent self-noise pressure based on multi-core optical fiber space division multiplexing signals further includes: step 3, 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:

[0055]

[0056] In formula (1), a is the proportional coefficient, and the average noise power is:

[0057]

[0058] make The average signal-to-noise ratio of the array can be expressed as:

[0059]

[0060] 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;

[0061]

[0062] 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:

[0063]

[0064] 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,

[0065]

[0066] 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,

[0067] The first fiber core Core1, the second fiber core Core2, the third fiber core Core3, the fourth fiber core Core4, the fifth fiber core Core5, the sixth fiber core Core6 and the seventh fiber core Core7 are spatially multiplexed, that is, there are seven array elements. When a certain position of the sensing fiber is subjected to vibration, the seven cores of the sensing fiber all demodulate the vibration signal and superimpose it. First, the noise correlation of the system is analyzed, and then it is analyzed whether the equivalent self-noise pressure of the system before and after superposition is suppressed under the premise that the noise is correlated.

[0068] A method for suppressing equivalent self-noise pressure based on multi-core optical fiber space division multiplexing signals further includes: step 4, calculating the cross-correlation coefficient matrix of seven independent optical fiber core noises using formula (5):

[0069]

[0070] 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 the seven optical fiber cores, that is, the signals are completely correlated, (ρ s ) ij =1, after the seven independent optical 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 seven cores are accumulated is lower than the theoretical value AG=10lg7=8.45dB.

[0071] A method for suppressing equivalent self-noise pressure based on multi-core optical fiber space division multiplexing signals further includes: step 5, assuming the vibration frequency is 500 Hz, and accumulating the time domain signal before and after the demodulated phase signal. Figure 3 ) and PSD curves ( Figure 4 ), the PSD of the seven independent fiber cores at 500Hz is -33.68dB, the noise average is -80.92dB, and the signal-to-noise ratio is 47.24dB; after the signals of the seven independent fiber cores 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 independent fiber cores at four frequency points (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.

[0072] Table 1 PSD data (dB / Hz) at each frequency point before and after the accumulation of seven independent fiber core signals

[0073]

[0074]

[0075] This invention, based on a dual-channel DAS system, uses 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. Results show that the sum of the noise correlation coefficients of the seven independent optical fiber cores is 11.28. Multiplexing the seven independent optical fiber core signals achieves a 6.33dB reduction in the system's equivalent self-noise pressure within the 100-1000Hz frequency band. This suppression of equivalent self-noise pressure is crucial for improving the accuracy of the system's vibration signal reconstruction and enhancing the system's ability to detect weak, low-frequency signals.

[0076] like Figure 1 and Figure 2As shown, an embodiment of the present invention further provides an equivalent self-noise pressure suppression device based on a multi-core optical fiber space division multiplexing signal, comprising a narrow linewidth laser Laser, a first coupler C1, an acousto-optic modulator AOM, a second coupler C2, a first amplifier EDFA, two erbium-doped fiber amplifiers EDFA, two filters Filter, two circulators Circulator, a fan-in coupler Fan-in Coupler, a multi-core optical fiber Multi-core Fiber, a third coupler C3, a fourth coupler C4, a first balanced detector BPD1 and a second balanced detector BPD2.

[0077] The narrow-linewidth laser emits light that is split by the first coupler C1 and then generates a pulse signal through the acousto-optic modulator AOM. The light is then split into two transmission beams by the second coupler C2. The two beams are respectively injected into the connected sensing fiber through two sets of erbium-doped fiber amplifiers EDFA, two sets of filters, two circulators, and fan-in couplers. The fan-in coupler is optically connected to the multi-core fiber.

[0078] Figure 2 Combine Figure 1 The first circulator Circulator1 of the two circulators is connected to the third coupler C3, the first coupler C1 is connected to the third coupler C3, and the third coupler C3 is connected to the first balanced detector BPD1; the second circulator Circulator2 of the two circulators is connected to the fourth coupler C4, the first coupler C1 is connected to the fourth coupler C4, and the fourth coupler C4 is connected to the second balanced detector BPD2; wherein, the splitting ratio of the first coupler C1 is 98:1:1, and the splitting ratios of the second coupler C2, the third coupler C3, and the fourth coupler C4 are all 1:1.

[0079] The sensing optical fiber is provided with a first fiber core Core1, a second fiber core Core2, a third fiber core Core3, a fourth fiber core Core4, a fifth fiber core Core5, a sixth fiber core Core6 and a seventh fiber core Core7; the first fiber core Core1, the second fiber core Core2, the third fiber core Core3, the fourth fiber core Core4, the fifth fiber core Core5 and the sixth fiber core Core6 are arranged in a ring, and the seventh fiber core Core7 is located at the center of the ring.

[0080] Among them, the first circulator Circulator1 injects the incident light of the first channel CH1 into the first fiber core Core1, and the tail end of the first fiber core Core1 is connected in series with the tail end of the second fiber core Core2. At this time, the incident light is transmitted back to the head end of the second fiber core Core2, the head end of the second fiber core Core2 is connected in series with the head end of the third fiber core Core3, and the tail end of the third fiber core Core3 is connected in series with the tail end of the fourth fiber core Core4. The incident light of the first channel CH1 passes through the first fiber core Core1, the second fiber core Core2, the third fiber core Core3, and the fourth fiber core Core4 in sequence and is transmitted in a serpentine shape.

[0081] The second circulator Circulator2 injects the incident light of the second channel CH2 into the fifth fiber core Core5. The tail end of the fifth fiber core Core5 is connected in series with the tail end of the sixth fiber core Core6. At this time, the incident light is transmitted back to the head end of the sixth fiber core Core6. The head end of the sixth fiber core Core6 is then connected in series with the head end of the seventh fiber core Core7. The incident light of the second channel CH2 passes through the fifth fiber core Core5, the sixth fiber core Core6 and the seventh fiber core Core7 in sequence and is transmitted in a serpentine shape.

[0082] 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 suppressing equivalent self-noise pressure based on multi-core optical fiber space division multiplexing signals, 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 through 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. Step 2: When the sensing fiber is subjected to acoustic pressure, the phase of the backscattered light 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 first coupler C1 is 98:1:1, and the splitting ratios of the second coupler C2, the third coupler C3, and the fourth coupler C4 are all 1:

1. The sensing optical fiber of step 1 is provided with a first fiber core Core1, a second fiber core Core2, a third fiber core Core3, a fourth fiber core Core4, a fifth fiber core Core5, a sixth fiber core Core6 and a seventh fiber core Core7; The first circulator Circulator1 in the two circulator groups injects the incident light of the first channel CH1 into the first fiber core Core1. The tail end of the first fiber core Core1 is connected in series with the tail end of the second fiber core Core2. At this time, the incident light is transmitted back to the head end of the second fiber core Core2. The head end of the second fiber core Core2 is then connected in series with the head end of the third fiber core Core3. The tail end of the third fiber core Core3 is then connected in series with the tail end of the fourth fiber core Core4. The incident light of the first channel CH1 is transmitted in a serpentine shape through the first fiber core Core1, the second fiber core Core2, the third fiber core Core3, and the fourth fiber core Core4 in sequence. The second circulator 2 in the two circulator groups injects the incident light from the second channel CH2 into the fifth fiber core Core5. The tail end of the fifth fiber core Core5 is connected in series with the tail end of the sixth fiber core Core6. At this time, the incident light is transmitted back to the head end of the sixth fiber core Core6. The head end of the sixth fiber core Core6 is then connected in series with the head end of the seventh fiber core Core7. The incident light from the second channel CH2 is transmitted in a serpentine shape through the fifth fiber core Core5, the sixth fiber core Core6, and the seventh fiber core Core7 in sequence. The first fiber core, Core 1, the second fiber core, Core 2, the third fiber core, Core 3, the fourth fiber core, Core 4, the fifth fiber core, Core 5, the sixth fiber core, and the seventh fiber core, Core 7, are all in a single optical fiber. The underwater acoustic pressure acts evenly on each fiber core, and the phase change of the transmitted light caused by the strain change of each fiber core is consistent. This is the prerequisite for signal accumulation. Also includes: step 3, set Array element signals with consistent sensitivity , , , Linear addition, the average power of the output signal is: (1)； In formula (1), is the proportional coefficient, and the average noise power is: (2)； make , , then the average signal-to-noise ratio of the array can be expressed as: (3)； in, 、 Respectively represent Array elements and The cross-correlation coefficient between the signal and noise of each array element; (4)； (5)； 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: (6)； 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, (7)； The array gain after the accumulation of array elements is , when the signals are perfectly correlated but the noise is partially correlated, (8)； The first fiber core Core1, the second fiber core Core2, the third fiber core Core3, the fourth fiber core Core4, the fifth fiber core Core5, the sixth fiber core Core6 and the seventh fiber core Core7 are spatially multiplexed, that is, there are seven array elements. When a certain position of the sensing fiber is subjected to vibration, the seven cores of the sensing fiber all demodulate the vibration signal and superimpose it. First, the noise correlation of the system is analyzed, and then it is analyzed whether the equivalent self-noise pressure of the system before and after superposition is suppressed under the premise that the noise is correlated.

2. The method for suppressing equivalent self-noise pressure based on multi-core optical fiber space division multiplexing signals according to claim 1, characterized in that: Also includes: Step 4: The cross-correlation coefficient matrix of the seven independent fiber core noises is calculated by formula (5): （9）； in, , ,but When the same signal acts on the seven optical fiber cores, the signals are completely correlated. After the seven independent optical fiber cores are added together, the system equivalent self-noise pressure can be reduced to Due to the existence of noise correlation, the equivalent self-noise pressure suppression effect after seven cores are accumulated is lower than the theoretical value. .

3. The method for suppressing equivalent self-noise pressure based on multi-core optical fiber space division multiplexing signals according to claim 2, characterized in that: The method further includes: step 5, 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 independent optical 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 independent optical 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 independent optical 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.

4. An equivalent self-noise pressure suppression device based on a multi-core optical fiber space division multiplexing signal, used to implement the equivalent self-noise pressure suppression method based on a multi-core optical fiber space division multiplexing signal according to any one of claims 1 to 3, characterized in that: It includes a narrow linewidth laser, a first coupler C1, an acousto-optic modulator AOM, a second coupler C2, a first amplifier EDFA, two erbium-doped fiber amplifiers EDFA, two filters, two circulators, a fan-in / fan-out coupler, a multi-core fiber, a third coupler C3, a fourth coupler C4, a first balanced detector BPD1 and a second balanced detector BPD2; The narrow-linewidth laser emits light that is split by the first coupler C1 and then generates a pulse signal through the acousto-optic modulator AOM. The light is then split into two beams by the second coupler C2 and respectively injected into the connected sensing fiber through two sets of erbium-doped fiber amplifiers EDFA, two sets of filters, two circulators, and a fan-in coupler. The fan-in coupler is then optically connected to the multi-core fiber. The first circulator Circulator1 of the two circulators is connected to the third coupler C3, the first coupler C1 is connected to the third coupler C3, and the third coupler C3 is connected to the first balanced detector BPD1; the second circulator Circulator2 of the two circulators is connected to the fourth coupler C4, the first coupler C1 is connected to the fourth coupler C4, and the fourth coupler C4 is connected to the second balanced detector BPD2; wherein, the splitting ratio of the first coupler C1 is 98:1:1, and the splitting ratios of the second coupler C2, the third coupler C3, and the fourth coupler C4 are all 1:

1.

5. The equivalent self-noise pressure suppression device based on multi-core optical fiber space division multiplexing signal according to claim 4, characterized in that: The sensing optical fiber is provided with a first fiber core Core1, a second fiber core Core2, a third fiber core Core3, a fourth fiber core Core4, a fifth fiber core Core5, a sixth fiber core Core6 and a seventh fiber core Core7; the first fiber core Core1, the second fiber core Core2, the third fiber core Core3, the fourth fiber core Core4, the fifth fiber core Core5 and the sixth fiber core Core6 are arranged in a ring, and the seventh fiber core Core7 is located at the center of the ring; Among them, the first circulator Circulator1 injects the incident light of the first channel CH1 into the first fiber core Core1, and the tail end of the first fiber core Core1 is connected in series with the tail end of the second fiber core Core2. At this time, the incident light is transmitted back to the head end of the second fiber core Core2, the head end of the second fiber core Core2 is connected in series with the head end of the third fiber core Core3, and the tail end of the third fiber core Core3 is connected in series with the tail end of the fourth fiber core Core4. The incident light of the first channel CH1 passes through the first fiber core Core1, the second fiber core Core2, the third fiber core Core3, and the fourth fiber core Core4 in sequence in a serpentine transmission; The second circulator Circulator2 injects the incident light of the second channel CH2 into the fifth fiber core Core5. The tail end of the fifth fiber core Core5 is connected in series with the tail end of the sixth fiber core Core6. At this time, the incident light is transmitted back to the head end of the sixth fiber core Core6. The head end of the sixth fiber core Core6 is then connected in series with the head end of the seventh fiber core Core7. The incident light of the second channel CH2 passes through the fifth fiber core Core5, the sixth fiber core Core6 and the seventh fiber core Core7 in sequence and is transmitted in a serpentine shape.

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