A system for canceling fading noise in a distributed fiber acoustic sensing system

Abstract

The application discloses a system for eliminating fading noise in a distributed optical fiber acoustic wave sensing system, which comprises a light source, a light frequency shift modulator, a light amplifier, an optical circulator, a photoelectric detector, a data acquisition card and a data demodulator; the laser emitted by the light source is modulated into double-pulse light with a frequency difference by the light frequency shift modulator, the double-pulse light is amplified by the light amplifier and then injected into an optical fiber through the optical circulator, the Rayleigh backscattering light generated in the optical fiber is emitted back to the optical circulator, the light emitted from the optical circulator is detected by the photoelectric detector and converted into an electric signal, the electric signal is collected by the data acquisition card and converted into a digital signal, and the data demodulator demodulates the digital signal by using a fading noise elimination method to obtain a signal in which the fading noise is eliminated. Therefore, the application can effectively eliminate the fading noise in the distributed optical fiber acoustic wave sensing system, improve the quality of the signal and eliminate the interference of the error signal.

Description

Technical Field

[0001] This invention relates to a fading noise elimination system in a distributed fiber optic acoustic wave sensing system, and pertains to the field of distributed fiber optic sensing. Background Technology

[0002] Distributed fiber-optic acoustic sensing (DAS) technology utilizes Rayleigh backscattering in optical fibers for sensing and localization. A DAS system requires injecting highly coherent pulsed light into the fiber, and the detected backscattered light is the Rayleigh scattered light at all locations within the pulse width. Since the amplitude and phase of Rayleigh scattering are randomly distributed along the fiber axis, and the detected signal is the superposition of these random scatterings, the detected signal will exhibit significant intensity fluctuations; these fluctuations constitute fading noise.

[0003] Fading noise is inherent in distributed fiber optic acoustic wave sensing systems and is caused by the randomness of Rayleigh scattering. As the performance of distributed fiber optic acoustic wave sensing systems continues to improve, fading noise has become a key issue limiting its performance. Fading noise not only reduces the signal-to-noise ratio but also causes signal errors, leading to significant errors in measurement results and hindering the system's application effectiveness.

[0004] Hartog et al. used multiple optical frequencies to reduce fading noise. Their system incorporated multiple different optical frequencies, with signals from different frequencies superimposed to reduce noise caused by fading. Zhao et al. used spatial division multiplexing in few-mode fibers to suppress fading noise. These methods employ the average of multidimensional (frequency, phase, or spatial) signals to suppress random fading noise, increasing the complexity of the hardware or modulated signal. Dong et al. proposed a denoising network based on game theory, called convolutional adversarial denoising, to suppress random noise in DAS data. Zhao et al. proposed a DAS data denoiser based on convolutional neural networks, which can suppress fading noise and various other types of noise. However, the denoiser requires a training set, limiting the adaptability of this method. Summary of the Invention

[0005] The present invention aims to at least solve one of the technical problems existing in the prior art. Therefore, in view of the above-mentioned problems, the object of the present invention is to provide a fading noise cancellation system in a distributed fiber optic acoustic wave sensing system, which can eliminate fading noise in distributed fiber optic acoustic wave sensing and improve signal quality.

[0006] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows:

[0007] In a first aspect, the present invention provides a fading noise cancellation system in a distributed fiber optic acoustic sensing system. The system includes a light source, an optical frequency-shift modulator, an optical amplifier, an optical circulator, a photodetector, a data acquisition card, and a data demodulator. The laser emitted by the light source is modulated by the optical frequency-shift modulator into a double-pulse light with a frequency difference. The double-pulse light is amplified by the optical amplifier and injected into an optical fiber through the optical circulator. Rayleigh backscattered light generated within the optical fiber is emitted back to the optical circulator. The light emitted from the optical circulator is detected by the photodetector and converted into an electrical signal. The electrical signal is acquired by the data acquisition card and converted into a digital signal. The data demodulator demodulates the digital signal using a fading noise cancellation method to obtain a signal with fading noise eliminated.

[0008] Furthermore, the continuous laser emitted by the light source at a frequency of f0 is modulated by the optical frequency shift modulator into a double-pulse light with a frequency difference. The width of the two pulses is w, and the pulse interval is d. The frequency of the first modulated optical pulse is f0+f1, and the frequency of the second modulated optical pulse is f0+f2.

[0009] Furthermore, the implementation process of the fading noise cancellation method includes:

[0010] Place the optical fiber in a Cartesian coordinate system, with the fiber along the Z-axis, and obtain the light field E of the Rayleigh backscattered light at position z of the optical fiber. RS (z,t);

[0011] Obtain the interference intensity of the Rayleigh backscattered light at position z in the optical fiber, where the interference intensity is the light field E. RS The square of (z,t);

[0012] Heterodyne demodulation is performed on the interference intensity of Rayleigh backscattered light to obtain the change in phase of the optical wave in the optical fiber caused by external vibration. Where γ(z,t) is the fading factor at position z, d is the pulse interval, and sin(2πf) is the fading factor at position z. s t) represents the actual phase change of the light wave caused by external vibration. This is the common-mode noise term;

[0013] The fading factor γ(z,t) is obtained using common-mode noise. Dividing Φ(z,t) by the value of the fading factor γ(z,t) yields:

[0014]

[0015] A high-pass filter is selected based on the drift frequency of the common-mode noise. The low-frequency, slowly drifting common-mode noise term is filtered out by the high-pass filter, resulting in an optical wave phase demodulation result with fading noise eliminated.

[0016]

[0017] Furthermore, the light field E of the Rayleigh backscattered light at position z of the optical fiber... RS (z,t) is:

[0018]

[0019] Where E1 is the amplitude of the first optical pulse, E2 is the amplitude of the second optical pulse, r(p,t) is the Rayleigh scattering rate at position p on the optical fiber at time t, θ(p,t) is the Rayleigh scattering phase at position p on the optical fiber at time t, and r and θ are random quantities. Let be the total phase change experienced by the Rayleigh backscattered light at position p. Let be the change in the phase of the light wave at position l on the optical fiber at time t caused by external vibration. The common-mode noise experienced by the first light pulse. This refers to the common-mode noise experienced by the second optical pulse.

[0020] Furthermore, the specific process of obtaining the fading factor γ(z,t) value using common-mode noise is as follows:

[0021] Add a low-pass filter to the signal at all positions z of Φ(z,t) to extract the low-frequency slow drift term of common-mode noise;

[0022] The effective value of the filtered signal is obtained according to its position and denoted as R(z). Then the fading factor at different positions is γ(z)=R(z) / max(R(z)), where max(*) is the calculated maximum value.

[0023] Secondly, the present invention provides a fading noise cancellation system for a distributed fiber optic acoustic wave sensing system. The system includes a light source, a first optical coupler, a second optical coupler, a first optical frequency-shifting modulator, a second optical frequency-shifting modulator, an optical phase modulator, an optical amplifier, an optical circulator, a photodetector, a data acquisition card, and a data demodulator. The light source emits a continuous laser beam with a frequency of f0, which is split into two paths by the first optical coupler. The first path of light is frequency-shifted by the first optical frequency-shifting modulator to a frequency of f0+f1, and the continuous laser beam is modulated into a pulse beam with a width of w. The second path of light is frequency-shifted by the second optical frequency-shifting modulator to a frequency of f0+f2, and the continuous laser beam is modulated into a pulse beam with a width of w. The light beam with a frequency of f0+f2 undergoes sinusoidal phase modulation by the optical phase modulator, resulting in a frequency of f... cSimultaneously, the second optical frequency shift modulator also delays the pulsed light, so that the delay interval between the first and second optical pulses is d. The two modulated lights are combined into one by the second optical coupler. The combined light is sent to the optical amplifier for amplification and then injected into the optical fiber through the optical circulator. The Rayleigh backscattered light generated in the optical fiber is emitted back to the optical circulator. The light emitted from the optical circulator is detected by the photodetector and converted into an electrical signal. The electrical signal is acquired by the data acquisition card and converted into a digital signal. The data demodulator demodulates the digital signal using a fading noise cancellation method to obtain a signal with fading noise eliminated.

[0024] Furthermore, the implementation process of the fading noise cancellation method includes:

[0025] The optical fiber is placed in a Cartesian coordinate system, and the optical field E of the Rayleigh backscattered light at position z of the fiber is obtained along the Z-axis. RS (z,t);

[0026] Obtain the interference intensity of the Rayleigh backscattered light at position z in the optical fiber, where the interference intensity of the Rayleigh backscattered light at position z in the optical fiber is the optical field E. RS The square of (z,t);

[0027] Heterodyne demodulation is performed on the interference intensity of the Rayleigh backscattered light at position z in the optical fiber to obtain the change in the phase of the optical wave in the fiber caused by external vibration:

[0028] Φ(z,t)=-dγ(z,t)sin(2πf s t)+γ(z,t)sin(2πf c t);

[0029] Where γ(z,t) is the fading factor at position z, sin(2πf s t) represents the actual phase change of the light wave caused by external vibration, sin(2πf) c t) is a fixed-frequency phase modulation signal loaded by an optical phase modulator;

[0030] The fading factor γ(z,t) is obtained using fixed modulation, and Φ(z,t) is divided by the fading factor γ(z,t) to obtain:

[0031]

[0032] Add a low-pass filter cutoff frequency to filter out the fixed modulation term sin(2πf) of Φ(z,t). c t), we get:

[0033]

[0034] Among them, the cutoff frequency f of the low-pass filter cut <f c .

[0035] Furthermore, the optical field E of the Rayleigh backscattered light at position z in the optical fiber RS (z,t);

[0036]

[0037] Where E1 is the amplitude of the first optical pulse, E2 is the amplitude of the second optical pulse, r(p,t) is the Rayleigh scattering rate at position p on the optical fiber at time t, θ(p,t) is the Rayleigh scattering phase at position p on the optical fiber at time t, and r(p,t) and θ(p,t) are random quantities along spatial position p. Let be the total phase change experienced by the Rayleigh backscattered light at position p. It represents the change in the phase of the optical wave at position l on the optical fiber at time t caused by external vibration.

[0038] Furthermore, the method for obtaining the fading factor γ(z,t) using fixed modulation is as follows: First method: Perform Fourier transform on the signal at all positions z to obtain the power spectral density, and sequentially extract the signal at position z at frequency f. c Power spectral intensity P at the location c (z), then the fading factor at different positions is γ(z,t)=P c (z) / max(P c (z)), where max(*) is the calculated maximum value; the second method: add a bandpass filter to the signal at all positions z, with a passband frequency of f. c Then, the effective value of the filtered signal is obtained according to the position and denoted as R(z). The fading factor at different positions is γ(z,t)=R(z) / max(R(z)), where max(*) is the calculated maximum value.

[0039] This invention, by adopting the above technical solution, has the following characteristics: Since Rayleigh scattering varies randomly with position, fading noise also varies randomly with position. This invention can effectively eliminate fading noise in distributed fiber optic acoustic sensing systems, eliminate interference from erroneous signals, and improve signal quality. In summary, this invention has excellent application prospects in distributed acoustic sensing, seismic exploration, and oilfield logging. Attached Figure Description

[0040] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts. In the drawings:

[0041] Figure 1 This is a structural diagram of the fading noise elimination system in the distributed optical fiber acoustic wave sensing system of Embodiment 1 of the present invention.

[0042] Figure 2 This is a schematic diagram of the signal before fading noise is eliminated in Embodiment 1 of the present invention.

[0043] Figure 3 This is a schematic diagram of the signal after eliminating fading noise according to Embodiment 1 of the present invention.

[0044] Figure 4 This is a structural diagram of the fading noise elimination system in the distributed optical fiber acoustic wave sensing system of Embodiment 2 of the present invention.

[0045] Figure 5 This is a schematic diagram of the signal before fading noise is eliminated in Embodiment 2 of the present invention.

[0046] Figure 6 This is a schematic diagram of the signal after eliminating fading noise according to Embodiment 2 of the present invention. Detailed Implementation

[0047] It should be understood that the terminology used herein is for the purpose of describing particular exemplary embodiments only and is not intended to be limiting. Unless the context clearly indicates otherwise, the singular forms “a,” “an,” and “described” as used herein may also include the plural forms. The terms “comprising,” “including,” “containing,” and “having” are inclusive and therefore indicate the presence of the stated features, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, elements, components, and / or combinations thereof. The method steps, processes, and operations described herein are not construed as requiring them to be performed in a particular order described or illustrated unless the order of performance is explicitly indicated. It should also be understood that additional or alternative steps may be used.

[0048] Although terms such as first, second, third, etc., may be used in this document to describe multiple elements, components, regions, layers, and / or segments, these elements, components, regions, layers, and / or segments should not be limited by these terms. These terms may be used only to distinguish one element, component, region, layer, or segment from another. Unless the context clearly indicates otherwise, terms such as "first," "second," and other numerical terms used herein do not imply order or sequence. Therefore, the first element, component, region, layer, or segment discussed below may be referred to as the second element, component, region, layer, or segment without departing from the teachings of the exemplary embodiments.

[0049] For ease of description, spatial relative terms may be used in the text to describe the relationship of one element or feature relative to another element or feature as shown in the figure. These relative terms include, for example, "inside," "outside," "middle," "outer," "below," "above," etc. Such spatial relative terms are intended to include different orientations of the device in use or operation, other than those depicted in the figure.

[0050] This invention provides a fading noise cancellation system for a distributed fiber optic acoustic wave sensing system. The system includes a light source, an optical frequency-shift modulator, an optical amplifier, an optical circulator, a photodetector, a data acquisition card, and a data demodulator. Laser light emitted from the light source is modulated into a double-pulse light with a frequency difference by the optical frequency-shift modulator. The double-pulse light is amplified by the optical amplifier and injected into the optical fiber through the optical circulator. Rayleigh backscattered light generated within the optical fiber is emitted back to the optical circulator. The light emitted from the optical circulator is detected by the photodetector and converted into an electrical signal. The electrical signal is acquired by the data acquisition card and converted into a digital signal. The data demodulator demodulates the digital signal using a fading noise cancellation method to obtain a signal with fading noise eliminated. Therefore, this invention can effectively eliminate fading noise in a distributed fiber optic acoustic wave sensing system, improve signal quality, and eliminate erroneous signal interference.

[0051] Exemplary embodiments of the invention will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the invention are shown in the drawings, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the invention and to fully convey the scope of the invention to those skilled in the art.

[0052] Example 1: As Figure 1As shown, the fading noise cancellation system in the distributed fiber optic acoustic wave sensing system provided in this embodiment includes a light source 101, a first optical coupler 102, a second optical coupler 103, a first optical frequency shift modulator 104, a second optical frequency shift modulator 105, an optical phase modulator 106, an optical amplifier 107, an optical circulator 108, a photodetector 109, a data acquisition card 110, and a data demodulator 111, wherein:

[0053] Light source 101 emits continuous laser light with a frequency of f0, which is split into two paths by the first optical coupler 102. The first path light is frequency-shifted by the first optical frequency-shift modulator 104 to a frequency of f0+f1, and the continuous laser light is simultaneously modulated into a pulse light with a width of w. The second path light is frequency-shifted by the second optical frequency-shift modulator 105 to a frequency of f0+f2, and the continuous laser light is simultaneously modulated into a pulse light with a width of w. The frequency-shifted modulated light with a frequency of f0+f2 is then sinusoidally modulated by the optical phase modulator 106 to a frequency of f0+f1. c Simultaneously, the second optical frequency shift modulator 105 delays the pulsed light, making the delay interval between the first and second optical pulses d. The two modulated lights are combined into one beam by the second optical coupler 103. The combined light is amplified by the optical amplifier 107 and injected into the optical fiber through the optical circulator 108. The Rayleigh backscattered light generated in the optical fiber is emitted to the optical circulator 108. The light emitted from the optical circulator 108 is detected by the photodetector 109 and converted into an electrical signal. The electrical signal is acquired by the data acquisition card 110 and converted into a digital signal. The data demodulator 111 demodulates the digital signal using a fading noise cancellation method to obtain a signal with fading noise eliminated.

[0054] Furthermore, the specific implementation process of the fading noise cancellation method includes:

[0055] S1. Place the optical fiber in a Cartesian coordinate system, with the fiber aligned along the Z-axis, and obtain the light field E of the Rayleigh backscattered light at position z on the fiber. RS (z,t).

[0056] In this embodiment, the light field of the Rayleigh backscattered light at position z of the optical fiber is represented as follows:

[0057]

[0058] Where E1 is the amplitude of the first optical pulse, E2 is the amplitude of the second optical pulse, r(p,t) is the Rayleigh scattering rate at position p on the optical fiber at time t, θ(p,t) is the Rayleigh scattering phase at position p on the optical fiber at time t, and r(p,t) and θ(p,t) are random quantities along spatial position p. Let be the total phase change experienced by the Rayleigh backscattered light at position p. The phase change of the light wave at position l on the optical fiber at time t caused by external vibration; sin(2πf c t) is the phase modulation signal loaded by the optical phase modulator.

[0059] S2. Obtain the interference intensity of the Rayleigh backscattered light at position z of the optical fiber.

[0060] In this embodiment, the interference intensity of the Rayleigh backscattered light at position z of the optical fiber can be determined by the optical field E. RS The square of (z,t) is obtained, expressed as:

[0061]

[0062] In the formula, Δf = f1 - f2 is the heterodyne frequency, and x and y are the integral variables.

[0063] S3. Apply the heterodyne demodulation algorithm to equation (2) to obtain the change in the phase of the optical wave in the optical fiber caused by external vibration. The heterodyne demodulation algorithm process is as follows:

[0064] Generate digital reference signal I r1 =sin(2πΔft) and I r2 =cos(2πΔft).

[0065] Combine equation (2) with I r1 After mixing and low-pass filtering, the signal can be obtained:

[0066]

[0067] Combine equation (2) with I r2 After mixing and low-pass filtering, the signal can be obtained:

[0068]

[0069] The cutoff frequency f of the low-pass filter in the above steps cut Satisfying the relation: 0 <f cut <Δf.

[0070] Dividing equation (3) by equation (4) and performing an arctangent operation yields the change in phase of the optical wave in the optical fiber caused by external vibration, Φ(z,t):

[0071]

[0072] In equation (5), the Rayleigh scattering phase θ is uniformly distributed along space, so the integral along space is zero;

[0073] Further assuming the external vibration signal has a frequency of f sFor a single-frequency plane wave, the phase change of the light wave caused by external vibration can be expressed as: Therefore, Equation (5) can be simplified to:

[0074]

[0075] In equation (6), the Rayleigh scattering rate r follows a Rayleigh distribution along space and contains multiple integrals, resulting in random fluctuations in the demodulated phase along space, which is the fading noise. Expressing the combined effect of the Rayleigh scattering rate r as the fading factor γ(z,t), equation (6) can be further expressed as:

[0076]

[0077] In equation (7), Φ(z,t) is the phase change of the demodulated light wave, γ(z,t) is the fading factor at position z, d is the pulse interval, and sin(2πf) is the phase change of the light wave. s t) represents the actual phase change of the light wave caused by external vibration, sin(2πf) c t) represents the fixed-frequency phase modulation signal loaded by the optical phase modulator. If there is no fixed-frequency phase modulation signal loaded by the optical phase modulator, then equation (7) is expressed as Φ(z,t)=-dγ(z,t)sin(2πf s The demodulated signal, in addition to containing the true phase change sin(2πf), also includes the actual phase change. s In addition to t), it also includes coefficients -d and γ(z,t). The coefficient -d only scales the actual phase change proportionally and does not cause fading changes along the spatial position; while γ(z,t) introduces fading changes along the spatial position, resulting in distortion of the demodulated signal.

[0078] S4. Obtain the value of the fading factor γ using fixed modulation.

[0079] In this embodiment, it can be seen from the above formula (7) that by using fixed modulation sin(2πf) c The value of the fading factor γ(z,t) can be obtained in two ways:

[0080] The first method involves performing a Fourier transform on the signals at all positions z in equation (7) to obtain the power spectral density, and then sequentially extracting the signals at position z at frequency f. c Power spectral intensity P at the location c (z), then the fading factor at different positions can be calculated as γ(z,t)=P c (z) / max(P c (z)), where max(*) is used to calculate the maximum value.

[0081] The second method involves adding a bandpass filter to the signal at all positions z in equation (7), with the filter passband frequency being f. c Then, the effective value (also known as the root mean square value) of the filtered signal is obtained according to the position (the effective value is obtained by averaging all the data and then taking the square root). It is denoted as R(z). The fading factor at different positions can be calculated as γ(z,t)=R(z) / max(R(z)), where max(*) is the calculated maximum value.

[0082] S5. After obtaining the fading factor γ(z,t), first divide equation (7) by the fading factor γ(z,t) to obtain:

[0083]

[0084] Then, a low-pass filter is added to equation (8), and the cutoff frequency of the filter is f. cut <f c This filters out the fixed modulation term sin(2πf) c t), we get:

[0085]

[0086] Equation (9) is the phase demodulation result after eliminating fading noise, where d is the pulse interval, sin(2πf s t) represents the actual phase change of the light wave caused by external vibration. It can be seen that the demodulation result only scales and inverts the coefficient d of the actual phase change, which has completely eliminated fading noise.

[0087] like Figure 2 The signal shown is before fading noise is eliminated. The fiber length is 180m. A 500Hz sine wave signal is applied by the optical phase modulator. A 30Hz sine wave signal is applied within a range of 60m to 90m on the fiber. Figure 2 The left figure shows a time-domain waterfall plot of the change in the phase of the optical wave in the demodulated optical fiber, and the right figure shows the amplitude of the signal at different locations. As can be seen from the right figure, the amplitude fluctuates randomly along the location, which is caused by fading noise. Figure 3 The image shows the signal obtained after applying the above-described method for eliminating fading noise. Figure 3 The left figure shows a time-domain waterfall plot of the change in the phase of the optical wave in the optical fiber after fading noise has been eliminated, and the right figure shows the amplitude of the signal at different locations. It can be seen that the amplitude fluctuation of the signal, i.e., the fading noise, has been well eliminated.

[0088] Example 2: Figure 4As shown, this embodiment also provides a fading noise cancellation system in a distributed fiber optic acoustic sensing system, including a light source 201, an optical frequency shift modulator 202, an optical amplifier 203, an optical circulator 204, a photodetector 205, a data acquisition card 206, and a data demodulator 207, wherein:

[0089] Light source 201 emits continuous laser light at frequency f0, which is modulated by optical frequency shift modulator 202 into a double-pulse light with a frequency difference. The width of each pulse is w, and the pulse interval is d. The frequency of the first optical pulse is f0+f1, and the frequency of the second optical pulse is f0+f2. The double-pulse light is amplified by optical amplifier 203 and then injected into optical fiber through optical circulator 204. The Rayleigh backscattered light generated in the optical fiber passes through optical circulator 204 to photodetector 205, where it is converted into an electrical signal. This signal is then acquired by data acquisition card 206 and converted into a digital signal. Data demodulator 207 demodulates the digital signal using a fading noise cancellation method to obtain a signal with fading noise eliminated.

[0090] Furthermore, the specific implementation process of the fading noise cancellation method is as follows:

[0091] A1. Place the optical fiber in a Cartesian coordinate system, with the fiber along the Z-axis, and obtain the light field E of the Rayleigh backscattered light at position z on the fiber. RS (z,t).

[0092] In this embodiment, the light field of the Rayleigh backscattered light at position z on the optical fiber can be expressed as:

[0093]

[0094] Where E1 is the amplitude of the first optical pulse, E2 is the amplitude of the second optical pulse, w is the width of the optical pulse, d is the delay interval between the two optical pulses, r(p,t) is the Rayleigh scattering rate at position p on the optical fiber at time t, θ(p,t) is the Rayleigh scattering phase at position p on the optical fiber at time t, and r and θ are random quantities. Let be the total phase change experienced by the Rayleigh backscattered light at position p. It represents the change in the phase of the optical wave at position l on the optical fiber at time t caused by external vibration. The common-mode noise experienced by the first light pulse. This refers to the common-mode noise experienced by the second optical pulse.

[0095] A2. Obtain the interference intensity of the Rayleigh backscattered light at position z in the optical fiber.

[0096] In this embodiment, the interference intensity of the Rayleigh backscattered light at position z on the optical fiber can be determined by the optical field E. RS The square of (z,t) is obtained, expressed as:

[0097]

[0098] In the formula, Δf = f1 - f2 is the heterodyne frequency.

[0099] A3. Applying the heterodyne demodulation algorithm to equation (11) yields the change in the phase of the optical wave in the optical fiber caused by external vibrations. The heterodyne demodulation algorithm flow is as follows:

[0100] Generate digital reference signal I r1 =sin(2πΔft) and I r2 =cos(2πΔft).

[0101] Combine equation (11) with I r1 After mixing and low-pass filtering, the signal can be obtained:

[0102]

[0103] Combine equation (12) with I r2 After mixing and low-pass filtering, the signal can be obtained:

[0104]

[0105] The cutoff frequency f of the low-pass filter in the above steps cut Satisfying the relation: 0 <f cut <Δf.

[0106] Dividing equation (12) by equation (13) and performing an arctangent operation yields the change in phase of the optical wave in the optical fiber caused by external vibration, Φ(z,t):

[0107]

[0108] In equation (14), The Rayleigh scattering phase θ is uniformly distributed along space, so the integral along space is zero;

[0109] Further assuming the external vibration signal has a frequency of f s For a single-frequency plane wave, the phase change of the light wave caused by external vibration can be expressed as: Therefore, Equation (14) can be simplified to:

[0110]

[0111] In equation (15), the Rayleigh scattering rate r follows a Rayleigh distribution along space and contains multiple integrals, which leads to random fluctuations in the demodulated phase along space, which is the fading noise.

[0112] If the combined effect of Rayleigh scattering rate r is expressed as the fading factor γ(z,t), then equation (15) can be further expressed as:

[0113]

[0114] In equation (16), Φ(z,t) is the phase change of the demodulated light wave, γ(z,t) is the fading factor at position z, d is the pulse interval, and sin(2πf) is the phase change of the light wave. s t) represents the change in the phase of the light wave caused by external vibration. The common-mode noise term is typically introduced by errors in the light source and optical frequency shift modulator, and manifests as a slow, low-frequency drift signal. If common-mode noise is ignored, equation (16) can be expressed as Φ(z,t)=-dγ(z,t)sin(2πf s The demodulated signal, in addition to containing the true phase change sin(2πf), also includes the actual phase change. s In addition to t), it also includes coefficients -d and γ(z,t). The coefficient -d only scales the actual phase change proportionally and does not cause fading changes along the spatial position; while γ(z,t) introduces fading changes along the spatial position, resulting in distortion of the demodulated signal.

[0115] A4. Obtain the value of the fading factor γ using common-mode noise.

[0116] In this embodiment, it can be seen from the above formula (16) that the fading factor γ(z,t) can be obtained using the common-mode noise term. A low-pass filter is added to the signal at all positions z in formula (16) to extract the low-frequency slow drift term of the common-mode noise. Then, the effective value of the filtered signal is obtained according to the position and denoted as R(z). The fading factor at different positions can be calculated as γ(z)=R(z) / max(R(z)), where max(*) is the calculated maximum value.

[0117] A5. After obtaining the fading factor γ(z,t), first divide equation (16) by the fading factor γ(z,t) to obtain:

[0118]

[0119] Then, a high-pass filter is added to equation (17). The cutoff frequency of the filter is determined based on the drift frequency of the common-mode noise, and is usually chosen to be below 3Hz. This filters out the low-frequency, slowly drifting common-mode noise term, resulting in:

[0120]

[0121] Equation (18) is the phase demodulation result after eliminating fading noise, where d is the pulse interval, sin(2πf st) represents the actual phase change of the light wave caused by external vibration. It can be seen that the demodulation result only scales and inverts the coefficient d of the actual phase change, which has completely eliminated fading noise.

[0122] Figure 5 The signal before fading noise was eliminated is shown. The fiber length is 180m, the ambient noise is low-frequency drift below 1Hz, and a 30Hz sine wave signal is applied within a range of 60m to 90m on the fiber. Figure 5 The left figure shows a time-domain waterfall plot of the change in the phase of the optical wave in the demodulated optical fiber, and the right figure shows the amplitude of the signal at different locations. As can be seen from the right figure, the amplitude fluctuates randomly along the location, which is caused by fading noise. Figure 6 The image shows the signal obtained after applying the above-described method for eliminating fading noise. Figure 6 The left figure shows a time-domain waterfall plot of the change in the phase of the optical wave in the optical fiber after eliminating fading noise, and the right figure shows the amplitude of the signal at different locations. It can be seen that the amplitude fluctuation of the signal, i.e., the fading noise, has been well eliminated.

[0123] The various embodiments in this specification are described in a progressive manner. Similar or identical parts between embodiments can be referred to interchangeably. Each embodiment focuses on describing the differences from other embodiments. In the description of this specification, the terms "a preferred embodiment," "furthermore," "specifically," "in this embodiment," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the embodiments in this specification. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described can be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0124] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A system for eliminating fading noise in a distributed fiber optic acoustic sensing system, characterized in that, The system includes a light source, an optical frequency shift modulator, an optical amplifier, an optical circulator, a photodetector, a data acquisition card, and a data demodulator; The laser emitted by the light source is modulated into a double-pulse light with a frequency difference by the optical frequency shift modulator. The double-pulse light is amplified by the optical amplifier and injected into the optical fiber through the optical circulator. The Rayleigh backscattered light generated in the optical fiber is emitted back to the optical circulator. The light emitted from the optical circulator is detected by the photodetector and converted into an electrical signal. The electrical signal is acquired by the data acquisition card and converted into a digital signal. The data demodulator demodulates the digital signal using a fading noise cancellation method to obtain a signal with fading noise eliminated. The implementation process of the fading noise cancellation method includes: Place the optical fiber in a Cartesian coordinate system, with the fiber along the Z-axis, and obtain the light field E of the Rayleigh backscattered light at position z of the optical fiber. RS (z,t); Obtain the interference intensity of the Rayleigh backscattered light at position z in the optical fiber, where the interference intensity is the light field E. RS The square of (z,t); Heterodyne demodulation is performed on the interference intensity of Rayleigh backscattered light to obtain the change in phase of the optical wave in the optical fiber caused by external vibration. Where γ(z,t) is the fading factor at position z, d is the pulse interval, and sin(2πf) is the fading factor at position z. s t) represents the actual phase change of the light wave caused by external vibration. For common-mode noise, f s The frequency of light waves caused by external vibrations; The fading factor γ(z,t) is obtained using common-mode noise. Dividing Φ(z,t) by the value of the fading factor γ(z,t) yields: A high-pass filter is selected based on the drift frequency of the common-mode noise. The low-frequency, slowly drifting common-mode noise term is filtered out by the high-pass filter, resulting in an optical wave phase demodulation result with fading noise eliminated. The specific process of obtaining the fading factor γ(z,t) value using common-mode noise is as follows: Add a low-pass filter to the signal at all positions z of Φ(z,t) to extract the low-frequency slow drift term of common-mode noise; The effective value of the filtered signal is obtained according to its position and denoted as R(z). Then the fading factor at different positions is γ(z)=R(z) / max(R(z)), where max(*) is the calculated maximum value.

2. The fading noise cancellation system in the distributed fiber optic acoustic sensing system according to claim 1, characterized in that, The continuous laser emitted by the light source at a frequency of f0 is modulated by the optical frequency shift modulator into a double-pulse light with a frequency difference. The width of the two pulses is w, and the pulse interval is d. The frequency of the first modulated light pulse is f0+f1, and the frequency of the second modulated light pulse is f0+f2.

3. The fading noise cancellation system in the distributed fiber optic acoustic sensing system according to claim 1, characterized in that, The light field E of the Rayleigh backscattered light at position z of the optical fiber. RS (z,t) is: Where E1 is the amplitude of the first optical pulse, E2 is the amplitude of the second optical pulse, r(p,t) is the Rayleigh scattering rate at position p on the optical fiber at time t, θ(p,t) is the Rayleigh scattering phase at position p on the optical fiber at time t, and r and θ are random quantities. Let be the total phase change experienced by the Rayleigh backscattered light at position p. Let be the change in the phase of the light wave at position l on the optical fiber at time t caused by external vibration. The common-mode noise experienced by the first optical pulse. This refers to the common-mode noise experienced by the second optical pulse.

4. A system for eliminating fading noise in a distributed fiber optic acoustic sensing system, characterized in that, The system includes a light source, a first optical coupler, a second optical coupler, a first optical frequency shift modulator, a second optical frequency shift modulator, an optical phase modulator, an optical amplifier, an optical circulator, a photodetector, a data acquisition card, and a data demodulator; The light source emits continuous laser light with a frequency of f0. This light is split into two paths by the first optical coupler. The first path, after being frequency-shifted by the first optical frequency-shift modulator, has a frequency of f0 + f1, and the continuous laser light is modulated into a pulse with a width of w. The second path, after being frequency-shifted by the second optical frequency-shift modulator, has a frequency of f0 + f2, and the continuous laser light is modulated into a pulse with a width of w. The light with a frequency of f0 + f2 undergoes sinusoidal phase modulation by the optical phase modulator, resulting in a frequency of f... c Simultaneously, the second optical frequency shift modulator also delays the pulsed light, making the delay interval between the first and second optical pulses d; the two modulated lights are combined into one by the second optical coupler, and the combined light is sent to the optical amplifier for amplification and then injected into the optical fiber through the optical circulator. The Rayleigh backscattered light generated in the optical fiber is emitted back to the optical circulator, and the light emitted from the optical circulator is detected by the photodetector and converted into an electrical signal. The electrical signal is acquired by the data acquisition card and converted into a digital signal. The data demodulator demodulates the digital signal using a fading noise cancellation method to obtain a signal with fading noise eliminated. The implementation process of the fading noise cancellation method includes: The optical fiber is placed in a Cartesian coordinate system, and the optical field E of the Rayleigh backscattered light at position z of the fiber is obtained along the Z-axis. RS (z,t); Obtain the interference intensity of the Rayleigh backscattered light at position z in the optical fiber, where the interference intensity of the Rayleigh backscattered light at position z in the optical fiber is the optical field E. RS The square of (z,t); Heterodyne demodulation is performed on the interference intensity of the Rayleigh backscattered light at position z in the optical fiber to obtain the change in the phase of the optical wave in the fiber caused by external vibration: Φ(z,t)=-dγ(z,t)sin(2πf s t)+γ(z,t)sin(2πf c t); Where γ(z,t) is the fading factor at position z, sin(2πf s t) represents the actual phase change of the light wave caused by external vibration, sin(2πf) c t) is the fixed-frequency phase modulation signal loaded by the optical phase modulator, f s The frequency of light waves caused by external vibrations; The fading factor γ(z,t) is obtained using fixed modulation, and Φ(z,t) is divided by the fading factor γ(z,t) to obtain: Add a low-pass filter cutoff frequency to filter out the fixed modulation term sin(2πf) of Φ(z,t). c t), we get: Among them, the cutoff frequency f of the low-pass filter cut <f c ; The method for obtaining the fading factor γ(z,t) using fixed modulation is as follows: Perform a Fourier transform on the signal at all positions z to obtain the power spectral density, and then sequentially extract the power spectral density at position z at frequency f. c Power spectral intensity P at the location c (z), then the fading factor at different positions is γ(z,t)=P c (z) / max(P c (z)), where max(*) is used to calculate the maximum value.

5. The fading noise cancellation system in the distributed fiber optic acoustic sensing system according to claim 4, characterized in that, The light field E of Rayleigh backscattered light at position z in the fiber RS (z,t); Where E1 is the amplitude of the first optical pulse, E2 is the amplitude of the second optical pulse, r(p,t) is the Rayleigh scattering rate at position p on the optical fiber at time t, θ(p,t) is the Rayleigh scattering phase at position p on the optical fiber at time t, and r(p,t) and θ(p,t) are random quantities along spatial position p. Let be the total phase change experienced by the Rayleigh backscattered light at position p. It represents the change in the phase of the optical wave at position l on the optical fiber at time t caused by external vibration.

6. The fading noise cancellation system in the distributed fiber optic acoustic sensing system according to claim 4, characterized in that, Another method for obtaining the fading factor γ(z,t) using fixed modulation: Add a bandpass filter to the signal at all positions z, with a passband frequency of f. c Then, the effective value of the filtered signal is obtained according to the position and denoted as R(z). The fading factor at different positions is γ(z,t)=R(z) / max(R(z)), where max(*) is the calculated maximum value.

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