CN116086591B - Distributed acoustic wave sensing method and system using time-frequency multiplexing and phase base value reference - Google Patents
Distributed acoustic wave sensing method and system using time-frequency multiplexing and phase base value reference Download PDFInfo
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Abstract
The invention discloses a distributed acoustic wave sensing method and a system adopting time-frequency multiplexing and phase base value reference, which use a time-frequency multiplexing mode to introduce an optical pulse sequence with offset on both time domain and frequency domain, separate the optical pulse sequence at a detection end by utilizing the offset to carry out phase demodulation, and utilize two extra side frequencies to inhibit the influence of coherent fading based on the phase base value reference method, thereby realizing the distributed acoustic wave sensing with large bandwidth and low fading.
Description
Technical Field
The invention belongs to the technical field of distributed optical fiber sensing, and particularly relates to a distributed acoustic wave sensing method and system adopting time-frequency multiplexing and phase base value reference.
Background
In recent years, the distributed acoustic wave sensing technology based on phase-sensitive optical time domain reflection has been widely used in various fields such as perimeter security, rail transit, oil and gas exploration and the like due to the characteristics of long sensing distance, high sensitivity, electromagnetic interference resistance, low cost and the like. The implementation mode of detecting the sound wave is to pump the detection light pulse into the sensing optical fiber, and monitor the sound wave signal by analyzing information such as amplitude, phase, polarization and the like carried by the back Rayleigh scattering light signal generated by the detection light pulse in the transmission process of the sensing optical fiber. The current distributed acoustic wave sensing technology based on phase demodulation is the most common signal demodulation method of the distributed acoustic wave sensing technology because of the characteristics of being capable of quantitatively recovering acoustic wave signals, flexible in structure and the like, for example, a fiber distributed acoustic wave sensing system disclosed in patent document CN110440900A and a random position point fiber distributed acoustic wave sensing system disclosed in patent document CN 103575379A.
The existing phase demodulation type distributed acoustic wave sensing technology also faces some problems, such as coherent fading and polarization fading problems in the superposition process of the backward Rayleigh scattering signals can influence the demodulation precision of the phase signals, and even the demodulation signals can be distorted when serious. In addition, the system bandwidth of the distributed acoustic wave sensing technology is also mutually restricted with the maximum sensing distance of the system.
In order to solve the above problems, a series of system structure and demodulation algorithm improvement schemes are proposed. For example, the signal fading problem can be solved by combining a polarization diversity method, a frequency division multiplexing technology, a chirped pulse technology and the like, and the system bandwidth limitation can be solved by adopting a pulse coding or combining an optical frequency comb mode. However, these solutions cannot combine low fading with large bandwidth distributed acoustic measurements.
Disclosure of Invention
In view of the above technical shortcomings, the present invention aims to provide a distributed acoustic wave sensing method and system using time-frequency multiplexing and phase base value reference, so as to achieve both low-fading and large-bandwidth optical fiber distributed acoustic wave sensing.
To achieve the above object, an embodiment of the present invention provides a distributed acoustic wave sensing system using time-frequency multiplexing and phase base value reference, including a light source, further including:
the first optical fiber coupler is used for dividing light source light output by the light source into two paths;
the modulation unit is used for performing frequency shift and delay on one path of light source light output by the first optical fiber coupler in a time-frequency multiplexing mode to generate an optical pulse sequence with offset in both time domain and frequency domain;
an electro-optic modulator for modulating each optical pulse in the optical pulse train to an optical pulse comprising an original frequency and two sidebands shifted in frequency;
the interference unit is used for optically coupling and coherent one path of optical signal formed by the back Rayleigh scattered light generated by the transmission of the optical pulse in the optical fiber to be detected with the other path of optical source to form an interference optical signal;
and the acquisition demodulation unit is used for converting the interference light signals into digital interference signals, and then comparing the phase information of each frequency position of each frame of signal with the phase base value according to the interference signals to obtain the change of the phase information of each sensing unit on the tested optical fiber along with the frame number, so as to obtain the acoustic wave signals detected by the tested optical fiber.
Preferably, the modulation unit comprises a first acousto-optic modulator, a second optical fiber coupler, a second acoustic optical modulator, a first optical fiber amplifier and a delay optical fiber;
one path of light source light output by the first optical fiber coupler is modulated into frequency shift delta f through the first acousto-optic modulator AOM1 With repetition frequency f rep The first optical pulse with pulse width delta tau is divided into two paths at the input end of the second optical fiber coupler, wherein one path of the first optical pulse sequentially passes through the first optical fiber amplifier, the delay optical fiber and the frequency shift delta f AOM2 The second optical modulator is amplified, delayed and modulated and then is input to the input end of the second optical fiber coupler, and the second optical fiber coupler is coupled with the other path of first optical pulse and then outputs a frequency interval delta f AOM And the time interval is Δt.
Preferably, the electro-optic modulator is subjected to a frequency Δf EOM Sinusoidal signal driving, modulating input optical pulse by controlling sinusoidal signal amplitude and bias voltage of electro-optical modulator, sideband frequency shift of output optical pulse is equal to frequency of sinusoidal signal, and meeting Δf EOM ≤Δf AOM 3 and Δf EOM >0.5/Deltaτ, deltaτ represents the time interval.
Preferably, the length L of the delay fiber needs to satisfy:
wherein v is g Group velocity, f, of transmitted light in an optical fiber PD Is the bandwidth of the photodetector.
Preferably, the interference unit comprises a second optical fiber amplifier, a circulator, a polarization controller, a third optical fiber coupler,
the optical pulse output by the electro-optical modulator is amplified by the second optical fiber amplifier and then is input into the first port of the circulator, the optical pulse is injected into the tested optical fiber from the second port of the circulator and is transmitted in the optical fiber to be tested to generate backward Rayleigh scattering light, the backward Rayleigh scattering light forms an optical signal in the circulator and is output through the third port of the circulator, the other path of light source light output by the first optical fiber coupler is input into two ports of the third optical fiber coupler respectively with the optical signal output by the circulator after passing through the polarization controller, and interference optical signals are formed by coupling coherence in the third optical fiber coupler.
Preferably, in the data acquisition system, the step of obtaining the change of the phase information of each sensing unit on the measured optical fiber along with the frame number by demodulating the phase information of each frequency position of each frame signal and comparing the phase information with the corresponding reference phase according to the interference signal includes:
(a) At a sampling rate f for the kth frame of interference signal s After sampling, obtaining a sampling sequence with the length of N, filtering a kth frame interference signal by using 3M band-pass filters to obtain 3M sampling sequences with the length of N, grouping the 3M sampling sequences according to a frequency sequence and with 3 frequencies as 1 group to obtain M groups of interference signal sampling sequences, wherein the center frequency of each band-pass filter is mDeltaf AOM +nΔf EOM M=1, 2, … M, n= -1,0,1, the M-th set of sampling sequences comprises frequency components { mΔf AOM -Δf EOM ,mΔf AOM ,mΔf AOM +Δf EOM }；
(b) The method comprises the following steps of:
(b-1) performing hilbert transformation and phase calculation on the interference signals of each frequency, completing phase signal demodulation and obtaining interference signal envelopes, and obtaining a phase sequence corresponding to the interference signals through phase unwrapping on the phase signals;
(b-2) if the 1 st frame interference signal is being demodulated, recording the phase sequence obtained in (b-1) as a phase base value at the current frequency interference signal;
(b-3) subtracting the phase base value from the phase sequence obtained in (b-1) to obtain a corrected phase sequence, and calculating a corrected complex sequence by combining the corrected phase sequence with an envelope curve of the interference signal envelope;
(c) Adding the complex sequences corresponding to the three frequencies to obtain an added complex sequence;
(d) Carrying out phase calculation and phase unwrapping on the final sum complex sequence in the step (c), and then carrying out difference on the phase sequence obtained by the position unwrapping to obtain a phase difference sequence with the length of N-1;
(e) Repeating the step (b) -the step (d) until all M groups of interference signal sampling sequences are demodulated;
(f) And (c) continuing to repeat the process from the step (a) until each frame of interference signal is demodulated, namely, the phase information of each sensing unit on the tested optical fiber changing along with time can be obtained, and meanwhile, the phase change caused by the acoustic wave signal sensed by the tested optical fiber is also obtained.
Preferably, in step (a), the sampling rate f s Not less than 5f PD ，f PD Represented as the bandwidth of the photodetector.
Preferably, in step (b-2), further comprising: and (3) taking the phase sequence obtained in the step (b-1) as a phase base value after sliding average.
Preferably, in step (e), a phase difference sequence between M adjacent sensing units with length N-1 is obtained, and the actual distance Δd corresponding to each sensing unit is:
the measurement time corresponding to each phase difference sequence is as follows:
in order to achieve the above object, an embodiment of the present invention further provides a distributed acoustic wave sensing method using time-frequency multiplexing and phase base value reference, where the distributed acoustic wave sensing method uses the above distributed acoustic wave sensing system, and the method includes the following steps:
dividing light source light output by a light source into two paths through a first optical fiber coupler;
performing frequency shift and time delay on one path of light source light output by the first optical fiber coupler through a modulation unit to generate an optical pulse sequence with offset in both time domain and frequency domain;
modulating each optical pulse in the optical pulse sequence into an optical pulse containing the original frequency and the frequency shift of the two sidebands by an electro-optical modulator;
an interference unit is used for optically coupling and coherent one path of optical signals formed by back Rayleigh scattered light generated by the transmission of the optical pulse in the optical fiber to be detected with another path of optical source to form interference optical signals;
after the interference light signals are converted into digital interference signals through the acquisition demodulation unit, the phase information of each frequency position of each frame of signal is compared with the corresponding reference phase according to the interference signals, so that the change of the phase information of each sensing unit on the tested optical fiber along with the frame number is obtained, and further the acoustic wave signals detected by the tested optical fiber are obtained.
Compared with the prior art, the invention has the beneficial effects that at least the following steps are included:
the time-frequency multiplexing mode is used in the modulation unit to introduce an optical pulse sequence with offset in both time domain and frequency domain, the offset is utilized to separate the optical pulse sequence from each other at the detection end for phase demodulation, and the additional two side frequencies are utilized to inhibit the influence of coherent fading based on a phase base value reference method, so that the distributed acoustic wave sensing with large bandwidth and low fading can be realized.
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In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings that are required in the embodiments or the description of the prior art will be briefly described, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
FIG. 1 is a schematic diagram of a distributed acoustic wave sensing system provided by an embodiment;
FIG. 2 is another schematic structural diagram of a distributed acoustic wave sensing system provided by an embodiment;
FIG. 3 is a flow chart of demodulation of an interference signal provided by an embodiment;
fig. 4 is a flow chart of a distributed acoustic wave sensing method provided by an embodiment.
Detailed Description
Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings. When the following description refers to the accompanying drawings, the same numbers in different drawings refer to the same or similar elements, unless otherwise indicated. The implementations described in the following exemplary examples are not representative of all implementations consistent with the present application. Rather, they are merely examples of apparatus and methods consistent with some aspects of the present application as detailed in the accompanying claims.
The terminology used in the present application is for the purpose of describing particular embodiments only and is not intended to be limiting of the present application. As used in this application and the appended claims, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It should also be understood that the term "and/or" as used herein refers to and encompasses any or all possible combinations of one or more of the associated listed items.
It should be understood that although the terms first, second, third, etc. may be used herein to describe various information, these information should not be limited by these terms. These terms are only used to distinguish one type of information from another. For example, a first light pulse may also be referred to as a second light pulse, and similarly, a second light pulse may also be referred to as a first light pulse, without departing from the scope of the present application.
In order to realize distributed acoustic wave sensing with large bandwidth and low fading, an embodiment provides a distributed acoustic wave sensing system adopting time-frequency multiplexing and phase base value reference, as shown in fig. 1 and 2, which comprises a light source 1, a first optical fiber coupler 2, a modulation unit 3, an electro-optical modulator 4, an interference unit 5 and an acquisition demodulation unit 6.
The light source 1 may employ a narrow linewidth laser for emitting light source light of a narrow linewidth. The first optical fiber coupler 2 has two output ports for dividing the light source light outputted from the light source into two paths through the two output ports, wherein one path of light source light is inputted to the modulating unit 3, and the other path of light source light is inputted to the interference unit 5.
The modulation unit 3 is configured to shift and delay one path of light source light output by the first optical fiber coupler 2 by using a time-frequency multiplexing manner, so as to generate an optical pulse sequence with offset in both time domain and frequency domain. As shown in fig. 2, the modulation unit includes a first acousto-optic modulator 31, a second optical fiber coupler 32, a first optical fiber amplifier 33, a delay optical fiber 34, and a second acoustic optical modulator 35, wherein the second optical fiber coupler 32, the second first optical fiber amplifier 33, the delay optical fiber 34, and the second acoustic optical modulator 35 form an optical fiber loop, and one path of light source light output by the first optical fiber coupler 2 is modulated into a shift frequency Δf through the first acoustic optical modulator 31 AOM1 With repetition frequency f rep The first light pulse with pulse width delta tau is divided into two paths at the input end of the second optical fiber coupler 32, wherein one path of the first light pulse sequentially passes through the first optical fiber amplifier 33, the delay optical fiber 34 and the frequency shift delta f AOM2 Is amplified, delayed and modulated and then input to the input end of the second optical fiber coupler 32, and is coupled with another path of first optical pulse in the second optical fiber coupler 32 to output a series of frequency intervals of Deltaf AOM And the time interval is a sequence of light pulses at, the time interval being fixed and dependent on the length of the delay fiber.
Wherein the first acousto-optic modulator 31 and the second acousto-optic modulator 35 are controlled by an acousto-optic modulator drive 36, and the acousto-optic modulator drive 36 is controlled by a signal generator 38. The first optical fiber amplifier 33 may be an erbium doped optical fiber amplifier. The length L of the delay fiber 34 needs to satisfy:
wherein v is g Is an optical fiberGroup velocity, f of medium transmitted light PD Is the bandwidth of the photodetector.
The electro-optical modulator 4 is arranged to modulate each optical pulse in the sequence of optical pulses to an optical pulse comprising the original frequency and the frequency shift of the two sidebands. The frequency of the electro-optic modulator 4 output by the signal generator 38 is delta f EOM Sinusoidal signal driving, modulating input optical pulse by controlling sinusoidal signal amplitude and bias voltage of electro-optical modulator 4, so that each optical pulse in the optical pulse sequence is added with two frequency sidebands based on original frequency, and sideband frequency shift of output optical pulse is equal to frequency of sinusoidal signal, i.e. sideband frequency shift is delta f EOM And meet Δf EOM ≤Δf AOM 3 and Δf EOM >0.5/Deltaτ, deltaτ represents the time interval.
The interference unit 5 is used for optically coupling and coherent one path of optical signals formed by the backward rayleigh scattered light generated by transmitting the optical pulse in the optical fiber to be tested with the other path of optical source to form an interference optical signal by using the optical pulse output by the electro-optical modulator 4. As shown in fig. 2, the interference unit 5 includes a second optical fiber amplifier 51, a circulator 52, a polarization controller 53, and a third optical fiber coupler 54, where the circulator 52 includes three ports, an optical pulse output by the electro-optical modulator 4 is amplified by the second optical fiber amplifier 51 and then input to a first port of the circulator, an optical fiber to be measured is injected from the second port of the circulator and transmitted in the optical fiber to be measured to generate backward rayleigh scattered light, the backward rayleigh scattered light forms an optical signal in the circulator and is output through the third port of the circulator, another optical source light output by the first optical fiber coupler 2 is input to two ports of the third optical fiber coupler 54 after passing through the polarization controller 53, and mixing coherence is performed in the third optical fiber coupler 54 to form an interference optical signal.
The second optical fiber amplifier 51 may be an erbium-doped optical fiber amplifier.
The acquisition demodulation unit 6 is used for converting the interference light signals into digital interference signals, and then obtaining the change of the phase information of each sensing unit on the tested optical fiber along with the frame number by demodulating the phase information of each frequency position of each frame signal and comparing the phase information with the corresponding phase basic value according to the interference signals, so as to obtain the acoustic wave signals detected by the tested optical fiber. As shown in fig. 2, the acquisition demodulation unit 6 includes a photodetector 61 and a data acquisition system 62. Wherein the photodetector 61 is used to convert the interference light signal into a digital form of the interference signal. The data acquisition system 62 is configured to acquire interference signals in the form of data and demodulate the interference signals, as shown in fig. 3, and includes the following steps:
(a) At a sampling rate f for the kth frame of interference signal s After sampling, obtaining a sampling sequence with the length of N, filtering a kth frame interference signal by using 3M band-pass filters to obtain 3M sampling sequences with the length of N, grouping the 3M sampling sequences according to a frequency sequence and with 3 frequencies as 1 group to obtain M groups of interference signal sampling sequences, wherein the center frequency of each band-pass filter is mDeltaf AOM +nΔf EOM M=1, 2, … M, n= -1,0,1, the M-th set of sampling sequences comprises frequency components { mΔf AOM -Δf EOM ,mΔf AOM ,mΔf AOM +Δf EOM -a }; sampling rate f s Not less than 5f PD ，f PD Represented as the bandwidth of the detector;
(b) The method comprises the following steps of:
(b-1) performing Hilbert transform and phase calculation (specifically, arctangent operation) on the interference signals of each frequency, completing phase signal demodulation and obtaining interference signal envelopes, and obtaining phase sequences corresponding to the interference signals through phase unwrapping on the phase signals;
(b-2) if the 1 st frame of interference signal is being demodulated, then the phase sequence obtained in (b-1) is recorded as a phase base value under the current frequency interference signal, and of course, the phase sequence can be used as the phase base value after being subjected to sliding average;
(b-3) subtracting the phase base value from the phase sequence obtained in (b-1) to obtain a corrected phase sequence, and calculating a corrected complex sequence by combining the corrected phase sequence with an envelope curve of the interference signal envelope;
for the corrected complex sequence, respectively calculating a real part and an imaginary part of the complex sequence, and specifically, multiplying the sine value and the cosine value of the corrected phase sequence with an envelope curve to obtain the real part and the imaginary part of the complex sequence;
(c) Adding the complex sequences corresponding to the three frequencies to obtain an added complex sequence;
specifically, adding real parts in complex sequences corresponding to three frequencies, and adding imaginary parts to obtain an added complex sequence;
(d) Performing phase calculation (specifically adopting arctangent operation) and phase unwrapping on the added complex sequence in the step (c), and then performing difference on the phase sequence obtained by the position unwrapping to obtain a phase difference sequence with the length of N-1;
(e) Repeating the step (b) -the step (d) until all M groups of interference signal sampling sequences are demodulated;
through the step (e), a phase difference sequence between M adjacent sensing units with the length of N-1 can be obtained, and the actual distance delta D corresponding to each sensing unit is as follows:
the measurement time corresponding to each phase difference sequence is as follows:
(f) And (c) continuing to repeat the process from the step (a) until each frame of interference signal is demodulated, namely, the phase information of each sensing unit on the tested optical fiber changing along with time can be obtained, and meanwhile, the phase change caused by the acoustic wave signal sensed by the tested optical fiber is also obtained.
Wherein the phase change is converted into time-varying strain information by the following equation.
Where ε is the stress perceived by the fiber, Δφ is the demodulated phase, and β is the propagation coefficient.
The distributed acoustic wave sensing system provided in the above embodiment uses a loop including an acousto-optic modulator to perform frequency shift and delay to generate a pulse sequence having an offset in both the time domain and the frequency domain, and combines an electro-optic modulator to modulate two sidebands. In the demodulation process, the bandwidth is improved by separating signals with different frequencies and demodulating the phase change of the signals with time. And two side frequency signals modulated by the electro-optical modulator are combined with phase base value reference to reduce the influence of coherent fading, thereby realizing the optical fiber distributed acoustic wave sensing with large bandwidth and low fading.
In order to realize distributed acoustic wave sensing with large bandwidth and low fading, an embodiment provides a distributed acoustic wave sensing method adopting time-frequency multiplexing and phase base value reference, wherein the distributed acoustic wave sensing method adopts the distributed acoustic wave sensing system, as shown in fig. 4, and comprises the following steps:
s10, dividing light source light output by a light source into two paths through a first optical fiber coupler;
s20, performing frequency shift and time delay on one path of light source light output by the first optical fiber coupler through a modulation unit to generate an optical pulse sequence with offset in both time domain and frequency domain;
s30, modulating each optical pulse in the optical pulse sequence into an optical pulse containing the original frequency and the frequency shift of two sidebands through an electro-optical modulator;
s40, an interference unit is used for optically coupling and coherent one path of optical signal formed by the back Rayleigh scattered light generated by the transmission of the optical pulse in the optical fiber to be detected with another path of optical source to form an interference optical signal;
s50, after the interference light signals are converted into digital interference signals through the acquisition demodulation unit, the phase information of each frequency position of each frame of signal is compared with the corresponding reference phase according to the interference signals, so that the change of the phase information of each sensing unit on the tested optical fiber along with the frame number is obtained, and further the acoustic wave signals detected by the tested optical fiber are obtained.
The distributed acoustic wave sensing system and the method provided by the embodiment introduce the pulse sequences with offset in both time domain and frequency domain by using a time-frequency multiplexing mode, separate the pulse sequences at the detection end by using the offset to perform phase demodulation, and utilize two additional side frequencies to inhibit the influence of coherent fading based on a phase base value reference method, so that the distributed acoustic wave sensing with large bandwidth and low fading can be realized.
The foregoing detailed description of the preferred embodiments and advantages of the invention will be appreciated that the foregoing description is merely illustrative of the presently preferred embodiments of the invention, and that no changes, additions, substitutions and equivalents of those embodiments are intended to be included within the scope of the invention.
Claims (8)
1. A distributed acoustic wave sensing system employing time-frequency multiplexing and phase-based value referencing, comprising a light source, further comprising:
the first optical fiber coupler is used for dividing light source light output by the light source into two paths;
the modulation unit is used for performing frequency shift and delay on one path of light source light output by the first optical fiber coupler in a time-frequency multiplexing mode to generate an optical pulse sequence with offset in both time domain and frequency domain; the modulation unit comprises a first acousto-optic modulator, a second optical fiber coupler, a second acousto-optic modulator, a first optical fiber amplifier and a delay optical fiber; one path of light source light output by the first optical fiber coupler is modulated into frequency shift delta f through the first acousto-optic modulator AOM1 With repetition frequency f rep The first optical pulse with the pulse width delta tau is divided into two paths at the input end of the second optical fiber coupler, wherein one path of the first optical pulse sequentially passes through the first optical fiber amplifier, the delay optical fiber and the frequency shift delta f AOM2 The second optical modulator is modulated, amplified and delayed and then is input to the input end of the second optical fiber coupler, and the second optical fiber coupler is coupled with the other path of first optical pulse and then outputs a frequency interval delta f AOM And the time interval is an optical pulse sequence of delta t;
an electro-optic modulator for modulating each optical pulse in the optical pulse train to an optical pulse comprising an original frequency and two sidebands shifted in frequency;
the interference unit is used for optically coupling and coherent one path of optical signal formed by the back Rayleigh scattered light generated by the transmission of the optical pulse in the optical fiber to be detected with the other path of optical source to form an interference optical signal;
the acquisition demodulation unit is used for obtaining the change of the phase information of each sensing unit on the tested optical fiber along with the frame number by demodulating the phase information of each frequency position of each frame signal and comparing the phase information with a phase base value according to the interference signal after converting the interference optical signal into the interference signal in a digital form, and further obtaining the acoustic wave signal detected by the tested optical fiber, and comprises the following steps:
(a) At a sampling rate f for the kth frame of interference signal s After sampling, obtaining a sampling sequence with the length of N, filtering a kth frame interference signal by using 3M band-pass filters to obtain 3M sampling sequences with the length of N, grouping the 3M sampling sequences according to a frequency sequence and with 3 frequencies as 1 group to obtain M groups of interference signal sampling sequences, wherein the center frequency of each band-pass filter is mDeltaf AOM +nΔf EOM M=1, 2, … M, n= -1,0,1, the M-th set of sampling sequences comprises frequency components { mΔf AOM -Δf EOM ,mΔf AOM ,mΔf AOM +Δf EOM }；
(b) The method comprises the following steps of:
(b-1) performing hilbert transformation and phase calculation on the interference signals of each frequency, completing phase signal demodulation and obtaining interference signal envelopes, and obtaining a phase sequence corresponding to the interference signals through phase unwrapping on the phase signals;
(b-2) if the 1 st frame interference signal is being demodulated, recording the phase sequence obtained in (b-1) as a phase base value at the current frequency interference signal;
(b-3) subtracting the phase base value from the phase sequence obtained in (b-1) to obtain a corrected phase sequence, and calculating a corrected complex sequence by combining the corrected phase sequence with an envelope curve of the interference signal envelope;
(c) Adding the complex sequences corresponding to the three frequencies to obtain an added complex sequence;
(d) Carrying out phase calculation and phase unwrapping on the final sum complex sequence in the step (c), and then carrying out difference on the phase sequence obtained by phase unwrapping to obtain a phase difference sequence with the length of N-1;
(e) Repeating the step (b) -the step (d) until all M groups of interference signal sampling sequences are demodulated;
(f) And (c) continuing to repeat the process from the step (a) until each frame of interference signal is demodulated, namely, the phase information of each sensing unit on the tested optical fiber changing along with time can be obtained, and meanwhile, the phase change caused by the acoustic wave signal sensed by the tested optical fiber is also obtained.
2. The distributed acoustic wave sensing system employing time-frequency multiplexing and phase basis reference according to claim 1, wherein said electro-optic modulator is subjected to a frequency Δf EOM Sinusoidal signal driving, modulating input optical pulse by controlling sinusoidal signal amplitude and bias voltage of electro-optical modulator, sideband frequency shift of output optical pulse is equal to frequency of sinusoidal signal, and meeting Δf EOM ≤Δf AOM 3 and Δf EOM >0.5/Deltaτ, deltaτ represents the time interval.
3. The distributed acoustic wave sensing system using time-frequency multiplexing and phase basis reference according to claim 1, wherein the length L of the delay fiber needs to satisfy:
wherein v is g Group velocity, f, of transmitted light in an optical fiber PD Is the bandwidth of the photodetector.
4. The distributed acoustic wave sensing system employing time-frequency multiplexing and phase basis reference according to claim 1, wherein the interference unit comprises a second fiber amplifier, a circulator, a polarization controller, a third fiber coupler,
the optical pulse output by the electro-optical modulator is amplified by the second optical fiber amplifier and then is input into the first port of the circulator, the optical pulse is injected into the tested optical fiber from the second port of the circulator and is transmitted in the optical fiber to be tested to generate backward Rayleigh scattering light, the backward Rayleigh scattering light forms an optical signal in the circulator and is output through the third port of the circulator, the other path of light source light output by the first optical fiber coupler is input into two ports of the third optical fiber coupler respectively with the optical signal output by the circulator after passing through the polarization controller, and interference optical signals are formed by coupling coherence in the third optical fiber coupler.
5. The distributed acoustic wave sensing system employing time-frequency multiplexing and phase basis value referencing of claim 1, wherein in step (a), the sampling rate f s Not less than 5f PD ，f PD Represented as the bandwidth of the photodetector.
6. The distributed acoustic wave sensing system employing time-frequency multiplexing and phase basis value referencing of claim 1, further comprising in step (b-2): and (3) taking the phase sequence obtained in the step (b-1) as a phase base value after sliding average.
7. The distributed acoustic wave sensing system using time-frequency multiplexing and phase base reference according to claim 1, wherein in step (e), a phase difference sequence between M adjacent sensing units with length N-1 is obtained, and an actual distance Δd corresponding to each sensing unit is:
the measurement time corresponding to each phase difference sequence is as follows:
8. a distributed acoustic wave sensing method using time-frequency multiplexing and phase basis value reference, characterized in that the distributed acoustic wave sensing method uses the distributed acoustic wave sensing system according to any one of claims 1-7, comprising the steps of:
dividing light source light output by a light source into two paths through a first optical fiber coupler;
performing frequency shift and time delay on one path of light source light output by the first optical fiber coupler through a modulation unit to generate an optical pulse sequence with offset in both time domain and frequency domain;
modulating each optical pulse in the optical pulse sequence into an optical pulse containing the original frequency and the frequency shift of the two sidebands by an electro-optical modulator;
an interference unit is used for optically coupling and coherent one path of optical signals formed by back Rayleigh scattered light generated by the transmission of the optical pulse in the optical fiber to be detected with another path of optical source to form interference optical signals;
after the interference light signals are converted into digital interference signals through the acquisition demodulation unit, the phase information of each frequency position of each frame of signal is compared with the corresponding reference phase according to the interference signals, so that the change of the phase information of each sensing unit on the tested optical fiber along with the frame number is obtained, and further the acoustic wave signals detected by the tested optical fiber are obtained.
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