System for eliminating fading noise in distributed optical fiber sound wave sensing system

Abstract

The invention discloses a system for eliminating fading noise in a distributed optical fiber sound wave sensing system. The system comprises a light source, an optical frequency shift modulator, an optical amplifier, an optical circulator, a photoelectric detector, a data acquisition card and a data demodulator, laser emitted by the light source is modulated into double-pulse light with frequency difference through the light frequency shift modulator, the double-pulse light is injected into the optical fiber through the optical circulator after being amplified by the optical amplifier, Rayleigh backscattered light generated in the optical fiber is emitted back to the optical circulator, light emitted by the optical circulator is detected and converted into an electric signal through the photoelectric detector, and the electric signal is transmitted to the optical fiber through the optical circulator. The electric signals are collected and converted into digital signals through the data collection card, and the data demodulator demodulates the digital signals through a fading noise elimination method to obtain signals with fading noise eliminated. Therefore, fading noise in the distributed optical fiber acoustic wave sensing system can be effectively eliminated, the signal quality is improved, and error signal interference is eliminated.

Description

Technical Field

[0001] The invention relates to a system for eliminating fading noise in a distributed optical fiber acoustic wave sensing system, and relates to the field of distributed optical fiber sensing. Background Art

[0002] Distributed fiber-optic acoustic sensing (DAS) technology uses Rayleigh backscattering in optical fibers to achieve sensing and positioning. In a distributed fiber-optic acoustic sensing system, highly coherent pulsed light needs to be injected into the optical fiber, and the detected backscattered light is the Rayleigh scattered light at all positions within the pulse width. Since the amplitude and phase of Rayleigh scattering are randomly distributed along the axis of the optical fiber, and the detected signal is the superposition of these random scatterings, the detected signal will have large fluctuations in intensity, and this fluctuation is the fading noise.

[0003] Fading noise is inherent in the distributed fiber optic acoustic wave sensing system and is caused by the randomness of Rayleigh scattering. With the continuous improvement of the performance of distributed fiber optic acoustic wave sensing systems, fading noise has become a key issue restricting the performance of the system. Fading noise not only leads to a decrease in the signal-to-noise ratio, but also causes signal errors, which brings large errors to the measurement results and restricts the application effect of the system.

[0004] Hartog et al. used multiple optical frequencies to reduce fading noise. The system is equipped with multiple different optical frequencies, and the signals at different optical frequencies are superimposed to reduce the noise caused by fading. Zhao et al. used space division multiplexing in few-mode optical fiber to suppress fading noise. These methods use the average value of multi-dimensional (frequency domain, phase domain or spatial domain) signals to suppress random fading noise, which increases the complexity of hardware or modulated signals. Dong et al. proposed a denoising network based on game theory, called convolution adversarial denoising method, which is used to suppress random noise in DAS data. Zhao et al. proposed a DAS data denoiser based on convolutional neural network, which can suppress fading noise and various other noises. However, the denoiser requires a training set, and the adaptability of this method is limited. Summary of the invention

[0005] The present invention aims to solve at least one of the technical problems existing in the prior art. To this end, in view of the above problems, the purpose of the present invention is to provide a system for eliminating fading noise in a distributed optical fiber acoustic wave sensing system, which can eliminate the fading noise in the distributed optical fiber acoustic wave sensing and improve the signal quality.

[0006] In order to achieve the above-mentioned invention object, the technical solution adopted by the present invention is:

[0007] In a first aspect, the present invention provides a system for eliminating fading noise in a distributed optical fiber acoustic wave sensing system, the system comprising 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 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 by the optical circulator is detected by the photodetector and converted into an electrical signal, the electrical signal is collected by the data acquisition card and converted into a digital signal, the data demodulator demodulates the digital signal using a fading noise elimination method to obtain a signal with fading noise eliminated.

[0008] Furthermore, the frequency emitted by the light source is f 0 The continuous laser is modulated into a double pulse light with a frequency difference by the optical frequency shift modulator, the width of the two pulses is w, the pulse interval is d, and the frequency of the first optical pulse after modulation is f 0 +f 1 , the frequency of the second optical pulse after modulation is f 0 +f 2 .

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

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

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

[0012] The interference intensity of Rayleigh backscattered light is calculated by heterodyne demodulation to obtain the phase change of the light wave in the optical fiber caused by external vibration. Among them, γ(z,t) is the attenuation factor at position z, d is the interval of pulse light, sin(2πf s t) is the actual phase change of the light wave caused by external vibration, is the common mode noise term;

[0013] Using the common mode noise to obtain the value of the attenuation factor γ(z, t), divide Φ(z, t) by the value of the attenuation factor γ(z, t) to obtain:

[0014]

[0015] A high-pass filter is selected according to the drift frequency of the common-mode noise. The low-frequency and slow-drifting common-mode noise term is filtered out by the high-pass filter, and the optical wave phase demodulation result with the fading noise eliminated is obtained:

[0016]

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

[0018]

[0019] Among them, E 1 is the amplitude of the first light pulse, E 2 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, r and θ are random quantities; is the total phase change experienced by the Rayleigh backscattered light at position p, is the phase change of the light wave caused by external vibration at time t at position l on the optical fiber, is the common mode noise experienced by the first optical pulse, is the common mode noise experienced by the second optical pulse.

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

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

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

[0023] In a second aspect, the present invention provides a system for eliminating fading noise in a distributed optical fiber acoustic wave sensing system, the system comprising 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 a frequency of f 0 The continuous laser is divided into two paths by the first optical coupler, and the frequency of the first path of light after the first optical frequency shift modulator is shifted to f 0 +f 1 , the continuous laser is modulated into a pulse light with a width of w; the frequency of the second light after the second optical frequency shift modulator is shifted is f 0 +f2 , the continuous laser is modulated into a pulse light with a width of w and a light wave frequency of f 0 +f 2 The light passes through the optical phase modulator to perform sinusoidal modulation on the phase of the light wave, with a frequency of f c At the same time, the second optical frequency shift modulator also delays the pulse light, so that the delay interval between the first light and the second light pulse is d; the two modulated lights are combined into one light beam through the second optical coupler, the combined light beam 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 by the optical circulator is detected by the photodetector and converted into an electrical signal, the electrical signal is collected by the data acquisition card and converted into a digital signal, the data demodulator demodulates the digital signal using a fading noise elimination method to obtain a signal with fading noise eliminated.

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

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

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

[0027] The interference intensity of Rayleigh backscattered light at the optical fiber position z is calculated by heterodyne demodulation, and the change in the phase of the optical wave in the optical fiber caused by the external vibration is obtained:

[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) is 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 the optical phase modulator;

[0030] Using fixed modulation to obtain the fading factor γ(z,t), and dividing Φ(z,t) by the fading factor γ(z,t), we get:

[0031]

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

[0033]

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

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

[0036]

[0037] Among them, E 1 is the amplitude of the first optical pulse, E 2 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, r(p,t) and θ(p,t) are random quantities along the spatial position p; is the total phase change experienced by the Rayleigh backscattered light at position p, It is the phase change of the light wave caused by external vibration at position l on the optical fiber at time t.

[0038] Furthermore, the method of obtaining the fading factor γ(z,t) using fixed modulation is as follows: The first method: Perform Fourier transform on the signals at all positions z to obtain the power spectrum density, and extract the signals at position z at frequency f in turn. c The power spectrum intensity P at 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, and the filter passband frequency is f c , and then the effective value of the filtered signal is calculated according to the position, recorded 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] The present invention adopts the above technical solution, and has the following characteristics: Since Rayleigh scattering varies randomly with position, the fading noise also varies randomly with position. The present invention can effectively eliminate the fading noise in the distributed optical fiber acoustic wave sensing system, eliminate the interference of erroneous signals, and improve the signal quality. In summary, the present invention has a good application prospect in distributed acoustic wave sensing, seismic wave exploration, oil field logging, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Various other advantages and benefits will become apparent to those of ordinary skill in the art by reading the detailed description of the preferred embodiments below. The accompanying drawings are only for the purpose of illustrating the preferred embodiments and are not to be considered as limiting the present invention. Throughout the accompanying drawings, the same reference numerals are used to represent the same components. In the accompanying drawings:

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

[0042] Figure 2 This is a schematic diagram of a signal before eliminating fading noise according to the first embodiment of the present invention.

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

[0044] Figure 4 This is a structural diagram of a system for eliminating fading noise in a distributed optical fiber acoustic wave sensing system according to the second embodiment of the present invention.

[0045] Figure 5 This is a schematic diagram of a signal before eliminating fading noise according to the second embodiment of the present invention.

[0046] Figure 6 This is a schematic diagram of a signal after eliminating fading noise according to the second embodiment of the present invention. DETAILED DESCRIPTION

[0047] It should be understood that the terms used herein are only for the purpose of describing specific example embodiments and are not intended to be limiting. Unless the context clearly indicates otherwise, the singular forms "one", "an" and "said" as used herein may also be meant to include plural forms. The terms "include", "comprise", "contain", and "have" are inclusive, and therefore specify the existence of stated features, steps, operations, elements and / or parts, but do not exclude the existence or addition of one or more other features, steps, operations, elements, parts, and / or combinations thereof. The method steps, processes, and operations described herein are not interpreted as necessarily requiring them to be performed in the specific order described or illustrated, unless the execution order is clearly indicated. It should also be understood that additional or alternative steps may be used.

[0048] Although the terms first, second, third, etc. can be used in the text to describe multiple elements, components, regions, layers and / or sections, these elements, components, regions, layers and / or sections should not be limited by these terms. These terms can only be used to distinguish an element, component, region, layer or section from another region, layer or section. Unless the context clearly indicates, terms such as "first", "second" and other numerical terms do not imply order or sequence when used in the text. Therefore, the first element, component, region, layer or section discussed below can be referred to as the second element, component, region, layer or section without departing from the teaching of the example embodiments.

[0049] For ease of description, spatially relative terms may be used herein to describe the relationship of one element or feature relative to another element or feature as shown in the figures, such as "inside", "outside", "inner side", "outer side", "below", "above", etc. Such spatially relative terms are intended to include different orientations of the device in use or operation in addition to the orientation depicted in the figures.

[0050] The present invention provides a system for eliminating fading noise in a distributed optical fiber acoustic wave sensing system, the system comprising 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 by the optical circulator is detected by the photodetector and converted into an electrical signal, the electrical signal is collected and converted into a digital signal by the data acquisition card, and the data demodulator demodulates the digital signal using a fading noise elimination method to obtain a signal with fading noise eliminated. Therefore, the present invention 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 wrong signal.

[0051] The exemplary embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although the exemplary embodiments of the present invention are shown in the accompanying drawings, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments described herein. On the contrary, these embodiments are provided in order to enable a more thorough understanding of the present invention and to fully convey the scope of the present invention to those skilled in the art.

[0052] Embodiment 1: Figure 1As shown, the system for eliminating fading noise in a distributed optical fiber 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] The light source 101 emits a frequency of f 0 The continuous laser light is divided into two paths by the first optical coupler 102. The frequency of the first path of light after the first optical frequency shift modulator 104 shifts the frequency to f 0 +f 1 At the same time, the continuous laser is modulated into a pulse light with a width of w. The frequency of the second light after the second optical frequency shift modulator 105 shifts the frequency is f 0 +f 2 At the same time, the continuous laser is modulated into a pulse light with a width of w, and the frequency of the light wave modulated by frequency shift is f 0 +f 2 The light passes through the optical phase modulator 106 to perform sinusoidal modulation on the light wave phase, with a frequency of f c . At the same time, the second optical frequency shift modulator 105 also delays the pulse light, so that the delay interval between the first light and the second light pulse is d. The two modulated lights are combined into one light beam by the second optical coupler 103, and the combined light beam 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 by the optical circulator 108 is detected by the photodetector 109 and converted into an electrical signal. The electrical signal is collected and converted into a digital signal by the data acquisition card 110. The data demodulator 111 demodulates the digital signal through the fading noise elimination method to obtain a signal with the fading noise eliminated.

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

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

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

[0057]

[0058] Among them, E 1 is the amplitude of the first optical pulse, E 2is 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, r(p,t) and θ(p,t) are random quantities along the spatial position p; is the total phase change experienced by the Rayleigh backscattered light at position p, is the phase change of the light wave caused by external vibration at time t at position l on the optical fiber; sin(2πf c t) is the phase modulation signal loaded by the optical phase modulator.

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

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

[0061]

[0062] Where Δf = f 1 -f 2 is the heterodyne frequency, and x and y are the integral variables.

[0063] S3. Perform heterodyne demodulation algorithm on equation (2) to obtain the change in the phase of the light wave in the optical fiber caused by the 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] By combining formula (2) with I r1 After mixing and low-pass filtering, we can get the signal:

[0066]

[0067] By combining formula (2) with I r2 After mixing and low-pass filtering, we can get the signal:

[0068]

[0069] The cutoff frequency f of the low-pass filter in the above step is cut Satisfy the relationship: 0 <f cut <Δf.

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

[0071]

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

[0073] Further assume that the external vibration signal has a frequency of f s The phase change of the light wave caused by external vibration can be expressed as Thus there is Formula (5) can be simplified as:

[0074]

[0075] In formula (6), the Rayleigh scattering rate r is Rayleigh distributed along the space, and the formula contains multiple integrals, which leads to random fluctuations in the demodulated phase along the space, which is the fading noise. The comprehensive effect of the Rayleigh scattering rate r is expressed as the fading factor γ(z,t), then formula (6) can be further expressed as:

[0076]

[0077] In formula (7), Φ(z, t) is the phase change of the demodulated light wave, γ(z, t) is the attenuation factor at position z, d is the interval of the pulse light, sin(2πf s t) is 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. 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 t), the demodulated signal contains not only the actual phase change sin(2πf s In addition to t), it also includes coefficients -d and γ(z,t). The coefficient -d only scales the actual phase change and does not cause fading changes along the spatial position; while γ(z,t) will introduce 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 t) can get the value of the fading factor γ(z,t) in two ways:

[0080] The first method is to perform Fourier transform on the signal at all positions z in equation (7) to obtain the power spectrum density, and then extract the power spectrum of position z at frequency f. c The power spectrum intensity P atc (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 the calculated maximum value.

[0081] The second method is to add a bandpass filter to the signal at all positions z in equation (7), and the filter passband frequency is f c , and then calculate the effective value of the filtered signal according to the position (the effective value is also called the root mean square value, which is the effective value after squaring all the data and then taking the square root), recorded as R(z), then 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 add a low-pass filter to equation (8), and the filter cutoff frequency f cut <f c , thereby filtering out the fixed modulation term sin(2πf c t), we get:

[0085]

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

[0087] like Figure 2 The signal before eliminating the fading noise is shown. The optical fiber length is 180m, the optical phase modulator applies a sine wave signal with a frequency of 500Hz, and the sine wave signal with a frequency of 30Hz is applied in the range of 60m to 90m on the optical fiber. Figure 2 The left figure shows the time domain waterfall diagram of the phase change of the light wave in the optical fiber obtained by demodulation, and the right figure shows the amplitude of the signal at different positions. It can be seen from the right figure that the amplitude fluctuates randomly along the position, which is caused by fading noise. Figure 3 shows the signal obtained after using the above method to eliminate fading noise, Figure 3The left figure shows the time domain waterfall diagram of the phase change of the light wave in the optical fiber after the fading noise is eliminated, and the right figure shows the amplitude of the signal at different positions. It can be seen that the amplitude fluctuation of the signal, that is, the fading noise, has been well eliminated.

[0088] Embodiment 2: Figure 4 As shown, this embodiment also provides a system for eliminating fading noise in a distributed optical fiber acoustic wave 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] The light source 201 emits a frequency of f 0 The continuous laser is modulated into a double pulse light with a frequency difference by the optical frequency shift modulator 202. The width of the two pulses is w, the pulse interval is d, and the frequency of the first optical pulse is f 0 +f 1 , the second light pulse frequency is f 0 +f 2 The double pulse light is amplified by the optical amplifier 203 and then injected into the optical fiber through the optical circulator 204. The Rayleigh backscattered light generated in the optical fiber passes through the optical circulator 204 and reaches the photodetector 205 to be converted into an electrical signal. Then, it is collected and converted into a digital signal by the data acquisition card 206. The data demodulator 207 demodulates the digital signal through the fading noise elimination method to obtain a signal with the fading noise eliminated.

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

[0091] A1. Place the optical fiber in a Cartesian coordinate system. The optical fiber is along the Z axis. Obtain the light field E of the Rayleigh backscattered light at the optical fiber position z. RS (z,t).

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

[0093]

[0094] Among them, E 1 is the amplitude of the first light pulse, E 2 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. is the total phase change experienced by the Rayleigh backscattered light at position p, It is the phase change of the light wave caused by external vibration at position l on the optical fiber at time t. is the common mode noise experienced by the first optical pulse, is the common mode noise experienced by the second optical pulse.

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

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

[0097]

[0098] Where Δf = f 1 -f 2 is the heterodyne frequency.

[0099] A3. Perform heterodyne demodulation algorithm on equation (11) to obtain the change in the phase of the light wave in the optical fiber caused by external vibration. The heterodyne demodulation algorithm process is:

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

[0101] By combining formula (11) with I r1 After mixing and low-pass filtering, we can get the signal:

[0102]

[0103] By combining formula (12) with I r2 After mixing and low-pass filtering, we can get the signal:

[0104]

[0105] The cutoff frequency f of the low-pass filter in the above step is cut Satisfy the relationship: 0 <f cut <Δf.

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

[0107]

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

[0109] Further assume that the external vibration signal has a frequency of f sThe phase change of the light wave caused by external vibration can be expressed as Thus there is Formula (14) can be simplified as:

[0110]

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

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

[0113]

[0114] In formula (16), Φ(z, t) is the phase change of the demodulated light wave, γ(z, t) is the attenuation factor at position z, d is the interval of the pulse light, sin(2πf s t) is the phase change of the light wave caused by external vibration, is the common mode noise term, which is usually introduced by the error of the light source and optical frequency shift modulator, and appears as a low-frequency slowly drifting signal. If the common mode noise is not considered, equation (16) can be expressed as Φ(z,t)=-dγ(z,t)sin(2πf s t), the demodulated signal contains not only the actual phase change sin(2πf s In addition to t), it also includes coefficients -d and γ(z,t). The coefficient -d only scales the actual phase change and does not cause fading changes along the spatial position; while γ(z,t) will introduce fading changes along the spatial position, resulting in distortion of the demodulated signal.

[0115] A4. Use common mode noise to obtain the value of the attenuation factor γ.

[0116] In this embodiment, it can be seen from the above formula (16) that the value of the fading factor γ(z, t) can be obtained by using the common mode noise term. Low-pass filtering is added to the signal at all positions z in formula (16), the low-frequency slow floating term of the common mode noise is extracted, and then the effective value of the filtered signal is obtained according to the position, recorded as R(z), then 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 add high-pass filtering to equation (17). The filter cutoff frequency is determined according to the drift frequency of the common mode noise, usually selected to be below 3 Hz, to filter out the low-frequency and slow-drifting common mode noise term, and obtain:

[0120]

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

[0122] Figure 5 The signal before eliminating the fading noise is shown. The fiber length is 180m, the environmental noise is low-frequency slow drift below 1Hz, and a sine wave signal with a frequency of 30Hz is applied in the range of 60m to 90m on the fiber position. Figure 5 The left figure shows the time domain waterfall diagram of the phase change of the light wave in the optical fiber obtained by demodulation, and the right figure shows the amplitude of the signal at different positions. It can be seen from the right figure that the amplitude fluctuates randomly along the position, which is caused by fading noise. Figure 6 shows the signal obtained after using the above method to eliminate fading noise, Figure 6 The left figure shows the time domain waterfall diagram of the phase change of the light wave in the optical fiber after the fading noise is eliminated, and the right figure shows the amplitude of the signal at different positions. It can be seen that the amplitude fluctuation of the signal, that is, the fading noise, has been well eliminated.

[0123] Each embodiment in this specification is described in a progressive manner, and the same and similar parts between the embodiments can be referred to each other, and each embodiment focuses on the differences from other embodiments. In the description of this specification, the description of reference terms "a preferred embodiment", "further", "specifically", "in the present embodiment", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the embodiment of this specification. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples without contradiction.

[0124] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions 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 optical fiber acoustic wave 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 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 by the optical circulator is detected by the photoelectric detector and converted into an electrical signal, the electrical signal is collected by the data acquisition card and converted into a digital signal, and the data demodulator demodulates the digital signal using a fading noise elimination method to obtain a signal with fading noise eliminated.

2. The system for eliminating fading noise in a distributed optical fiber acoustic wave sensing system according to claim 1, characterized in that: The continuous laser with a frequency of f0 emitted by the light source 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, wherein the frequency of the first light pulse after modulation is f0+f1, and the frequency of the second light pulse after modulation is f0+f2.

3. The system for eliminating fading noise in a distributed optical fiber acoustic wave sensing system according to claim 1, characterized in that: The implementation process of the fading noise elimination method includes: The optical fiber is placed in a Cartesian coordinate system, and the optical fiber is along the Z axis to obtain the light field E of the Rayleigh backscattered light at the optical fiber position z. RS (z,t); Obtain the interference light intensity of the Rayleigh backscattered light at the optical fiber position z, where the interference light intensity is the light field E RS The square of (z,t); The interference intensity of Rayleigh backscattered light is calculated by heterodyne demodulation to obtain the phase change of the light wave in the optical fiber caused by external vibration. Among them, γ(z,t) is the attenuation factor at position z, d is the interval of pulse light, sin(2πf s t) is the actual phase change of the light wave caused by external vibration, is the common mode noise term; Using the common mode noise to obtain the value of the attenuation factor γ(z, t), divide Φ(z, t) by the value of the attenuation factor γ(z, t) to obtain: A high-pass filter is selected according to the drift frequency of the common-mode noise. The low-frequency and slow-drifting common-mode noise term is filtered out by the high-pass filter, and the optical wave phase demodulation result with the fading noise eliminated is obtained:

4. The system for eliminating fading noise in a distributed optical fiber acoustic wave sensing system according to claim 3, characterized in that: The light field E of the Rayleigh backscattered light at the optical fiber position z RS (z,t) is: Where, E1 is the amplitude of the first light pulse, E2 is the amplitude of the second light 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; is the total phase change experienced by the Rayleigh backscattered light at position p, is the phase change of the light wave caused by external vibration at time t at position l on the optical fiber, is the common mode noise experienced by the first optical pulse, is the common mode noise experienced by the second optical pulse.

5. The system for eliminating fading noise in a distributed optical fiber acoustic wave sensing system according to claim 3, characterized in that: The specific process of obtaining the attenuation factor γ(z,t) value using common mode noise is: Add low-pass filtering to the signal at all positions z of Φ(z,t) to extract the low-frequency slow drift term of the common mode noise; The effective value of the filtered signal is calculated according to the position, recorded as R(z), and the fading factor at different positions is γ(z)=R(z) / max(R(z)), where max(*) is the calculated maximum value.

6. A system for eliminating fading noise in a distributed optical fiber acoustic wave 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 a continuous laser with a frequency of f0, which is divided into two paths by the first optical coupler. The frequency of the light wave of the first path of light after the first optical frequency shift modulator is shifted to f0+f1, and the continuous laser is modulated into a pulse light with a width of w; the frequency of the light wave of the second path of light after the second optical frequency shift modulator is shifted to f0+f2, and the continuous laser is modulated into a pulse light with a width of w. The light wave frequency of f0+f2 is sinusoidally modulated by the optical phase modulator, and the frequency is f c At the same time, the second optical frequency shift modulator also delays the pulse light, so that the delay interval between the first light and the second light pulse is d; the two modulated lights are combined into one light beam through the second optical coupler, the combined light beam 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 by the optical circulator is detected by the photodetector and converted into an electrical signal, the electrical signal is collected by the data acquisition card and converted into a digital signal, the data demodulator demodulates the digital signal using a fading noise elimination method to obtain a signal with fading noise eliminated.

7. The system for eliminating fading noise in a distributed optical fiber acoustic wave sensing system according to claim 6, characterized in that: The implementation process of the fading noise elimination method includes: The optical fiber is placed in a Cartesian coordinate system, and the optical fiber is along the Z axis to obtain the light field E of the Rayleigh backscattered light at the optical fiber position z. RS (z,t); Obtain the interference light intensity of the Rayleigh backscattered light at the optical fiber position z, where the interference light intensity of the Rayleigh backscattered light at the optical fiber position z is the light field E RS The square of (z,t); The interference intensity of Rayleigh backscattered light at the optical fiber position z is calculated by heterodyne demodulation, and the change in the phase of the optical wave in the optical fiber caused by the external vibration is obtained: 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) is 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 the optical phase modulator; Using fixed modulation to obtain the fading factor γ(z,t), and dividing Φ(z,t) by the fading factor γ(z,t), we get: Add the cutoff frequency of the low-pass filter to filter out the fixed modulation term sin(2πf c t), we get: Among them, the cut-off frequency of the low-pass filter is f cut <f c .

8. The system for eliminating fading noise in a distributed optical fiber acoustic wave sensing system according to claim 7, characterized in that: The light field E of the Rayleigh backscattered light at the optical fiber position z RS (z,t); Wherein, 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, r(p, t) and θ(p, t) are random quantities along the spatial position p; is the total phase change experienced by the Rayleigh backscattered light at position p, It is the phase change of the light wave caused by external vibration at position l on the optical fiber at time t.

9. The system for eliminating fading noise in a distributed optical fiber acoustic wave sensing system according to claim 7, characterized in that: The method to obtain the fading factor γ(z,t) using fixed modulation is: The first method is to perform Fourier transform on the signals at all positions z to obtain the power spectrum density, and extract the power spectrum at position z at frequency f in turn. c The power spectrum intensity P at 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 is to add a bandpass filter to the signal at all positions z, and the filter passband frequency is f c , and then the effective value of the filtered signal is calculated according to the position, recorded 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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