Optical fiber distributed sound wave sensing method and system based on multi-frequency signal delay phase control

Abstract

The invention discloses an optical fiber distributed sound wave sensing system and method based on multi-frequency signal delay phase control, and the system comprises a dry light source, a first coupler, an acousto-optic modulator, a circulator, an electro-optic modulator, a second coupler, a balance detector, a drive circuit, and a data collection and signal processing module. A backscattering light signal returned by a sensing optical fiber is converted into a multi-frequency signal by modulation, and the phase shift amount is controlled by controlling the time delay of the signal to a balance detector by utilizing the characteristic that light with different frequencies is different in phase time delay after being transmitted through the same optical path. Therefore, signals with different frequencies after heterodyne detection have specific fixed phase delays, and signal detection and processing can be simplified. The method is characterized in that multi-channel same-frequency signals with set phase shift can be obtained through single-channel detection, and the phase shift amount can be adjusted.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical fiber sensing, and in particular relates to an optical fiber distributed acoustic wave sensing method and system based on multi-frequency signal delay phase control. Background Art

[0002] Fiber-optic distributed acoustic wave sensing technology usually obtains the characteristics of the acoustic wave signal sensed by the optical fiber by injecting light pulses into the sensing optical fiber and detecting some parameters of the backscattered light excited by the pulse in the sensing optical fiber. The parameters of the backscattered light usually detected include but are not limited to the amplitude, phase and polarization state of the light. The position of the sound wave is calculated by the time the signal reaches the detector. Among them, the phase detection method is widely used and has the advantages of high sensitivity and strong quantitative monitoring capabilities. Furthermore, by performing this process on the sensing optical fiber, such as writing quasi-continuous or discrete scattering enhancement points, enhancing the signal strength and improving the signal-to-noise ratio, the sensitivity of the sensing system can also be greatly improved. At present, fiber-optic distributed acoustic wave sensing technology has been applied in fields such as energy, security, electricity and transportation.

[0003] There are many phase detection methods at present, and one of the commonly used methods is heterodyne detection, which interferes the backscattered light with the local reference light with a fixed frequency difference from the backscattered light, and then calculates the phase of the electrical signal after interference to achieve measurement. Another detection method is to use pulse pair detection, or to perform delayed interference on the scattered light at the receiving end, introduce phase shift with the help of a special optical path, and then calculate the amplitude signal after the homodyne interference to obtain the required phase information.

[0004] Whether heterodyne detection or homodyne detection, it is generally necessary to obtain a set of signals with specific phase shifts, and then use the characteristics between these specific phase shift signals to calculate the phase value. The most commonly used phase shift is π / 2 or 2π / 3. If the phase shift is π / 2, it is a pair of orthogonal signals, and the phase can be calculated by the inverse tangent method. If the phase shift is 2π / 3, the phase can also be calculated by mathematical operations on a set of signals that constitute this phase shift and then inverse tangent. In addition to inverse tangent, the phase can also be obtained by the differential-cross-multiplication-integration (DCM) method. Therefore, it can be seen that it is very important to obtain multiple signals with specific phase shifts. In existing systems, the acquisition of multi-channel signals is usually obtained by using methods such as interferometers, optical bridges, or 3×3 couplers, which requires the same multi-channel detectors and subsequent analog-to-digital converters with the same number of channels. Another idea is to obtain the single-channel heterodyne signal by Hilbert transform during signal processing after detection and sampling, so that a simpler optical path and more complex signal processing can be used to achieve phase demodulation.

[0005] The above existing methods face the situation of having many detection channels or relatively complex signal processing methods. Summary of the invention

[0006] In view of the above, the purpose of the present invention is to provide a fiber optic distributed acoustic wave sensing method and device based on multi-frequency signal delay phase control, which can achieve phase demodulation by performing multi-frequency modulation at the receiving end, single-path detection and relatively simple data processing.

[0007] To achieve the above-mentioned purpose of the invention, an optical fiber distributed acoustic wave sensing system based on multi-frequency signal delay phase control is provided in the embodiment, comprising a coherent light source, a first coupler, an acousto-optic modulator, a circulator, an electro-optic modulator, a second coupler, a balanced detector, a driving circuit, and a data acquisition and signal processing module.

[0008] The output light of the coherent light source is divided into detection light and local oscillator light through a first coupler. The detection light passes through an acousto-optic modulator and is modulated under the control of a first modulation electrical signal of a driving circuit to output a detection light pulse. The detection light pulse is input into a sensing optical fiber through a circulator. The backscattered light returned by the sensing optical fiber is input into an electro-optic modulator through a circulator. The backscattered light is modulated with different modulation depths and bias points in the electro-optic modulator under the control of a second modulation electrical signal of a driving circuit. The output light of the electro-optic modulator is transmitted through a transmission optical fiber, and during transmission, it interferes with the local oscillator light through a second coupler, is input into a balanced detector through two ports, and is converted into an interference electrical signal, and is then input into a data acquisition and signal processing module. The length of the transmission optical fiber is the length of the optical fiber from the output of the electro-optic modulator to the input of the photodetector, and its value is set according to the phase delay difference between the multi-frequency signal components output by the electrical modulator.

[0009] The data acquisition and signal processing module is used to control the first modulated electrical signal and the second modulated electrical signal output by the driving circuit to realize modulation control of the acousto-optic modulator and the electro-optic modulator, and is also used to phase demodulate the interference electrical signal according to the phase delay difference and the frequency signal component.

[0010] Preferably, the acousto-optic modulator is controlled to adjust the detection light with a frequency of f0 to a repetition frequency of f s , the frequency shift is f AOM , a light pulse with a pulse width of Δτ, where the frequency shift of the input detection light by the acousto-optic modulator is up-shifted or down-shifted, then the output light pulse frequency is f0+f AOM or f0-f AOM .

[0011] Preferably, the electro-optic modulator generates a plurality of frequency-shifted optical signals based on the detection light under control, and the frequency difference between the frequency of the frequency-shifted optical signals and the frequency of the detection light is f EOM The frequency of the light component output by the electro-optic modulator is f0+fAOM ±k·f EOM , where k is an integer. In the system, the frequency range can be intercepted according to the bandwidth of the balanced detector. If it is intercepted to k = 1, there are 3 frequency components. The intensity of each frequency light component a k Depending on the modulation depth and bias point of the electro-optic modulator, the electrical signal spectrum is observed at the output end of the detector, and the bias point and modulation depth of the electro-optic modulator are adjusted to minimize the difference in amplitude of each frequency peak within the selected frequency range.

[0012] The length of the transmission fiber is set according to the phase delay difference between the multi-frequency signal components output by the electrical modulator, including:

[0013] When k is 1, the three frequency components are f0+f AOM -f EOM 、f0+f AOM and f0+f AOM +f EOM , or f0-f AOM -f EOM 、f0-f AOM and f0-f AOM +f EOM ;

[0014] The phase shift delay difference between the three frequency components is 2mπ+2π / 3, so the length D of the transmission fiber should be:

[0015]

[0016] Control the D value so that the phase shift difference between the lowest and highest frequency components among the three frequency components is mπ+π / 2. At this time, the D value should be:

[0017]

[0018] The D value is controlled so that the phase shift difference between adjacent frequency components among the three frequency components is mπ+π / 2. At this time, the D value should be:

[0019]

[0020] Where m is a non-negative integer, c is the speed of light in a vacuum, and n is e is the equivalent refractive index of the probe light transmitted in the optical fiber.

[0021] Preferably, phase demodulating the interference electrical signal according to the phase delay difference and the frequency signal component comprises:

[0022] When the phase shift delay difference is 2mπ+2π / 3, and the three frequency components are f0+f AOM -f EOM 、f0+f AOM and f0+fAOM +f EOM , or f0-f AOM -f EOM 、f0-f AOM and f0-f AOM +f EOM When , the phase demodulation process is:

[0023] The interference signal S IN It is divided into 3 paths, each with a center frequency of f AOM -f EOM 、f AOM and f AOM +f EOM 3 band-pass filters are used to obtain filtered signals S1, S2 and S3;

[0024] The filtered signals S1 and S3 enter the mixer and the frequency f EOM The single frequency signal is multiplied to obtain the mixed signal S 1C With S 3C , the mixing signal has f AOM Quantity;

[0025] Mixing signal S 1C and S 3C , and the filtered signal S2 all pass through the center frequency f AOM The bandpass filter is used to obtain the filtered signal S 1E , S 2E and S 3E ;

[0026] The filtered signal S 1E , S 2E and S 3E Perform amplitude calibration respectively to obtain a signal S with consistent signal amplitude 1N , S 2N and S 3N ;

[0027] Based on the signal S 1N , S 2N and S 3N , the phase of each sampling point for:

[0028]

[0029] If the sensing fiber is a common fiber or a continuous scattering enhanced fiber, the phase difference output result is obtained by differential calculation of the phases of adjacent sampling points after unwinding.

[0030] If the sensing fiber is a point scattering enhanced fiber, then the phase After unwrapping, the phase values ​​of the sampling points belonging to the same pulse are added to obtain the phase sum Then the phase sum of adjacent pulses Subtract the phase difference to get the output result

[0031] Preferably, phase demodulating the interference electrical signal according to the phase delay difference and the frequency signal component comprises:

[0032] When the phase shift delay difference is mπ+π / 2, and the three frequency components are f0+f AOM -f EOM 、f0+f AOM and f0+f AOM +f EOM , or f0-f AOM -f EOM 、f0-f AOM and f0-f AOM +f EOM When , the phase demodulation process is:

[0033] The interference signal S IN It is divided into two paths, each with a center frequency of f AOM -f EOM and f AOM +f EOM Two band-pass filters are used to obtain filtered signals S1' and S2';

[0034] The filtered signals S1' and S2' enter the mixer and the frequency is f EOM The single frequency signal is multiplied to obtain the mixed signal S 1C 'With S 2C ', the mixing signal has f AOM Quantity;

[0035] Mixing signal S 1C 'With S 2C 'Respectively pass through the center frequency of f AOM The bandpass filter is used to obtain the filtered signal S 1E ' and S 2E ';

[0036] The filtered signal S 1E ' and S 2E 'After amplitude calibration, the signal amplitude is consistent. Model S 1N ' and S 2N ';

[0037] Based on S 1N ' and S 2N ', the phase of each sampling point for:

[0038]

[0039] If the sensing fiber is a common fiber or a continuous scattering enhanced fiber, the phase difference output result is obtained by differential calculation of the phases of adjacent sampling points after unwinding.

[0040] If the sensing fiber is a point scattering enhanced fiber, then the phase After unwrapping, the phase values ​​of the sampling points belonging to the same pulse are added to obtain the phase sum Then the phase sum of adjacent pulses Subtract the phase difference to get the output result

[0041] Preferably, the system further comprises an optical amplifier and a bandpass filter, and the detection light pulse output by the acousto-optic modulator is amplified and filtered in turn by the optical amplifier and the bandpass filter, and then input into the sensing optical fiber through a circulator.

[0042] Preferably, the balanced detector is replaced by a single-ended detector. In this case, the output light of the electro-optic modulator interferes with the local oscillator light and is input into the single-ended detector through a port, thereby also realizing the conversion of the interference light signal into the interference electrical signal.

[0043] Preferably, the electro-optic modulator is replaced by a phase modulator.

[0044] To achieve the above-mentioned object of the invention, an embodiment of the present invention further provides a fiber-optic distributed acoustic wave sensing method based on multi-frequency signal delay phase control, the method using the above-mentioned fiber-optic distributed acoustic wave sensing system, comprising the following steps:

[0045] The outgoing light of the coherent light source is divided into the detection light and the local oscillator light through the first coupler;

[0046] The data acquisition and signal processing module controls the driving circuit to output a first modulated electrical signal to control the acousto-optic modulator to modulate the detection light and output a detection light pulse, and the detection light pulse is input to the sensing optical fiber through the circulator;

[0047] The backscattered light returned by the sensing optical fiber is input to the electro-optical modulator through the circulator, and the data acquisition and signal processing module controls the driving circuit to output a second modulated electrical signal to control the electro-optical modulator to modulate the input light with the same modulation depth and bias point;

[0048] The output light of the electro-optic modulator is transmitted through the transmission optical fiber, and during the transmission, it interferes with the local oscillator light through the second coupler, and then is input into the balanced detector through two ports to be converted into an interference electrical signal, and then input into the data acquisition and signal processing module;

[0049] The data acquisition and signal processing module performs phase demodulation on the interference electrical signal according to the phase delay difference and the frequency signal component.

[0050] Compared with the prior art, the present invention has the following beneficial effects:

[0051] The system of the present invention adopts heterodyne detection mode, and converts the backscattered light signal returned by the sensing optical fiber into a multi-frequency signal through an electro-optical modulator, and utilizes the characteristics that light of different frequencies has different phase delays after being transmitted through the same optical path, and controls the delay of the interference light signal to the balanced detector to control the phase shift amount, so that different frequency signals have specific fixed phase delays after heterodyne detection, which can simplify signal detection and processing. In this way, multi-channel signals with the same frequency and set phase shift can be obtained through single-channel detection, and the phase shift amount can be adjusted. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0053] Figure 1 is a schematic structural diagram of an optical fiber distributed acoustic wave sensing system provided in an embodiment;

[0054] Figure 2 is a spectrum diagram of an electrical signal output by a balanced detector provided in an embodiment;

[0055] Figure 3 is a phase demodulation block diagram of three-way signals after filtering provided by an embodiment;

[0056] Figure 4 is a spectrum diagram of three-way signals after filtering provided by the embodiment;

[0057] Figure 5 are three mixed signals provided in the embodiment;

[0058] Figure 6 The embodiment provides three-way signals and details after the second filtering and amplitude calibration for ordinary optical fiber;

[0059] Figure 7 The embodiment provides three-way signals and details after the second filtering and amplitude calibration for the point scattering enhanced optical fiber;

[0060] Figure 8 is a phase demodulation block diagram of two signals after filtering provided by an embodiment;

[0061] Fig. 9is a spectrum diagram of two signals after filtering provided by the embodiment;

[0062] Fig.10 are two mixed signals provided in the embodiment;

[0063] Fig.11 The embodiment provides two signals and details after the second filtering and amplitude calibration for ordinary optical fiber;

[0064] Fig.12 These are two-way signals and details after the second filtering and amplitude calibration of the point scattering enhanced optical fiber provided in the embodiment. DETAILED DESCRIPTION

[0065] To make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific implementation methods described herein are only used to explain the present invention and do not limit the scope of protection of the present invention.

[0066] like Figure 1 As shown, an embodiment provides an optical fiber distributed acoustic wave sensing system based on multi-frequency signal delay phase control, including a coherent light source, a first coupler (referred to as coupler 1), an acousto-optic modulator, an optical amplifier, a bandpass filter, a circulator, an electro-optic modulator, a second coupler (referred to as coupler 2), a balanced detector, a driving circuit, and a data acquisition and signal processing module.

[0067] In the embodiment, the coherent light source can be a laser, and the light with an output frequency of f0 is divided into two paths by the coupler 1, one of which is connected to an input port of the coupler 2 as the local oscillator light, and the other is connected to the input of the acousto-optic modulator as the detection light, and is modulated by the acousto-optic modulator to a repetition frequency of f based on the first modulation electrical signal. s , the frequency shift is f AOM , a detection light pulse with a pulse width of Δτ, wherein the frequency shift of the input light by the AOM can be either upshift or downshift, that is, the detection light pulse frequency output by the AOM can be f0+f AOM or f0-f AOM The first modulation electrical signal of the acousto-optic modulator comes from the driving circuit.

[0068] In the embodiment, the detection light pulse output by the acousto-optic modulator is connected to the input of the optical amplifier, and the detection light pulse signal is amplified by the optical amplifier and then input into the bandpass filter to filter out the out-of-band noise, so as to obtain a detection light pulse with higher power and higher signal-to-noise ratio. The detection light pulse is input into port 1 of the optical circulator and then injected into the sensing optical fiber through port 2. It should be noted that if the output power of the coherent light source is sufficient, both the optical amplifier and the bandpass filter can be omitted. Among them, the optical amplifier can be an erbium-doped fiber amplifier, a semiconductor optical amplifier or other types of amplifiers that can increase the energy of the input optical signal.

[0069] In the embodiment, the sensing optical fiber can be an ordinary single-mode optical fiber, an optical fiber with continuous enhancement of scattered signals after special processing, a point-scattering enhanced optical fiber with discrete reflection points or fiber gratings, or other types of optical fibers that can generate backscattered light signals. The length of the optical fiber and the repetition frequency of the light pulse should satisfy the following relationship:

[0070]

[0071] Where c is the speed of light in vacuum, n e is the equivalent refractive index of the detection light transmitted in the optical fiber, f s is the repetition frequency of the detection light pulse.

[0072] In the embodiment, the detection light pulse injected into the sensing optical fiber excites backscattered light in the optical fiber, and the backscattered light carries the vibration sensing information and returns to port 2 of the circulator, and then enters the electro-optic modulator after being emitted from port 3 of the circulator. Under the modulation of the second modulation electrical signal output by the driving circuit, the electro-optic modulator loads the backscattered light with a frequency of f EOM By controlling the modulation depth and bias point, multiple frequency-shifted optical signals can be generated based on the detection light frequency. The frequency difference between these frequency-shifted optical signals and the detection light frequency is f EOM If the frequency shift of the coherent light source by the acousto-optic modulator is up-shift, then the frequency of each component after passing through the electro-optic modulator is f0+f AOM ±k·f EOM If the frequency shift of the coherent light source by the acousto-optic modulator is down-shift, then the frequency of each component after passing through the electro-optic modulator is f0-f AOM ±k·f EOM , where k is an integer. The modulation depth and bias point can be adjusted to retain the frequency component where k is 1. In this way, the optical signal output by the electro-optic modulator is mainly composed of the frequency f0+f AOM -f EOM 、f0+f AOM and f0+f AOM +f EOM The three components of AOM -fEOM 、f0-f AOM and f0-f AOM +f EOM The modulation depth and bias point can also be controlled to distribute the optical signal energy in more frequency components. The subsequent description of the embodiments assumes that the output optical energy of the electro-optic modulator is mainly concentrated in three frequency components.

[0073] In the embodiment, the output of the electro-optic modulator is connected to an input port of the coupler 2, and interferes with another local oscillator light with a frequency of f0 from the coupler 1 in the coupler 2. The interference light signal is connected to the two input ends of the balanced detector through the two output ports. Here, the splitting ratio of the coupler 2 is 50:50. The light of each frequency component generated by the electro-optic modulator will have different phase shifts after passing through the transmission optical fiber with a length of D. The transmission optical fiber length D can be controlled so that the phase shift difference between the three frequency components is 2mπ+2π / 3, then the D value should be:

[0074]

[0075] The D value can also be controlled so that the phase shift difference between the lowest and highest frequency components among the three frequency components is mπ+π / 2. In this case, the D value should be:

[0076]

[0077] The D value can also be controlled so that the phase shift difference between adjacent frequency components in the three frequency components is mπ+π / 2. In this case, the D value should be:

[0078]

[0079] Wherein, m is a non-negative integer.

[0080] In the embodiment, the light from the coherent light source and the light from the electro-optic modulator interfere, so that the interference electric signal output by the balanced detector only retains the electric signal with a frequency equal to the frequency difference of the two input lights. Regardless of whether the frequency shift of the coherent light source by the acousto-optic modulator is an upward frequency shift or a downward frequency shift, the electric signal output by the balanced detector includes f AOM ±kf EOM The frequency components, where k is an integer. This interference electrical signal containing multiple frequencies is input to the data acquisition and signal processing module for phase demodulation.

[0081] The data acquisition and signal processing module uses a bandpass filter to separate the various frequency components, and the phase delay difference introduced by the transmission fiber length D will continue to be retained. The calculation of the phase depends on this designed and controlled phase delay difference. If the phase delay difference of each frequency component introduced by the transmission fiber length D is 2mπ+2π / 3, and the electro-optic modulator is controlled so that the light energy is mainly concentrated on f0+fAOM -f EOM 、f0+f AOM and f0+f AOM +f EOM The three components of AOM -f EOM 、f0-f AOM and f0-f AOM +f EOM 3 components. Figure 2 Shown is f AOM 200MHz, f EOM The following is an example of the spectrum of the balanced detector output signal at 75MHz. Based on this, the phase calculation steps are as follows Figure 3 As shown, the details are as follows:

[0082] The interference signal S IN It is divided into 3 paths, each with a center frequency of f AOM -f EOM 、f AOM and f AOM +f EOM The three band-pass filters BPF are used to obtain the filtered signals S1, S2 and S3, and their respective spectra are shown in Figure 4 As shown;

[0083] The filtered signals S1 and S3 enter the mixer and the frequency f EOM The single frequency signal is multiplied to obtain the mixed signal S 1C With S 3C , the mixing signal has f AOM Quantity, such as Figure 5 As shown;

[0084] Mixing signal S 1C and S 3C , and the filtered signal S2 all pass through the center frequency f AOM The bandpass filter BPF is used to obtain the filtered signal S 1E , S 2E and S 3E ;

[0085] The filtered signal S 1E , S 2E and S 3E Perform amplitude calibration respectively to obtain a signal S with consistent signal amplitude 1N , S 2N and S 3N , and the phase shift difference introduced by the delay is retained in the electrical signal. If the sensing fiber is an ordinary fiber, then the three signals with the same frequency and a phase difference of 2π / 3 are as follows Figure 6 As shown; if the sensing fiber is a point-scattering enhanced fiber with equal spacing, the signal is as follows Figure 7 As shown;

[0086] Based on the signal S 1N , S 2N and S 3N , the phase of each sampling point for:

[0087]

[0088] If the sensing fiber is a common fiber or a continuous scattering enhanced fiber, the phase difference output result is obtained by differential calculation of the phases of adjacent sampling points after unwinding.

[0089] If the sensing fiber is a point scattering enhanced fiber, then the phase After unwrapping, the phase values ​​of the sampling points belonging to the same pulse are added to obtain the phase sum Then the phase sum of adjacent pulses Subtract the phase difference to get the output result

[0090] The phase delay difference of each frequency component introduced by the transmission fiber length D is mπ+π / 2. The calculation steps of the phase are as follows: Figure 8 As shown, the details are as follows:

[0091] The interference signal S IN It is divided into two paths, each with a center frequency of f AOM -f EOM and f AOM +f EOM The two band-pass filters BPF are used to obtain the filtered signals S1' and S2', and their respective spectra are shown in Fig. 9 As shown;

[0092] The filtered signals S1' and S2' enter the mixer and the frequency is f EOM The single frequency signal is multiplied to obtain the mixed signal S 1C 'With S 2C ', the mixing signal has f AOM Quantity, such as Fig.10 As shown;

[0093] Mixing signal S 1C 'With S 2C 'Respectively pass through the center frequency of f AOM The bandpass filter BPF is used to obtain the filtered signal S 1E ' and S 2E ';

[0094] The filtered signal S 1E ' and S 2E'After amplitude calibration, the signal amplitude is consistent. Model S 1N ' and S 2N ', because the phase shift difference introduced by the delay is retained in the electrical signal. If the sensing fiber is a common fiber, then two signals with the same frequency and a phase difference of π / 2 are as follows Fig.11 As shown; if the sensing fiber is a point-scattering enhanced fiber with equal spacing, the signal is as follows Fig.12 As shown;

[0095] Based on S 1N ' and S 2N ', the phase of each sampling point for:

[0096]

[0097] If the sensing fiber is a common fiber or a continuous scattering enhanced fiber, the phase difference output result is obtained by differential calculation of the phases of adjacent sampling points after unwinding.

[0098] If the sensing fiber is a point scattering enhanced fiber, then the phase After unwrapping, the phase values ​​of the sampling points belonging to the same pulse are added to obtain the phase sum Then the phase sum of adjacent pulses Subtract the phase difference to get the output result

[0099] In the embodiment, the driving circuit for controlling the acousto-optic modulator and the electro-optic modulator is also under the control of the signal acquisition and signal processing module to ensure signal synchronization, control of the bias point and modulation depth of the electro-optic modulator, etc.

[0100] In the embodiment, the balanced detector is replaced by a single-ended detector. At this time, the output light of the electro-optic modulator interferes with the local oscillator light and is input to the single-ended detector through a port, thereby also realizing the conversion of the interference light signal into the interference electrical signal.

[0101] In the embodiment, the electro-optic modulator is replaced by a phase modulator, and the subsequent signal detection and phase demodulation methods are not affected.

[0102] The embodiment further provides a sensing method using the above-mentioned optical fiber distributed acoustic wave sensing system based on multi-frequency signal delay phase control, comprising the following steps:

[0103] The outgoing light of the coherent light source is divided into the detection light and the local oscillator light through the first coupler;

[0104] The data acquisition and signal processing module controls the driving circuit to output a first modulated electrical signal to control the acousto-optic modulator to modulate the detection light and output a detection light pulse, and the detection light pulse is input to the sensing optical fiber through the circulator;

[0105] The backscattered light returned by the sensing optical fiber is input to the electro-optical modulator through the circulator, and the data acquisition and signal processing module controls the driving circuit to output a second modulated electrical signal to control the electro-optical modulator to modulate the input light with the same modulation depth and bias point;

[0106] The output light of the electro-optic modulator is transmitted through the transmission optical fiber, and during the transmission, it interferes with the local oscillator light through the second coupler, and then is input into the balanced detector through two ports to be converted into an interference electrical signal, and then input into the data acquisition and signal processing module;

[0107] The data acquisition and signal processing module performs phase demodulation on the interference electrical signal according to the phase delay difference and the frequency signal component.

[0108] The above method uses modulation to convert the backscattered light signal returned by the sensing optical fiber into a multi-frequency signal, and uses the characteristic that light of different frequencies has different phase delays after being transmitted through the same optical path. The phase shift is controlled by controlling the delay of the signal to the balanced detector, so that different frequency signals have a specific fixed phase delay after heterodyne detection, which can simplify signal detection and processing. The characteristic of this method is that multi-channel signals with the same frequency and a set phase shift can be obtained through single-channel detection, and the phase shift amount can be adjusted.

[0109] The specific implementation methods described above provide a detailed description of the technical solutions and beneficial effects of the present invention. It should be understood that the above is only the most preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, supplements and equivalent substitutions made within the scope of the principles of the present invention should be included in the protection scope of the present invention.

Claims

1. An optical fiber distributed acoustic wave sensing system based on multi-frequency signal delay phase control, characterized in that: The invention comprises a coherent light source, a first coupler, an acousto-optic modulator, a circulator, an electro-optic modulator, a second coupler, a balanced detector, a driving circuit, and a data acquisition and signal processing module. The output light of the coherent light source is divided into detection light and local oscillator light through a first coupler. The detection light passes through an acousto-optic modulator and is modulated under the control of a first modulation electrical signal of a driving circuit to output a detection light pulse. The detection light pulse is input into a sensing optical fiber through a circulator. The backscattered light returned by the sensing optical fiber is input into an electro-optic modulator through a circulator. The backscattered light is modulated with different modulation depths and bias points in the electro-optic modulator under the control of a second modulation electrical signal of a driving circuit. The output light of the electro-optic modulator is transmitted through a transmission optical fiber, and during transmission, it interferes with the local oscillator light through a second coupler, is input into a balanced detector through two ports, and is converted into an interference electrical signal, and is then input into a data acquisition and signal processing module. The length of the transmission optical fiber is the length of the optical fiber from the output of the electro-optic modulator to the input of the photodetector, and its value is set according to the phase delay difference between the multi-frequency signal components output by the electrical modulator. The data acquisition and signal processing module is used to control the first modulated electrical signal and the second modulated electrical signal output by the driving circuit to realize modulation control of the acousto-optic modulator and the electro-optic modulator, and is also used to phase demodulate the interference electrical signal according to the phase delay difference and the frequency signal component.

2. The optical fiber distributed acoustic wave sensing system based on multi-frequency signal delay phase control according to claim 1 is characterized in that: The acousto-optic modulator modulates the probe light with a frequency of f0 to a repetition frequency of f under control. s , the frequency shift is f AOM , a light pulse with a pulse width of Δτ, where the frequency shift of the input detection light by the acousto-optic modulator is up-shifted or down-shifted, then the output light pulse frequency is f0+f AOM or f0-f AOM .

3. The optical fiber distributed acoustic wave sensing system based on multi-frequency signal delay phase control according to claim 2 is characterized in that: The electro-optic modulator generates multiple frequency-shifted optical signals based on the detection light under control. The frequency difference between these frequency-shifted optical signals and the frequency of the detection light is f EOM The frequency of the light component output by the electro-optic modulator is f0+f AOM ±k·f EOM , where k is an integer.

4. The optical fiber distributed acoustic wave sensing system based on multi-frequency signal delay phase control according to claim 3 is characterized in that: The length of the transmission fiber is set according to the phase delay difference between the multi-frequency signal components output by the electrical modulator, including: When k is 1, the three frequency components are f0+f AOM -f EOM 、f0+f AOM and f0+f AOM +f EOM , or f0-f AOM -f EOM 、f0-f AOM and f0-f AOM +f EOM ; The phase shift delay difference between the three frequency components is 2mπ+2π / 3, so the length D of the transmission fiber should be: Control the D value so that the phase shift difference between the lowest and highest frequency components among the three frequency components is mπ+π / 2. At this time, the D value should be: The D value is controlled so that the phase shift difference between adjacent frequency components among the three frequency components is mπ+π / 2. At this time, the D value should be: Where m is a non-negative integer, c is the speed of light in a vacuum, and n is e is the equivalent refractive index of the probe light transmitted in the optical fiber.

5. The optical fiber distributed acoustic wave sensing system based on multi-frequency signal delay phase control according to claim 4 is characterized in that: Phase demodulation of the interfering electrical signal is performed according to the phase delay difference and the frequency signal component, including: When the phase shift delay difference is 2mπ+2π / 3, and the three frequency components are f0+f AOM -f EOM 、f0+f AOM and f0+f AOM +f EOM , or f0-f AOM -f EOM 、f0-f AOM and f0-f AOM +f EOM When , the phase demodulation process is: The interference signal S IN It is divided into 3 paths, each with a center frequency of f AOM -f EOM 、f AOM and f AOM +f EOM 3 band-pass filters are used to obtain filtered signals S1, S2 and S3; The filtered signals S1 and S3 enter the mixer and the frequency f EOM The single frequency signal is multiplied to obtain the mixed signal S 1C With S 3C , the mixing signal has f AOM Quantity; Mixing signal S 1C and S 3C , and the filtered signal S2 all pass through the center frequency f AOM The bandpass filter is used to obtain the filtered signal S 1E , S 2E and S 3E ; The filtered signal S 1E , S 2E and S 3E Perform amplitude calibration respectively to obtain a signal S with consistent signal amplitude 1N , S 2N and S 3N ; Based on the signal S 1N , S 2N and S 3N , the phase of each sampling point for: If the sensing fiber is a common fiber or a continuous scattering enhanced fiber, the phase difference output result is obtained by differential calculation of the phases of adjacent sampling points after unwinding. If the sensing fiber is a point scattering enhanced fiber, then the phase After unwrapping, the phase values ​​of the sampling points belonging to the same pulse are added to obtain the phase sum Then the phase sum of adjacent pulses Subtract the phase difference to get the output result 6. The optical fiber distributed acoustic wave sensing system based on multi-frequency signal delay phase control according to claim 4 is characterized in that: Phase demodulation of the interfering electrical signal is performed according to the phase delay difference and the frequency signal component, including: When the phase shift delay difference is mπ+π / 2, and the three frequency components are f0+f AOM -f EOM 、f0+f AOM and f0+f AOM +f EOM , or f0-f AOM -f EOM 、f0-f AOM and f0-f AOM +f EOM When , the phase demodulation process is: The interference signal S IN It is divided into two paths, each with a center frequency of f AOM -f EOM and f AOM +f EOM Two band-pass filters are used to obtain filtered signals S1' and S2'; The filtered signals S1' and S2' enter the mixer and the frequency is f EOM The single frequency signal is multiplied to obtain the mixed signal S 1C 'With S 2C ', the mixing signal has f AOM Quantity; Mixing signal S 1C 'With S 2C 'Respectively pass through the center frequency of f AOM The bandpass filter is used to obtain the filtered signal S 1E ' and S 2E '; The filtered signal S 1E ' and S 2E 'After amplitude calibration, the signal amplitude is consistent. Model S 1N ' and S 2N '; Based on S 1N ' and S 2N ', the phase of each sampling point for: If the sensing fiber is a common fiber or a continuous scattering enhanced fiber, the phase difference output result is obtained by differential calculation of the phases of adjacent sampling points after unwinding. If the sensing fiber is a point scattering enhanced fiber, then the phase After unwrapping, the phase values ​​of the sampling points belonging to the same pulse are added to obtain the phase sum Then the phase sum of adjacent pulses Subtract the phase difference to get the output result 7. The optical fiber distributed acoustic wave sensing system based on multi-frequency signal delay phase control according to any one of claims 1 to 6, characterized in that: It also includes an optical amplifier and a bandpass filter. The detection light pulse output by the acousto-optic modulator is amplified and filtered by the optical amplifier and the bandpass filter in turn, and then input into the sensing optical fiber through a circulator.

8. The optical fiber distributed acoustic wave sensing system based on multi-frequency signal delay phase control according to any one of claims 1 to 6, characterized in that: The balanced detector is replaced by a single-ended detector. At this time, the output light of the electro-optic modulator interferes with the local oscillator light and is input into the single-ended detector through a port, thereby also realizing the conversion of the interference light signal into the interference electrical signal.

9. The optical fiber distributed acoustic wave sensing system based on multi-frequency signal delay phase control according to any one of claims 1 to 6, characterized in that: The electro-optic modulator is replaced by a phase modulator.

10. A fiber-optic distributed acoustic wave sensing method based on multi-frequency signal delay phase control, characterized in that: The method uses the optical fiber distributed acoustic wave sensing system according to any one of claims 1 to 9, comprising the following steps: The outgoing light of the coherent light source is divided into the detection light and the local oscillator light through the first coupler; The data acquisition and signal processing module controls the driving circuit to output a first modulated electrical signal to control the acousto-optic modulator to modulate the detection light and output a detection light pulse, and the detection light pulse is input to the sensing optical fiber through the circulator; The backscattered light returned by the sensing optical fiber is input to the electro-optical modulator through the circulator, and the data acquisition and signal processing module controls the driving circuit to output a second modulated electrical signal to control the electro-optical modulator to modulate the input light with the same modulation depth and bias point; The output light of the electro-optic modulator is transmitted through the transmission optical fiber, and during the transmission, it interferes with the local oscillator light through the second coupler, and then is input into the balanced detector through two ports to be converted into an interference electrical signal, and then input into the data acquisition and signal processing module; The data acquisition and signal processing module performs phase demodulation on the interference electrical signal according to the phase delay difference and the frequency signal component.

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