Equivalent self-noise pressure suppression method and device based on multi-core optical fiber space division multiplexing signal

Abstract

The invention discloses an equivalent self-noise pressure suppression method and device based on a multi-core optical fiber space division multiplexing signal, light emitted by a narrow linewidth laser is split through a first coupler C1, one path of light passes through an acousto-optic modulator AOM to generate a pulse signal, and then the pulse signal passes through a second coupler C2 to generate a pulse signal; 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 the dual-channel DAS system, the multi-core optical fiber serves as the 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 suppression effect of the seven-core space division signals is analyzed, and the detection capacity of the system for low-frequency weak signals can be improved.

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 a 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 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 equivalent self-noise pressure suppression and sensitivity of single-mode fiber DAS systems, researchers have introduced special optical fibers to improve system performance, such as continuous scattering enhanced fiber, discrete scattering enhanced 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 optical 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 optical 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 still correlated, the equivalent self-noise pressure of the system cannot be suppressed, and the quality of the acquired signal cannot be guaranteed. In addition, since the energy decay rate of low-frequency noise is slow during propagation, it is easy to be mixed with low-frequency signals, making it difficult to detect low-frequency signals. Therefore, suppressing the low-frequency equivalent self-noise pressure level of the system is of great significance to achieve high-precision detection of weak signals.

[0005] In order to suppress the equivalent self-noise pressure spectrum level of the DAS system, reduce the low-frequency equivalent self-noise pressure of the system and ensure the small diameter characteristics of the hydrophone unit, based on the dual-channel DAS detection and demodulation system, the same detection pulse light is guaranteed to be emitted by the dual channels at the same time, which is the premise for the subsequent signal superposition; Multi-core Fiber (MCF) is used as the sensing fiber, and 3 cores and 4 cores are connected in series to form two sensing channels, and connected to the DAS module. Each core responds to external disturbances in a consistent manner but the noise is inconsistent. The seven core signals at the same position are superimposed to reduce the equivalent self-noise pressure of the system, which is of great significance for the detection of low-frequency weak signals. Summary of the invention

[0006] In view of the problems that the existing DAS system has high equivalent self-noise pressure and insufficient detection capability for weak signals, 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 technical solution provided by the present invention is as follows:

[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] Step 1, the light emitted by the narrow linewidth 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, two groups of circulators, and fan-in couplers. The pulse light undergoes back Rayleigh scattering in the sensing optical fiber;

[0010] 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 at 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 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 groups of circulators 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 connected in series with the head end of the third fiber core Core3. 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 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 Circulator2 in the two groups of circulators injects the incident light of the second channel CH2 into the fifth fiber core Core5, and 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 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. The phase change of the transmitted light caused by the strain change of each fiber core is consistent, which is the prerequisite for signal accumulation.

[0015] A preferred embodiment of the present invention further comprises: 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 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] 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 space-division multiplexed, that is, there are seven array elements. When a certain position of the sensor optical fiber is vibrated, the seven cores of the sensor optical 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 has a suppression effect under the premise that the noise is correlated.

[0029] A preferred embodiment of the present invention further includes: Step 4, calculating the correlation coefficient matrix of the seven independent optical fiber 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 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 added is lower than the theoretical value AG=10lg7=8.45dB.

[0032] A preferred embodiment of the present invention also 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 seven independent optical 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 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 optical fiber amplifiers EDFA, two filters Filter, two circulators Circulator, a fan-in coupler Fan-inCoupler, 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 line width laser emits light which is split by the first coupler C1, then generates a pulse signal by the acousto-optic modulator AOM, and then is split into two transmission lights 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, two circulators, and fan-in couplers. The fan-in coupler is 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, and 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;

[0038] The second circulator Circulator2 injects the incident light of the second channel CH2 into the fifth fiber core Core5, and 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 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.

[0039] Compared with the prior art, the embodiment of the present invention provides an equivalent self-noise pressure suppression method based on multi-core optical fiber space division multiplexing signal, which has the following beneficial effects: the present invention is based on a dual-channel DAS system, uses multiple independent optical fiber cores as sensing optical fibers, and connects three cores and four cores in series to form two sensing channels, multiplexes the seven-core space division signal, and analyzes its 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 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. The suppression of equivalent self-noise pressure is of great significance for improving the accuracy of the system in restoring vibration signals 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 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.

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

[0044] Figure 4 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 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.

[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] Step 1, the light emitted by the narrow linewidth 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, two groups of circulators, and fan-in couplers. The pulse light undergoes back Rayleigh scattering in the sensing optical fiber;

[0048] 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 at 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 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 groups of circulators 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 connected in series with the head end of the third fiber core Core3. 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 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.

[0051] The second circulator Circulator2 in the two groups of circulators Circulator injects the incident light of the second channel CH2 into the fifth fiber core Core5, and 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 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.

[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. The phase change of the transmitted light caused by the strain change of each fiber core is consistent, which is the prerequisite for signal accumulation.

[0053] The phase difference signals demodulated from the same disturbance position of seven independent optical fiber cores are superimposed. Theoretically, when the noise of the seven fiber cores is 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 source of the two channels comes 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 fiber core in the same channel comes from the same pulse and also has a certain correlation. Therefore, the mutual correlation coefficient of the seven-core noise is calculated first, and on this basis, the actual equivalent self-noise pressure suppression effect after the seven independent optical fiber core signals are superimposed is calculated.

[0054] A method for suppressing equivalent self-noise pressure based on multi-core optical fiber space division multiplexing signals also 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:

[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 the 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 = 10lgm. 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 space-division multiplexed, that is, there are seven array elements. When a certain position of the sensor optical fiber is vibrated, the seven cores of the sensor optical 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 has a suppression effect 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 also includes: step 4, calculating the mutual correlation coefficient matrix of seven independent optical fiber core noises by formula (5):

[0069]

[0070] 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 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 added 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 also includes: step 5, assuming that the vibration frequency is 500 Hz, and accumulating the time domain signal before and after the demodulated phase signal Time Domain 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 seven independent fiber 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 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] The present invention is based on a dual-channel DAS system, with multi-core optical fiber as the sensing optical fiber, and three cores and four cores 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 independent 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 100-1000Hz frequency band. The suppression of equivalent self-noise pressure is of great significance for improving the accuracy of the system in restoring vibration signals and enhancing the system's ability to detect low-frequency weak 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, including 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 by the second coupler C2 into two transmission lights that are injected into the connected sensing optical fiber through two groups of erbium-doped fiber amplifiers EDFA, two groups of filters, two circulators, and fan-in couplers. The fan-in coupler is optically connected to the multi-core fiber.

[0078] Figure 2 Combination 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 first fiber core Core1 through the incident light of the first channel CH1, 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, and 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, and 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 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.

[0082] 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 suppressing equivalent self-noise pressure based on multi-core optical fiber space division multiplexing signals, characterized in that: The following steps are involved: Step 1, the light emitted by the narrow linewidth 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, two groups of circulators, and fan-in couplers. The pulse light undergoes back Rayleigh scattering in the sensing optical fiber; 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 at 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 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.

2. The method for suppressing equivalent self-noise pressure based on multi-core optical fiber space division multiplexing signal according to claim 1, characterized in that: 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 groups of circulators 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 connected in series with the head end of the third fiber core Core3. 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 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 Circulator2 in the two groups of circulators injects the incident light of the second channel CH2 into the fifth fiber core Core5, and 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 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 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. The phase change of the transmitted light caused by the strain change of each fiber core is consistent, which is the prerequisite for signal accumulation.

3. The method for suppressing equivalent self-noise pressure based on multi-core optical fiber space division multiplexing signal according to claim 2, characterized in that: The method further comprises: 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: 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, 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 space-division multiplexed, that is, there are seven array elements. When a certain position of the sensor optical fiber is vibrated, the seven cores of the sensor optical 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 has a suppression effect under the premise that the noise is correlated.

4. The method for suppressing equivalent self-noise pressure based on multi-core optical fiber space division multiplexing signal according to claim 3, characterized in that: The method further comprises: step 4, calculating the correlation coefficient matrix of seven independent optical fiber 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 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 added is lower than the theoretical value AG=10lg7=8.45dB.

5. The method for suppressing equivalent self-noise pressure based on multi-core optical fiber space division multiplexing signal according to claim 4, characterized in that: It also includes: Step 5, 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 independent optical 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 optical 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 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 6.33dB, that is, the system equivalent self-noise pressure is reduced by 6.33dB, which is close to the calculated value.

6. An equivalent self-noise pressure suppression device based on multi-core optical fiber space division multiplexing signal, characterized in that: It includes 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 / fan-out coupler Fan-in / out Coupler, a multi-core fiber Multi-core Fiber, a third coupler C3, a fourth coupler C4, a first balanced detector BPD1 and a second balanced detector BPD2; The narrow line width laser emits light which is split by the first coupler C1, and then generates a pulse signal by the acousto-optic modulator AOM. Then, it is divided into two transmission lights by the second coupler C2, and respectively injected into the connected sensing optical fiber through two groups of erbium-doped fiber amplifiers EDFA, two groups of filters, two circulators, and fan-in couplers. The fan-in coupler is 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:

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7. The equivalent self-noise pressure suppression device based on multi-core optical fiber space division multiplexing signal according to claim 6, 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, and 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; The second circulator Circulator2 injects the incident light of the second channel CH2 into the fifth fiber core Core5, and 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 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.

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