Quasi-Distributed Vibration Sensing System and Method Based on Low-Coherence Light Source

Abstract

The present invention provides a quasi-distributed vibration sensing system and method based on a low-coherence light source, comprising: a low-coherence light source, a balanced Mach-Zehnder interferometer unit, a weak reflection point array, a photodetector, and a signal processing unit; after the optical signal emitted by the low-coherence light source passes through the unbalanced Mach-Zehnder interferometer unit to reach the weak reflection point array, it passes through the unbalanced Mach-Zehnder interferometer unit again and enters the photodetector, and the signal processing unit collects the voltage signal output by the photodetector to demodulate the phase information of the interference signal generated by the CW light and the CCW light reflected by the same reflection point and outputs the external vibration signal.

Description

Technical Field

[0001] The present invention relates to the technical field of fiber optic sensing, and in particular, to a quasi-distributed vibration sensing system and method based on a low-coherence light source. Background Art

[0002] A quasi-distributed vibration sensing system is a sensing system that uses a weak fiber grating array or a weak reflection point array in an optical fiber as a sensing probe, can detect and locate strain signals occurring between any reflection points, and linearly obtain the waveform of the strain signal. Most of the currently widely used distributed vibration sensing systems are based on the structure of an optical time domain reflectometer or an optical frequency domain reflectometer. By measuring the phase change of the optical signals reflected by adjacent reflection points, the fiber optic strain information caused by environmental vibration is obtained. In these systems, a highly coherent light source with a line width in the kHz level is required, and the price of this highly coherent light source is very expensive, making the cost of the sensing system high and restricting the wide application of the quasi-distributed acoustic wave sensing system.

[0003] Patent document CN207036249U (application number: 201621320115.X) discloses a highly sensitive distributed fiber optic vibration sensing system. The system of the present utility model uses a highly coherent narrow line width laser as a light source, which is split into two paths by a coupler. One of the paths is modulated into an optical pulse sequence by a pulse modulator, amplified by an optical amplifier, and then injected into the vibration sensing optical fiber through a circulator. The backward Rayleigh scattered light in the sensing optical fiber is transmitted back through the circulator and heterodyne beat with the local oscillator light that has been frequency shifted by Δf in the other path. The beat optical signal is converted into an electrical signal by a detector, and then data acquisition and processing are performed. Summary of the Invention

[0004] Aiming at the defects in the prior art, the purpose of the present invention is to provide a quasi-distributed vibration sensing system and method based on a low-coherence light source.

[0005] A quasi-distributed vibration sensing system based on a low-coherence light source according to the present invention includes: a low-coherence light source, an unbalanced Mach-Zehnder interferometer unit, a weak reflection point array, a photodetector, and a signal processing unit;

[0006] After the optical signal emitted by the low-coherence light source reaches the weak reflection point array through the unbalanced Mach-Zehnder interferometer unit, it passes through the unbalanced Mach-Zehnder interferometer unit again and enters the photodetector. The signal processing unit collects the voltage signal output by the photodetector, demodulates the phase information of the interference signal generated by the CW light and the CCW light reflected by the same reflection point, and outputs the external vibration signal to obtain the strain signal.

[0007] Preferably, the unbalanced Mach-Zehnder interferometer unit includes: a first coupler, a second coupler, an acousto-optic modulator, a delay fiber, and an optional polarization controller;

[0008] When incoherent light enters the unbalanced Mach-Zehnder interferometer unit, the CW light sequentially passes through the first coupler, the acousto-optic modulator, the delay fiber, the second coupler, and the weak reflection point array to obtain reflected light; the reflected light sequentially passes through the second coupler, the polarization controller, the first coupler, and enters the photodetector;

[0009] When incoherent light enters the unbalanced Mach-Zehnder interferometer unit, the CCW light sequentially passes through the first coupler, the polarization controller, the second coupler, and the weak reflection point array to obtain reflected light; the reflected light sequentially passes through the second coupler, the delay fiber, the acousto-optic modulator, the first coupler, and enters the photodetector.

[0010] Preferably, both the first coupler and the second coupler are 50:50 couplers.

[0011] Preferably, an electrical signal generator is further included, and the electrical signal generator generates a periodic linear frequency-sweeping pulse signal, or periodically generates a combination of a linear frequency-sweeping pulse signal and a single-frequency pulse signal to drive the acousto-optic modulator and apply a modulation signal to the CW light and the CCW light.

[0012] Preferably, when the electrical signal generator generates a periodic linear frequency-sweeping pulse signal to drive the acousto-optic modulator, it includes:

[0013] Module M1: The signal processing unit collects the electrical signal I(t) output by the photodetector and performs a Fourier transform on the collected electrical signal I(t);

[0014] Module M2: One-to-one correspondence between the reflection point positions and the frequency information after Fourier transform, and the correspondence is where L a represents the total length of the upper arm of the interferometer; L b represents the total length of the lower arm of the interferometer; L i represents the fiber length from the i-th reflection point to the starting end of the reflection point array; υ represents the propagation speed of light in the fiber; κ represents the frequency-sweeping rate of the acousto-optic modulator;

[0015] Module M3: Extract the phase information θ i (t), where i represents the reflection point serial number;

[0016] Module M4: Differentiate the phase signals at the peak positions of adjacent reflection points to obtain the differentiated phase signal △θ i (t); Trigger modules M1 to M4 for each signal within each period, and utilize the obtained differentiated phase signal Δθi (t) Calculate the strain signal;

[0017]

[0018] where τ vib represents the repetition period of the linear swept-frequency signal; τ represents the delay of the current quasi-distributed vibration sensing system; τ = (L a -L b ) / υ; L a represents the total length of the upper arm of the interferometer; L b represents the total length of the lower arm of the interferometer; υ is the propagation speed of light in the optical fiber; L i-1,i represents the reflection point L i-1 and L i the optical fiber length between them.

[0019] Preferably, within each detection period, the electrical signal generator successively generates a linear swept-frequency pulse signal and a single-frequency pulse signal to drive the acousto-optic modulator;

[0020] The CW light first passes through the acousto-optic modulator and is modulated into a linear swept-frequency pulse signal, where the starting frequency of the frequency sweep is f l , the frequency sweep rate is κ, the pulse duration is t p , after reaching the weak reflection point array, the reflected light passes through the unbalanced Mach-Zehnder interferometer again and enters the photodetector;

[0021] The CCW light passes through the acousto-optic modulator after being reflected by the weak reflection point array. At this time, the driving signal of the acousto-optic modulator is a single-frequency signal f s .

[0022] Preferably, when the electrical signal generator periodically generates a combination of a linear swept-frequency pulse signal and a single-frequency pulse signal to drive the acousto-optic modulator, it includes:

[0023] Module N1: The signal processing unit collects the electrical signal I(t) output by the photodetector and convolves the collected electrical signal I(t) with the matched filter u(-t) in the digital domain; the expression of the matched filter is u(t) = rect(t / t p )exp{j2π(f l -f s )t + jπκt 2}, where rect represents the rectangular window function; t p represents the pulse width; f l represents the starting frequency of the frequency sweep of the linear swept-frequency pulse signal; f s represents the frequency of the single-frequency signal;

[0024] Module N2: Establish a one-to-one correspondence between the positions of the reflection points and the time of the time-domain signal after matched filtering. The correspondence is as follows: where L a represents the total length of the upper arm of the interferometer; L b represents the total length of the lower arm of the interferometer; L i represents the optical fiber length from the i-th reflection point to the starting end of the reflection point array; υ represents the propagation speed of light in the optical fiber; i represents the reflection point serial number;

[0025] Module N3: Extract the phase information θ i (t) at the peak position of the reflection point, where i represents the reflection point serial number;

[0026] Module N4: Differentiate the signals extracted from adjacent reflection points to obtain the differential phase signal △θ i (t); Trigger Modules N1 to N4 for the signals in each period, and calculate the strain signal using the obtained differential phase signal △θ i (t);

[0027]

[0028] where τ vib is the emission interval of the light source pulses; τ is the system delay; τ = (L a - L b ) / υ.

[0029] A quasi-distributed vibration sensing method based on a low-coherence light source according to the present invention includes:

[0030] After the optical signal emitted by the low-coherence light source passes through the unbalanced Mach-Zehnder interferometer unit and reaches the weak reflection point array, it passes through the unbalanced Mach-Zehnder interferometer unit again and enters the photodetector. The signal processing unit collects the voltage signal output by the photodetector, demodulates the phase information of the interference signal generated by the CW light and the CCW light reflected by the same reflection point, and outputs the external vibration signal to obtain the strain signal.

[0031] Preferably, the unbalanced Mach-Zehnder interferometer unit includes: a first coupler, a second coupler, an acousto-optic modulator, a delay optical fiber, and an optional polarization controller;

[0032] When incoherent light enters the unbalanced Mach-Zehnder interferometer unit, the CW light sequentially passes through the first coupler, the acousto-optic modulator, the delay optical fiber, the second coupler, and the weak reflection point array to obtain the reflected light; the reflected light sequentially passes through the second coupler, the polarization controller, and the first coupler and enters the photodetector;

[0033] When incoherent light enters the unbalanced Mach-Zehnder interferometer unit, the CCW light passes through the first coupler, polarization controller, second coupler, and weak reflection point array in sequence to obtain reflected light; the reflected light passes through the second coupler, delay fiber, acousto-optic modulator, and first coupler in sequence and enters the photodetector.

[0034] Preferably, it further includes an electrical signal generator, which generates a periodic linear frequency-sweeping pulse signal, or periodically generates a combination of a linear frequency-sweeping pulse signal and a single-frequency pulse signal to drive the acousto-optic modulator and apply a modulation signal to the CW light and CCW light;

[0035] When the electrical signal generator generates a periodic linear frequency-sweeping pulse signal to drive the acousto-optic modulator, it includes:

[0036] Step S11: The signal processing unit collects the electrical signal I(t) output by the photodetector and performs Fourier transform on the collected electrical signal I(t);

[0037] Step S12: One-to-one correspondence between the reflection point position and the frequency information after Fourier transform, and the correspondence relationship is where L a represents the total length of the upper arm of the interferometer; L b represents the total length of the lower arm of the interferometer; L i represents the fiber length from the i-th reflection point to the starting end of the reflection point array; υ represents the propagation speed of light in the fiber; κ represents the frequency-sweeping rate of the acousto-optic modulator;

[0038] Step S13: Extract the phase information θ i (t), where i represents the reflection point serial number;

[0039] Step S14: Differentiate the phase signals at the peak positions extracted from adjacent reflection points to obtain the differentiated phase signal △θ i (t); Repeat triggering Step S11 to Step S14 for the signals in each period, and calculate the strain signal using the obtained differentiated phase signal Δθ i (t);

[0040]

[0041] where τ vib represents the repetition period of the linear frequency-sweeping signal; τ represents the delay of the current quasi-distributed vibration sensing system; τ = (L a -L b ) / υ; L a represents the total length of the upper arm of the interferometer; L b represents the total length of the lower arm of the interferometer; υ is the propagation speed of light in the fiber; L i-1,i represents the reflection point Li-1 The optical fiber length between L i ;

[0042] During each detection period, the electrical signal generator generates a linear frequency-swept pulse signal and a single-frequency pulse signal successively to drive the acousto-optic modulator;

[0043] The CW light first passes through the acousto-optic modulator and is modulated into a linear frequency-swept pulse signal. Among them, the starting frequency of the frequency sweep is f l , the frequency sweep rate is κ, and the pulse duration is t p . After reaching the weak reflection point array, the reflected light passes through the unbalanced Mach-Zehnder interferometer again and enters the photodetector;

[0044] The CCW light passes through the acousto-optic modulator after being reflected by the weak reflection point array. At this time, the driving signal of the acousto-optic modulator is a single-frequency signal f s ;

[0045] When the electrical signal generator periodically generates a combination of a linear frequency-swept pulse signal and a single-frequency pulse signal to drive the acousto-optic modulator, it includes:

[0046] Step S21: The signal processing unit collects the electrical signal I(t) output by the photodetector and convolves the collected electrical signal I(t) with the matched filter u(-t) in the digital domain; the expression of the matched filter is u(t) = rect(t / t p )exp{j2π(f l -f s )t + jπκt 2}), where rect represents the rectangular window function; t p represents the pulse width; f l represents the starting frequency of the frequency sweep of the linear frequency-swept pulse signal; f s represents the frequency of the single-frequency signal;

[0047] Step S22: Establish a one-to-one correspondence between the reflection point position and the time of the time-domain signal after matched filtering. The correspondence is: where L a represents the total length of the upper arm of the interferometer; L b represents the total length of the lower arm of the interferometer; L i represents the optical fiber length from the i-th reflection point to the starting end of the reflection point array; υ represents the propagation speed of light in the optical fiber; i represents the reflection point serial number;

[0048] Step S23: Extract the phase information θ i (t), i represents the reflection point serial number;

[0049] Step S24: Differentiate the signals extracted from adjacent reflection points to obtain the differential phase signal △θ i (t); Repeat the triggering of Steps S21 to S24 for the signals within each period, and calculate the strain signal using the obtained differential phase signal △θ i (t);

[0050]

[0051] where τ vib is the light source pulse emission interval; τ is the system delay; τ = (L a - L b ) / υ.

[0052] Compared with the prior art, the present invention has the following beneficial effects:

[0053] 1. By combining a non-equal-arm interferometer with a weak reflection point array and introducing a modulation device, through different modulation schemes, the positions of weak reflection points are mapped to the frequency domain or the time domain, and finally the phase change of CW light and CCW light within the time difference is measured, realizing the positioning and demodulation of vibration;

[0054] 2. Compared with the existing distributed and quasi-distributed acoustic wave sensing systems that require the use of high-coherence laser light sources, which are costly, the present invention can use a very low-cost low-coherence light source, simple devices and optical paths to achieve dynamic strain measurement, significantly reducing the system cost and complexity.

[0055] 3. In terms of measurement accuracy, the present invention can achieve the same order of measurement accuracy as the existing acoustic wave sensing systems based on high-coherence light. In addition, the system accuracy of the acoustic wave sensing system based on a high-coherence light source is limited by problems such as the line width of the laser, while the accuracy of the present invention is no longer limited by the line width of the laser and the drift of the laser wavelength. Description of the Drawings

[0056] By reading the detailed description of the non-limiting embodiments with reference to the following drawings, other features, objectives and advantages of the present invention will become more obvious:

[0057] Figure 1 is a block diagram of a quasi-distributed sensing system based on a low-coherence light source.

[0058] Figure 2 is a flowchart of the demodulation method in Embodiment 2.

[0059] Figure 3 is a distance-reflectivity diagram of the first 5 weak reflection points obtained by performing a Fourier transform on the data collected once in Embodiment 2.

[0060] Figure 4Power spectral density of the strain signals on two channels after demodulation in Embodiment 2.

[0061] Figure 5 Strain accuracy at 1 kHz for each channel when the system in Embodiment 2 is stationary.

[0062] Figure 6 Flow chart of the demodulation method in Embodiment 3.

[0063] Figure 7 Distance-reflectivity diagram of the first 500-meter weak reflection points obtained by performing Fourier transform on the data collected once in Embodiment 3.

[0064] Figure 8 Power spectral density of the strain signals on two channels after demodulation in Embodiment 3.

[0065] Figure 9 Strain accuracy at 1 kHz for each channel when the system in Embodiment 3 is stationary.

[0066] 1 is a low-coherence light source, 2 is a first coupler, 3 is an acousto-optic modulator, 4 is an electrical signal generator, 5 is a delay fiber, 6 is a second coupler, 7 is an array of weak reflection points, 8 is a polarization controller, 9 is a photodetector, and 10 is a signal processing unit. Detailed implementation manners

[0067] The present invention will be described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several changes and improvements can still be made. These all belong to the protection scope of the present invention.

[0068] The present invention proposes a quasi-distributed vibration sensing system and method based on a low-coherence light source, which reduces the cost of the sensing system while maintaining high strain resolution.

[0069] The present invention relates to a quasi - distributed acoustic wave sensing system based on a low - coherence light source, which measures the interference signals of two beams of light reflected by the same reflection point and detects the vibration signals applied to the reflection point array through the phase change of the interference signals. The low - coherence pulsed light enters the reflection point array after passing through the unbalanced Mach - Zehnder interferometer unit, and after being reflected, it enters the photodetector after passing through the unbalanced Mach - Zehnder interferometer unit again. The optical signals of the following two paths are used for sensing: The light in the clockwise direction (CW): It travels along the upper arm of the interferometer from the light source, reaches the reflection point array and is reflected to the lower arm of the interferometer, and then is output to the photodetector; The light in the counter - clockwise direction (CCW): It travels along the lower arm of the interferometer from the light source, is reflected and then enters the upper arm of the interferometer, and finally the light output to the photodetector. The CW light and the CCW light travel in different directions and reach the photodetector after passing through the same optical path, and interfere at the detector. Since the positions of each reflection point are different, the CW light and the CCW light reflected by different reflection points reach the detector at different times, which is t i =(L a +L b +2L i ) / υ, where L a is the total length of the upper arm of the interferometer, L b is the total length of the lower arm of the interferometer, L i is the optical fiber length from the i - th reflection point to the starting end of the reflection point array, and υ is the propagation speed of light in the optical fiber. Through the transmission time, the light reflected from different reflection points can be separated in the time domain. Since the lengths of the upper arm and the lower arm of the interferometer are different, the CW light and the CCW light reach the i - th reflection point at different times, and there is a time delay τ=(L a -L b ) / υ. When there is a strain ε(t) between the (i - 1) - th reflection point and the i - th reflection point, due to the time delay τ, relative to the time when the CCW light reaches the i - th reflection point, when the CW light reaches the i - th reflection point, the length from the reflection point to the starting section of the optical fiber changes by ΔL. Therefore, the phase of the interference signal of the light generated by the CW light and the CCW light from the i - th reflection point will change. According to the photo - elastic effect, the phase change amount is where ω c is the central angular frequency of the light source, L i-1,i is the optical fiber length between the (i - 1) - th reflection point and the i - th reflection point, and γ is the photo - elastic coefficient. By detecting Δθ i , the vibration signal ε(t) can be linearly recovered. Since the optical fiber lengths from each reflection point after the i - th reflection point to the starting end of the reflection point array all change, it is necessary to perform a difference on the phases extracted from adjacent reflection points.

[0070] Example 1

[0071] A quasi-distributed vibration sensing system based on a low-coherence light source according to the present invention includes: a low-coherence light source, an unbalanced Mach-Zehnder interferometer unit, a weak reflection point array, a photodetector, and a signal processing unit;

[0072] After the optical signal emitted by the low-coherence light source passes through the unbalanced Mach-Zehnder interferometer unit and reaches the weak reflection point array, it passes through the unbalanced Mach-Zehnder interferometer unit again and enters the photodetector. The light passing through the interferometer in the clockwise direction (CW) and the light passing through the interferometer in the counterclockwise direction (CCW) reach the photodetector simultaneously and interfere. Since the arrival times of the CW and CCW lights at the same weak reflection point are different, the phase difference is modulated by the vibration to be measured. The signal processing unit collects the voltage signal output by the photodetector and demodulates the phase information of the interference signal generated by the CW light and the CCW light reflected from the same reflection point through signal processing technology to accurately output the external vibration signal;

[0073] Specifically, the signal processing unit is used to demodulate the electrical signal output on the photodetector to obtain the vibration signal to be measured.

[0074] Specifically, the unbalanced Mach-Zehnder interferometer unit includes: a first coupler, a second coupler, an acousto-optic modulator, a delay fiber, and an optional polarization controller;

[0075] When incoherent light enters the unbalanced Mach-Zehnder interferometer unit, the CW light sequentially passes through the first coupler, the acousto-optic modulator, the delay fiber, the second coupler, and the weak reflection point array to obtain a reflected light; the reflected light sequentially passes through the second coupler, the polarization controller, and the first coupler and enters the photodetector;

[0076] When incoherent light enters the unbalanced Mach-Zehnder interferometer unit, the CCW light sequentially passes through the first coupler, the polarization controller, the second coupler, and the weak reflection point array to obtain a reflected light; the reflected light sequentially passes through the second coupler, the delay fiber, the acousto-optic modulator, and the first coupler and enters the photodetector.

[0077] Specifically, the unbalanced Mach-Zehnder interferometer consists of two couplers as the basic structure, an acousto-optic modulator and a delay fiber are added to the upper arm, and a polarization controller is added to the lower arm, and the splitting ratios of the two couplers are both 50:50.

[0078] Specifically, it further includes an electrical signal generator, and the electrical signal generator generates a periodic linear frequency-sweeping pulse signal, or periodically generates a combination of a linear frequency-sweeping pulse signal and a single-frequency pulse signal to drive the acousto-optic modulator to apply a modulation signal to the CW light and the CCW light.

[0079] When an acousto-optic modulator is added to the unbalanced Mach-Zehnder interferometer unit, and the signal driving the acousto-optic modulator is a periodic linear frequency-sweeping pulse signal. In one period, after the CW light and the CCW light enter the unbalanced Mach-Zehnder interferometer, because their transmission paths are opposite, they will reach the acousto-optic modulator at different times and be modulated into linear frequency-sweeping light. At the photodetector, the CW light and the CCW light reflected from the same reflection point interfere to form a beat signal, and finally the photodetector outputs an electrical signal I(t). The electrical signal I(t) enters the signal processing unit, and after the signal processing process, the strain information ε(t) applied to the sensing array is output.

[0080] Specifically, when the electrical signal generator generates a periodic linear frequency-sweeping pulse signal to drive the acousto-optic modulator, it includes:

[0081] Module M1: The signal processing unit collects the electrical signal I(t) output by the photodetector and performs Fourier transform on the collected electrical signal I(t);

[0082] Module M2: Establish a one-to-one correspondence between the reflection point positions and the frequency information after Fourier transform. The correspondence is where L a represents the total length of the upper arm of the interferometer; L b represents the total length of the lower arm of the interferometer; L i represents the optical fiber length from the i-th reflection point to the starting end of the reflection point array; υ represents the propagation speed of light in the optical fiber; κ represents the frequency-sweeping rate of the acousto-optic modulator; the specific position of the reflection point is obtained through the frequency information;

[0083] Module M3: Extract the phase information θ i (t) at the peak position of the reflection point, where i represents the reflection point serial number;

[0084] Module M4: Differentiate the phase signals at the peak positions extracted from adjacent reflection points to obtain the differentiated phase signal △θ i (t); Trigger modules M1 to M4 for each signal in a period, and calculate the strain signal using the obtained differentiated phase signal Δθ i (t);

[0085]

[0086] where τ vib represents the repetition period of the linear frequency-sweeping signal; τ represents the delay of the current quasi-distributed vibration sensing system; τ = (L a -L b ) / υ; L a represents the total length of the upper arm of the interferometer; L b represents the total length of the lower arm of the interferometer; υ is the propagation speed of light in the optical fiber; Li-1,i Indicates the reflection point L i-1 and L i The optical fiber length between them.

[0087] The upper arm and the lower arm of the interferometer respectively refer to the Figure 1 Optical paths on the upper side and the lower side in the figure. They are both ordinary single-mode optical fibers. After connecting the first coupler and the second coupler, the most basic interferometer can be obtained.

[0088] Specifically, an acousto-optic modulator is added to the unbalanced Mach-Zehnder interferometer unit, and the signal driving the acousto-optic modulator is divided into two parts. One part is a linear frequency-sweeping pulse signal, and the other part is a single-frequency pulse signal. After the CW light and the CCW light enter the unbalanced Mach-Zehnder interferometer unit, the CW light first passes through the acousto-optic modulator and is modulated into a linear frequency-sweeping pulse signal, where the starting frequency of the frequency sweep is f l , the frequency-sweeping rate is κ, and the pulse width is t p . After reaching the weak reflection point array, the reflected light passes through the unbalanced Mach-Zehnder interferometer again and enters the photodetector. The CCW light passes through the acousto-optic modulator after being reflected by the weak reflection point array. At this time, the driving signal of the acousto-optic modulator is a single-frequency signal f s . At the photodetector, the CW light and the CCW light reflected by the same reflection point interfere to form a beat signal. Finally, the photodetector outputs an electrical signal I(t). The electrical signal I(t) enters the data processing unit. After the signal processing process, the strain information ε(t) applied to the sensing array is output.

[0089] Specifically, when the electrical signal generator periodically generates a combination of a linear frequency-sweeping pulse signal and a single-frequency pulse signal to drive the acousto-optic modulator, it includes:

[0090] Module N1: The signal processing unit collects the electrical signal I(t) output by the photodetector and convolves the collected electrical signal I(t) with the matched filter u(-t) in the digital domain; the expression of the matched filter is u(t) = rect(t / t p )exp{j2π(f l -f s )t + jπκt 2 )}, where rect represents the rectangular window function; t p represents the pulse width; f l represents the starting frequency of the frequency sweep of the linear frequency-sweeping pulse signal; f s represents the frequency of the single-frequency signal;

[0091] Module N2: One-to-one correspondence between the reflection point position and the time of the time-domain signal after matched filtering, and the correspondence relationship is: where L a represents the total length of the upper arm of the interferometer; Lb Represents the total length of the lower arm of the interferometer; L i Represents the optical fiber length from the i-th reflection point to the starting end of the reflection point array; υ represents the propagation speed of light in the optical fiber; i represents the reflection point serial number;

[0092] Module N3: Extract the phase information θ i (t) at the peak position of the reflection point, where i represents the reflection point serial number;

[0093] Module N4: Differentiate the signals extracted from adjacent reflection points to obtain the differential phase signal △θ i (t); Trigger modules N1 to N4 for the signals in each period, and calculate the strain signal using the obtained differential phase signal △θ i (t);

[0094]

[0095] Among them, τ vib Is the emission interval of the light source pulse; τ is the system delay; τ = (L a -L b ) / υ.

[0096] The quasi-distributed vibration sensing system based on a low-coherence light source provided by the present invention can be realized through the step process in the quasi-distributed vibration sensing method based on a low-coherence light source provided by the present invention. Those skilled in the art can understand the quasi-distributed vibration sensing method based on a low-coherence light source as a preferred example of the quasi-distributed vibration sensing system based on a low-coherence light source.

[0097] The present invention as a whole solves the problems in the existing distributed and quasi-distributed acoustic wave sensing systems that a high-coherence laser light source needs to be used, the cost is expensive, and the system accuracy is limited by the line width of the laser. Compared with the prior art, the present invention can achieve high-precision dynamic strain measurement with very simple devices and optical paths, the demodulation process is simple, and the system cost and complexity are significantly reduced.

[0098] Example 2

[0099] Embodiment 2 is a preferred example of Embodiment 1

[0100] As Figure 1 shown, this embodiment relates to a quasi-distributed acoustic wave sensing system based on a low-coherence light source for implementing the above method, including: a low-coherence light source 1, a first coupler 2, an acousto-optic modulator 3, an electrical signal generator 4, a delay optical fiber 5, a second coupler 6, a weak reflection point array 7, a polarization controller 8, a photodetector 9, and a signal processing unit 10;

[0101] Among them: The first coupler 2, the acousto-optic modulator 3, the delay optical fiber 5, the second coupler 6 and the polarization controller 8 form an unbalanced interferometer structure. The b port of the first coupler 2 is sequentially connected to the acousto-optic modulator 3, the delay optical fiber 5 and the a port of the second coupler 6. The d port of the first coupler 2 is sequentially connected to the polarization controller 8 and the c port of the second coupler 6. The b port of the second coupler 6 is connected to the weak reflection point array, and the c port of the first coupler is connected to the photodetector 9. The pulsed optical signal emitted by the low-coherence broadband light source 1 is output to the a port of the first coupler 2, and after passing through the first coupler 2, it enters the unbalanced bar interferometer structure through the b and d ports of the first coupler respectively. The optical signal passing through the b port of the first coupler passes through the acousto-optic modulator 3 and the delay optical fiber 5 and is output to the a port of the second coupler, enters the weak reflection point array through the b port of the second coupler, is reflected by the reflection point and then passes through the c port of the second coupler 6 and the polarization controller 8, and enters the photodetector 9 through the c port of the first coupler 2, forming an optical signal transmitted in the clockwise direction; the optical signal passing through the d port of the first coupler passes through the unbalanced interferometer structure in the opposite direction and finally enters the photodetector 9, forming an optical signal transmitted in the counterclockwise direction. The signal processing unit 10 collects the input voltage signal of the photodetector 9 and performs synchronous demodulation.

[0102] In this embodiment, the signal driving the acousto-optic modulator is a linear swept pulse signal. At this time, the phase of the CW light reflected by the i-th reflection point when it reaches the photodetector is: The phase of the CCW light reflected by the i-th reflection point when it reaches the photodetector is: Where Ω 0 is the initial angular frequency of the acousto-optic modulator, and k is the sweep rate.

[0103] The phase difference between the CW light and the CCW light is: ΔL is the change in the position of the reflection point when the CW light and the CCW light reach the i-th reflection point, where τ = (L a -L b ) / υ, L a is the total length of the upper arm of the interferometer, and L b is the total length of the lower arm of the interferometer. The first term in the parentheses is related to the position of the reflection point and can be used for the positioning of the reflection point; ΔL is often at the nanostrain level, so the second term is very small and can be ignored; the third term is linearly related to the optical path change caused by the strain and can be used for the demodulation of the strain signal; the fourth term is a constant term and does not affect the demodulation of the strain. The AC part of the photocurrent signal detected at the photodetector is: I AC ∝r i cos(2πf i t + θ i + C i ), where r iis the reflectivity of the i-th reflection point, θ i = 2ΔL(ω c +Ω 0 ), C i = (L a +L b + 2L i )Ω 0 . After performing Fourier transform on the received electrical signal, both fi i and θ i values can be obtained. Since the optical path of each reflection point has changed, it is necessary to perform differential on the phases extracted from adjacent reflection points to obtain the phase difference Δθ i between the (i - 1)-th reflection point and the i-th reflection point.

[0104] The low-coherence light source described above is an erbium-doped superfluorescent fiber source with a central wavelength of 1550 nm, a spectral bandwidth of 35 nm, the output optical power is set to 80 mW, and the optical pulse width generated by the light source is 200 us;

[0105] The light source described above can be replaced with a low-cost light source such as an LED;

[0106] The driving signal of the acousto-optic modulator is a linear frequency-swept pulse signal with a frequency ranging from 150 MHz to 250 MHz, a pulse width of 200 us, a pulse emission interval of 250 us, and a frequency-sweeping rate of 500 GHz / s.

[0107] The length of the delay fiber described above is 2.1 km of ordinary single-mode fiber. At this time, the time delay τ between CW and CCW is 10.5 us. The delay fiber can be replaced with ordinary single-mode fibers of other lengths;

[0108] The first fiber coupler described above is a 50:50 fiber coupler, and the second fiber coupler is a 50:50 fiber coupler;

[0109] The weak reflection dot array described above is an array of weak reflection points with a relatively low reflectivity inscribed by focusing femtosecond laser at the core position point by point. The reflectivity of each reflection point to incident light of different wavelengths is almost the same;

[0110] For the weak reflection dot array described above, the distance between reflection points is 20 m, the average reflectivity is -40 dB, and there are a total of 40 reflection points;

[0111] The weak reflection dot array described above can be replaced with a weak fiber grating array and a fiber grating array with non-overlapping reflection spectra;

[0112] The specific steps of this embodiment are as follows, as Figure 2 shown:

[0113] Step 1: Output a pulsed light source with a width of 200 us and a period of 250 us to the input end of the unbalanced interferometer, and adjust the time of the radio frequency signal generated by the electrical signal generator so that the electrical signal applied to the acousto-optic modulator is aligned with the optical pulse signal in the time domain. At this time, the signal generated by the electrical signal generator is a linear frequency-swept pulse signal with a frequency ranging from 150 MHz to 250 MHz, and the pulse width and period are the same as those of the optical pulse signal;

[0114] Step 2: Collect the electrical signal I(t) generated by the photodetector once, and perform a Fourier transform on this electrical signal;

[0115] Step 3: Perform peak searching on the signal after Fourier transform to find the position of each weak reflection, as Figure 3 shown; the signal after transformation is complex, and the phase signal at the peak of each weak reflection point can be extracted; subtract the phase signals extracted from adjacent channels to obtain the phase difference Δθ between the (i - 1)th reflection point and the ith reflection point i .

[0116] Step 4: Repeat Step 2 and Step 3, and measure Δθ at different times i to form a time-domain signal Δθ i (t). Multiply the integrated Δθ i (t) in the time domain by the ratio of the sampling interval τ of the system for vibration to τ. Specifically: vib In this example, τ is the pulse emission interval of 250 us, and τ is 10.52 us. vib

[0117] Step 5: Apply a sine strain signal of 500 Hz between the 1st and 2nd reflection points, and apply a sine strain signal of 700 Hz between the 2nd and 3rd reflection points. Repeat Steps 2 to 4, and the power spectral density of the strain signals on the two channels after demodulation can be obtained, as Figure 4 shown; repeat Steps 2 to 4 without applying a strain signal, and the strain accuracy at 1 kHz for each channel in Figure 5 can be obtained. The accuracy of the first ten channels before measurement can be read as

[0118] The method for locating the reflection points in this embodiment is to map the positions of different reflection points to different frequencies of the beat frequency signal. Therefore, the frequency after Fourier transform corresponds one-to-one with the position of the reflection point.

[0119] The spatial resolution of this example is determined by the reflection point spacing. After Fourier transform, the full width at half maximum of the main lobe at the reflection point is determined by the pulse frequency-sweeping range, that is, Δz = v / (2B), where B is the frequency-sweeping range. In this embodiment, B is 100 MHz, so the width of the main lobe is 1 meter.

[0120] The vibration frequency response bandwidth of this embodiment is determined by the emission time interval τ of the swept-probe optical pulse vib and is 1 / 2τ vib . In this embodiment, τ vib is 250 us, so the vibration frequency response bandwidth is 2 kHz.

[0121] Compared with the prior art, this example is based on a low-coherence light source laser, can linearly recover the strain signal, and the strain accuracy is Moreover, the system complexity and cost are significantly reduced, having good practical value.

[0122] Example 3

[0123] Embodiment 3 is a preferred example of Embodiment 1

[0124] As Figure 1 shown, this embodiment two relates to a quasi-distributed acoustic wave sensing system based on a low-coherence light source for implementing the above method, and the system composition is the same as that of Embodiment 1.

[0125] In this embodiment, the signal driving the acousto-optic modulator consists of two segments in one cycle. The first segment is a linear swept-frequency pulse, so that the CW-direction pulsed light can be modulated into a linear swept-frequency pulse after passing through the acousto-optic modulator. The second segment is a single-frequency pulse signal, so that a fixed frequency shift can be applied to the CCW light when it passes through the acousto-optic modulator. After being reflected by the i-th reflection point and reaching the photodetector, the electric field signal of the CW light is where ai is the loss of the CW light during transmission, ri is the reflectivity of the i-th reflection point, rect is the rectangular window function,[[]]END]] is the transmission duration of the CW light, t p is the pulse width, ω c is the central angular frequency of the light source, f l is the starting frequency of the linear swept-frequency pulse, and κ is the sweeping rate. After being reflected by the i-th reflection point and reaching the photodetector, the electric field signal of the CCW light is where f s is the frequency shift modulated by the acousto-optic modulator on the CCW light when the CCW light passes through the acousto-optic modulator. When strain ε(t) occurs between the i-th reflection point and the i-th reflection point, due to the change in the length of the strained section of the optical fiber, the time for the light to pass through this area will also change, and the amount of change in time is The times for the CW light and the CCW light to be reflected by the i-th reflection point and transmitted to the photodetector are t l =(L b +L i ) / v and t s =(L a +Li ) / v, where L a and L b are the lengths of the upper and lower arms of the interferometer respectively. At this time, when reaching the photodetector, the phase difference between the CW light and the CCW light is: where τ(t) is the time change caused by vibration, and the value is very small. Therefore, in , τ(t) in the second and third terms can be ignored. The current signals generated by the CW light and the CCW light of the i-th reflection point at the photodetector are: where θ i (t) = exp{jω c [τ(t - t s ) - τt - tl, ut = rectttpexph2πfl - fst + jπkt2. The second term of Iit, that is, the AC term, contains phase information, and θ i (t) contains information related to strain. In order to extract θ i (t), it is necessary to convolve the collected electrical signal with the matched filter u(-t) in the digital domain. After convolving the AC term in I i (t) with u(-t), the convolved signal R i (t) can be obtained, and the specific expression is where is the convolution operation, and * is the conjugate symbol. Rit is a pulsed complex signal with a sinc function shape, and θ i (t) can be extracted at the peak. Since the optical path of each reflection point has changed, it is necessary to differentiate the phases extracted from adjacent reflection points to obtain the phase difference Δθ i (t) between the (i - 1)-th reflection point and the i-th reflection point.

[0126] The low-coherence light source mentioned above is an erbium-doped superfluorescent fiber light source, with a central wavelength of 1550 nm, a spectral bandwidth of 35 nm, the peak power of the output optical pulse is set to 500 mW, the pulse width of the light pulse generated by the light source is 2 us, and the pulse emission interval is 100 us;

[0127] The light source mentioned above can be replaced by low-cost light sources such as LEDs;

[0128] The driving signal period of the acousto-optic modulator is 100 us. The first half of the frequency-sweeping signal is a frequency-sweeping signal with a width of 2 us, and the frequency-sweeping range is from 190 MHz to 240 MHz. The second half of the signal is a single-frequency signal of 170 MHz, and the duration length is at least long enough to cover the time of the CCW light reflected from all reflection points. In the example, the time is 20 us, as Figure 6 shown.

[0129] The ordinary single-mode optical fiber with a delay optical fiber length of 3.906 km. At this time, the time delays τ of CW and CCW are 19.53 us. The delay optical fiber can be replaced with ordinary single-mode optical fibers of other lengths;

[0130] The first optical fiber coupler is a 50:50 optical fiber coupler, and the second optical fiber coupler is a 50:50 optical fiber coupler;

[0131] The weak reflection dot array is an array of weak reflection points with a low reflectivity inscribed by focusing femtosecond laser at the core position point by point. Its feature is that the reflectivity of each reflection point to incident light of different wavelengths is almost the same;

[0132] For the weak reflection dot array described above, the distance between reflection points is 20 m, the average reflectivity is -40 dB, and there are 40 reflection points in total;

[0133] The weak reflection dot array can be replaced with a weak fiber grating array and a fiber grating array with non-overlapping reflection spectra;

[0134] The specific steps of this embodiment are as follows:

[0135] Step 1: Output a pulse light source with a width of 2 us and a period of 100 us to the input end of the unbalanced interferometer. Adjust the time of the radio frequency signal generated by the electrical signal generator so that the first half of the linearly frequency-swept electrical signal applied to the acousto-optic modulator is aligned with the optical pulse signal in the time domain. At this time, the signal generated by the electrical signal generator is a linearly frequency-swept pulse signal with a frequency ranging from 190 MHz to 240 MHz. The pulse width and period are the same as those of the optical pulse signal, both being 2 us, and the second half of the signal is a single-frequency signal of 170 MHz;

[0136] Step 2: Collect the electrical signal I(t) generated by the photodetector once, and convolve this electrical signal with a matched filter. The matched filter is a linearly frequency-swept pulse signal with a length of 2 us and a frequency-sweeping range from 20 MHz to 70 MHz;

[0137] Step 3: Perform peak searching on the convolved signal to find the position of each weak reflection, as Figure 7 shown; the transformed signal is complex, and the phase signal at the peak of each weak reflection point can be extracted; subtract the phase signals extracted from adjacent channels to obtain the phase difference Δθ between the (i - 1)-th reflection point and the i-th reflection point i .

[0138] Step 4: Repeat Step 2 and Step 3 multiple times, measure Δθ at different times i to form a time-domain signal Δθ i (t), and the obtained Δθ i(t) Integrate in the time domain and then multiply by the sampling interval τ of the system for vibration vib The ratio to τ, specifically: In this example, τ vib Is the pulse emission interval of 100 μs, and τ is 19.53 μs.

[0139] Step Five: Apply a 1 kHz sinusoidal strain signal between the 1st and 2nd reflection points, and apply a 2 kHz sinusoidal strain signal between the 2nd and 3rd reflection points. Repeat Steps Two to Four, and the power spectral density of the strain signals on the two channels after demodulation can be obtained, as Figure 8 Shown; Repeat Steps Two to Four without applying a strain signal, and the Figure 9 Strain accuracy at 1 kHz for each channel in can be obtained. It can be read that the accuracy of measuring thirty-five channels is

[0140] The method of locating the reflection points in this embodiment is to map the positions of different reflection points to different transmission times.

[0141] The spatial resolution of this example is determined by the reflection point spacing. After matched filtering, the full width at half maximum of the main lobe at the reflection point is determined by the pulse sweep frequency range, that is, Δz = v / (2B), where B is the sweep frequency range. In this embodiment, B is 50 MHz, so the main lobe width is 2 meters.

[0142] The vibration frequency response bandwidth of this embodiment is determined by the emission time interval τ vib Of the swept-frequency detection optical pulse, which is 1 / 2τ vib In this embodiment, τ vib Is 100 μs, so the vibration frequency response bandwidth is 5 kHz.

[0143] Compared with the prior art, this example is based on a low-coherence light source laser, and can linearly recover the strain signal. The strain accuracy is And the system complexity and cost are significantly reduced, having good practical value.

[0144] Those skilled in the art know that in addition to implementing the systems, devices, and their respective modules provided by the present invention in the form of pure computer-readable program code, the method steps can be logically programmed to enable the systems, devices, and their respective modules provided by the present invention to be implemented in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers, etc. to implement the same program. Therefore, the systems, devices, and their respective modules provided by the present invention can be considered as a kind of hardware component, and the modules included therein for implementing various programs can also be regarded as the structure within the hardware component; the modules for implementing various functions can also be regarded as both software programs for implementing the method and the structure within the hardware component.

[0145] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Without conflict, the embodiments of the present application and the features in the embodiments can be arbitrarily combined with each other.

Claims

1. A quasi-distributed vibration sensing system based on a low-coherence light source, characterized in that, it includes: a low-coherence light source, an unbalanced Mach-Zehnder interferometer unit, a weak reflection point array, a photodetector, and a signal processing unit; After the optical signal emitted by the low-coherence light source passes through the unbalanced Mach-Zehnder interferometer unit and reaches the weak reflection point array, it passes through the unbalanced Mach-Zehnder interferometer unit again and enters the photodetector. The signal processing unit collects the voltage signal output by the photodetector, demodulates the phase information of the interference signal generated by the CW light and CCW light reflected by the same reflection point, and outputs the external vibration signal to obtain the strain signal; The unbalanced Mach-Zehnder interferometer unit includes: a first coupler, a second coupler, an acousto-optic modulator, a delay fiber, and an optional polarization controller; When incoherent light enters the unbalanced Mach-Zehnder interferometer unit, the CW light sequentially passes through the first coupler, the acousto-optic modulator, the delay fiber, the second coupler, and the weak reflection point array to obtain the reflected light; the reflected light sequentially passes through the second coupler, the polarization controller, and the first coupler and enters the photodetector; When incoherent light enters the unbalanced Mach-Zehnder interferometer unit, the CCW light sequentially passes through the first coupler, the polarization controller, the second coupler, and the weak reflection point array to obtain the reflected light; the reflected light sequentially passes through the second coupler, the delay fiber, the acousto-optic modulator, and the first coupler and enters the photodetector.

2. The quasi-distributed vibration sensing system based on a low-coherence light source according to claim 1, characterized in that, both the first coupler and the second coupler are 50:50 couplers.

3. The quasi-distributed vibration sensing system based on a low-coherence light source according to claim 1, characterized in that, it further includes an electrical signal generator, and the electrical signal generator generates a periodic linear frequency-sweeping pulse signal, or periodically generates a combination of a linear frequency-sweeping pulse signal and a single-frequency pulse signal to drive the acousto-optic modulator and apply a modulation signal to the CW light and CCW light.

4. The quasi-distributed vibration sensing system based on a low-coherence light source according to claim 3, characterized in that, when the electrical signal generator generates a periodic linear frequency-sweeping pulse signal to drive the acousto-optic modulator, it includes: Module M1: The signal processing unit collects the electrical signal I(t) output by the photodetector and performs Fourier transform on the collected electrical signal I(t); Module M2: Establish a one-to-one correspondence between the positions of the reflection points and the frequency information after Fourier transform. The correspondence is where L a represents the total length of the upper arm of the interferometer; L b represents the total length of the lower arm of the interferometer; L i represents the optical fiber length from the i-th reflection point to the starting end of the reflection point array; υ represents the propagation speed of light in the optical fiber; κ represents the frequency sweep rate of the acousto-optic modulator; Module M3: Extract the phase information θ at the peak position of the reflection point i (t), where i represents the reflection point serial number; Module M4: Differentiate the phase signals at the peak positions extracted from adjacent reflection points to obtain the differentiated phase signal Δθ i (t); Repeat triggering Modules M1 to M4 for the signals in each period, and calculate the strain signal using the obtained differentiated phase signal Δθ i (t); Among them, τ vib represents the repetition period of the linear frequency-swept signal; τ represents the delay of the current quasi-distributed vibration sensing system; τ = (L a - L b ) / υ; L a represents the total length of the upper arm of the interferometer; L b represents the total length of the lower arm of the interferometer; υ is the propagation speed of light in the optical fiber; L i-1,i represents the optical fiber length between the reflection point L i-1 and L i .

5. The quasi-distributed vibration sensing system based on a low-coherence light source according to claim 3, characterized in that, within each detection period, the electrical signal generator successively generates a linear frequency-sweeping pulse signal and a single-frequency pulse signal to drive the acousto-optic modulator; The CW light is first modulated by an acou-optic modulator into a linearly swept-frequency pulse signal, where the starting frequency of the frequency sweep is f l , the frequency sweep rate is κ, and the pulse duration is t p . After reaching the weak reflection point array, the reflected light passes through an unbalanced Mach-Zehnder interferometer again and enters a photodetector; The CCW light is reflected by the weak reflection point array and then passes through the acousto-optic modulator. At this time, the driving signal of the acousto-optic modulator is a single-frequency signal f s .

6. The quasi-distributed vibration sensing system based on a low-coherence light source according to claim 5, characterized in that, when the electrical signal generator periodically generates a combination of a linear frequency-sweeping pulse signal and a single-frequency pulse signal to drive the acousto-optic modulator, it includes: Module N1: The signal processing unit collects the electrical signal I(t) output by the photodetector and convolves the collected electrical signal I(t) with the matched filter u(-t) in the digital domain; the expression of the matched filter is u(t) = rect(t / t p )exp{j2π(f l -f s )t + jπκt 2 )}, where rect represents the rectangular window function; t p represents the pulse width; f l represents the starting frequency of the frequency sweep of the linearly frequency-swept pulse signal; f s represents the frequency of the single-frequency signal; Module N2: Establish a one-to-one correspondence between the positions of the reflection points and the time of the time-domain signal after matched filtering. The correspondence is as follows: where L a represents the total length of the upper arm of the interferometer; L b represents the total length of the lower arm of the interferometer; L i represents the optical fiber length from the i-th reflection point to the starting end of the reflection point array; υ represents the propagation speed of light in the optical fiber; i represents the reflection point serial number; Module N3: Extract the phase information θ at the peak position of the reflection point i (t), where i represents the reflection point number; Module N4: Differentiate the signals extracted from adjacent reflection points to obtain the differential phase signal Δθ i (t); Repeat triggering Modules N1 to N4 for the signals in each period, and calculate the strain signal using the obtained differential phase signal Δθ i (t); Among them, τ vib is the emission interval of the light source pulses; τ is the system delay; τ = (L a - L b ) / υ.

7. A quasi-distributed vibration sensing method based on a low-coherence light source, characterized in that, it includes: After the optical signal emitted by the low-coherence light source passes through the unbalanced Mach-Zehnder interferometer unit and reaches the weak reflection point array, it passes through the unbalanced Mach-Zehnder interferometer unit again and enters the photodetector. The signal processing unit collects the voltage signal output by the photodetector to demodulate the phase information of the interference signal generated by the CW light and the CCW light reflected from the same reflection point, and outputs the external vibration signal to obtain the strain signal; The unbalanced Mach-Zehnder interferometer unit includes: a first coupler, a second coupler, an acousto-optic modulator, a delay fiber, and an optional polarization controller; When incoherent light enters the unbalanced Mach-Zehnder interferometer unit, the CW light sequentially passes through the first coupler, the acousto-optic modulator, the delay fiber, the second coupler, and the weak reflection point array to obtain a reflected light; the reflected light sequentially passes through the second coupler, the polarization controller, and the first coupler and enters the photodetector; When incoherent light enters the unbalanced Mach-Zehnder interferometer unit, the CCW light sequentially passes through the first coupler, the polarization controller, the second coupler, and the weak reflection point array to obtain a reflected light; the reflected light sequentially passes through the second coupler, the delay fiber, the acousto-optic modulator, and the first coupler and enters the photodetector; It further includes an electrical signal generator, which generates a periodic linearly swept-frequency pulse signal, or periodically generates a combination of a linearly swept-frequency pulse signal and a single-frequency pulse signal to drive the acousto-optic modulator and apply a modulation signal to the CW light and the CCW light; When the electrical signal generator generates a periodic linearly swept-frequency pulse signal to drive the acousto-optic modulator, it includes: Step S11: The signal processing unit collects the electrical signal I(t) output by the photodetector and performs a Fourier transform on the collected electrical signal I(t); Step S12: Establish a one-to-one correspondence between the positions of the reflection points and the frequency information after Fourier transform, and the correspondence is where L a represents the total length of the upper arm of the interferometer; L b represents the total length of the lower arm of the interferometer; L i represents the optical fiber length from the i-th reflection point to the starting end of the reflection point array; υ represents the propagation speed of light in the optical fiber; κ represents the frequency sweeping rate of the acousto-optic modulator; Step S13: Extract the phase information θ at the peak position of the reflection point i (t), where i represents the reflection point serial number; Step S14: Differentiate the phase signals at the peak positions extracted from adjacent reflection points to obtain the differentiated phase signal Δθ i (t); Repeat the triggering of steps S11 to S14 for the signals in each period, and calculate the strain signal using the obtained differentiated phase signal Δθ i (t); Among them, τ vib represents the repetition period of the linear frequency-swept signal; τ represents the delay of the current quasi-distributed vibration sensing system; τ = (L a - L b ) / υ; L a represents the total length of the upper arm of the interferometer; L b represents the total length of the lower arm of the interferometer; υ is the propagation speed of light in the optical fiber; L i-1,i represents the optical fiber length between the reflection point L i-1 and L i . Within each detection period, the electrical signal generator sequentially generates a linearly swept-frequency pulse signal and a single-frequency pulse signal to drive the acousto-optic modulator; The CW light is first modulated by an acou-optic modulator into a linearly frequency-swept pulse signal, where the starting frequency of the frequency sweep is f l , the frequency-sweeping rate is κ, and the pulse duration is t p . After reaching the weak reflection point array, the reflected light passes through an unbalanced Mach-Zehnder interferometer again and enters a photodetector; The CCW light passes through the acousto-optic modulator after being reflected by the weak reflection point array. At this time, the driving signal of the acousto-optic modulator is a single-frequency signal f s ; When the electrical signal generator periodically generates a combination of a linearly swept-frequency pulse signal and a single-frequency pulse signal to drive the acousto-optic modulator, it includes: Step S21: The signal processing unit collects the electrical signal I(t) output by the photodetector, and convolves the collected electrical signal I(t) with the matched filter u(-t) in the digital domain; the expression of the matched filter is u(t) = rect(t / t p )exp(j2π(f l -f s )t + jπκt 2}, where rect represents the rectangular window function; t p represents the pulse width; f l represents the starting frequency of the frequency sweep of the linearly frequency-swept pulse signal; f s represents the frequency of the single-frequency signal. Step S22: Establish a one-to-one correspondence between the positions of the reflection points and the time of the time-domain signal after matched filtering. The correspondence is as follows: where L a represents the total length of the upper arm of the interferometer; L b represents the total length of the lower arm of the interferometer; L i represents the optical fiber length from the i-th reflection point to the starting end of the reflection point array; υ represents the propagation speed of light in the optical fiber; i represents the reflection point serial number; Step S23: Extract the phase information θ at the peak position of the reflection point i (t), where i represents the reflection point serial number; Step S24: Differentiate the signals extracted from adjacent reflection points to obtain the differential phase signal Δθ i (t); Repeat the triggering steps from step S21 to step S24 for the signals in each period, and calculate the strain signal by using the obtained differential phase signal Δθ i (t); Among them, τ vib is the light source pulse emission interval; τ is the system delay; τ = (L a - L b ) / υ.

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