DAS system based on linear frequency modulation pulse and phase demodulation method thereof

Abstract

The invention belongs to the technical field of optical fiber distributed sensing, and particularly relates to a DAS system based on linear frequency modulation pulses and a phase demodulation method of the DAS system based on the linear frequency modulation pulses. A part of light is divided by the polarization-maintaining coupler to serve as signal light, the signal light is modulated into LFM light pulses through a double-parallel Mach-Zehnder modulator driven by an LFM signal generator, and the LFM light pulses are output to a sensing optical fiber through an optical circulator after being filtered by an optical filter. Rayleigh backscattered light generated in the sensing optical fiber is output by the optical circulator and then enters the interference coupler; the other part of light separated by the polarization-maintaining coupler is used as reference light to enter an interference coupler; an interference signal output by the interference coupler is detected by the balanced photoelectric detector and is subjected to phase demodulation by the computer to obtain a vibration signal. According to the invention, rapid positioning and high-fidelity demodulation of the vibration signals are realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical fiber distributed sensing, and specifically relates to a DAS system based on linear frequency modulation pulse and a phase demodulation method thereof. The phase compensation effect of the linear frequency modulation pulse DAS system can be used to realize rapid positioning and high-fidelity demodulation of vibration signals. Background Art

[0002] In recent years, fiber-optic distributed acoustic sensing (DAS) technology has significant advantages and application prospects in the fields of seismic wave monitoring, perimeter security, large-scale infrastructure operation and maintenance, and urban underground space health monitoring due to its advantages such as wide sensing range, high sensing measurement sensitivity, high temporal and spatial resolution accuracy, and less environmental impact. Among them, optical time domain reflectometer based on phase sensitivity Based on the theoretical basis that the phase of the Rayleigh backscattered light after the detection pulse light is injected into the optical fiber is linearly related to the optical fiber strain, the phase information of the Rayleigh backscattered signal is demodulated to recover the vibration signal. Linear frequency modulation (LFM) has also been applied in DAS systems because it has the advantages of continuous waveform and pulse waveform. Introducing it into the DAS system can break the limitations of pulse width and spatial resolution.

[0003] The phase demodulation methods based on the linear frequency modulated pulse DAS system are mainly divided into two types: interferometer structure-based and heterodyne coherent structure-based. The passive demodulation method based on the interferometer structure mainly includes the 3×3 coupler demodulation method and the phase generation carrier demodulation method. Although this type of demodulation method has a simple structure, due to the need to use different numbers of photodetectors, the system cost and measurement error are increasing while the environmental stability is deteriorating. The demodulation methods based on the heterodyne coherent structure can be roughly divided into three categories. The first category is the IQ demodulation method, which divides the collected signal into I and Q polarization states through a 90° optical mixer, and realizes phase demodulation through the inverse tangent method (Optics express, 2016, 24 (2): 853-858.). Compared with the interferometer structure, although the environmental adaptability and detection sensitivity indicators are improved, the introduction of the 90° mixer will increase the system cost and there is a problem of 90° induction inaccuracy. The second type is the differential phase demodulation method, which performs point differential calculations on the phases before and after the vibration position to obtain the phase change along the optical fiber link (IEEE Photonics Journal, 2018, 10 (1): 1-9.). Compared with the IQ demodulation method, external noise can be eliminated through the differential step, and the vibration waveform recovery effect is better after arithmetic averaging, but its data volume and calculation amount are very large, and it takes longer time, making it difficult to achieve real-time demodulation. The third type is the matched filter demodulation method, which solves the contradiction between spatial resolution and sensing distance through pulse compression (Optics letters, 2017, 42 (3): 391-394.). Compared with the differential phase demodulation method, although the spatial resolution has been broken through to the sub-meter level, the vibration waveform recovery effect after demodulation is not good.

[0004] Due to the real-time requirements for phase information acquisition and processing, it is necessary to design a new phase demodulation device and method to overcome the existing technical difficulties, which can not only realize the rapid and accurate positioning of vibration signals, but also reflect external disturbance information and realize fast and high-fidelity demodulation. Summary of the invention

[0005] The present invention overcomes the shortcomings of the prior art and provides a DAS system based on linear frequency modulation pulses and a phase demodulation method to solve the problems of long calculation time and poor vibration waveform recovery effect in the existing phase demodulation technology based on linear frequency modulation pulse DAS system.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is: a DAS system based on linear frequency modulation pulse, including an LFM optical pulse modulation shaping module, the LFM optical pulse modulation shaping module including a narrow linewidth laser, a polarization-maintaining coupler, an LFM signal generator, and a dual parallel Mach-Zehnder modulator; the laser emitted by the narrow linewidth laser is divided into two parts by the polarization-maintaining coupler, a part of the light separated by the polarization-maintaining coupler is used as signal light and modulated into an LFM optical pulse by the dual parallel Mach-Zehnder modulator driven by the LFM signal generator, the LFM optical pulse is filtered by an optical filter and output to a sensing optical fiber through an optical circulator, the Rayleigh backscattered light generated in the sensing optical fiber is output by the optical circulator and then incident to one input end of an interference coupler; the other part of the light separated by the polarization-maintaining coupler is incident to the other input end of the interference coupler as a reference light; the Rayleigh backscattered light and the reference light interfere with each other at the interference coupler, the interference signal output by the interference coupler is detected by a balanced photodetector, and the detection signal is phase-demodulated by a computer to obtain a vibration signal.

[0007] The LFM signal generator is used to generate a digital single pulse signal with a pulse width of ns and a repetition period of μs, and the narrow linewidth laser is used to output a laser with a central wavelength of .nm.

[0008] The linear frequency modulated pulse based DAS system further includes a beam splitter and a modulator bias controller. The beam splitter is arranged at the output end of the dual parallel Mach-Zehnder modulator, and is used to split the output laser of the dual parallel Mach-Zehnder modulator into a part and send it to the modulator bias controller. The modulator bias controller is used to bias the intensity modulator to the zero point and the phase shifter to the orthogonal point according to the output laser of the dual parallel Mach-Zehnder modulator.

[0009] The linear frequency modulated pulse based DAS system further comprises an adjustable optical attenuator and a polarization controller. The reference light output by the polarization maintaining coupler is subjected to the adjustable optical attenuator to adjust the light intensity. The polarization controller adjusts the polarization and then enters the polarization controller. After passing through the polarization controller, the reference light is incident on the other input end of the interference coupler.

[0010] The linear frequency modulated pulse based DAS system further comprises an optical filter, a first erbium-doped fiber amplifier and a second erbium-doped fiber amplifier. The first erbium-doped fiber amplifier is used to amplify the LFM optical pulse at the output end of the dual parallel Mach-Zehnder modulator, and the optical filter is used to filter out the spontaneous radiation noise of the first erbium-doped fiber amplifier; the second erbium-doped fiber amplifier is used to amplify the Rayleigh backscattered light output by the optical circulator.

[0011] The phase demodulation method of the DAS system based on linear frequency modulation pulse comprises the following steps:

[0012] Step 1: Obtain the detected light intensity sensing signal;

[0013] Step 2: Perform Hilbert transform and phase unwinding processing on the light intensity sensing signal to obtain the instantaneous phase at each position of the optical fiber, calculate the differential phase based on the instantaneous phase, and then obtain the differential phase curve;

[0014] Step 3: Extract the differential phase curve of the vibration event coverage position, and determine the centroid solution area based on the reverse phase bulge on the differential phase curve;

[0015] Step 4: Calculate the centroid coordinates of the centroid solution area according to the centroid method;

[0016] Step 5: Repeat steps 1 to 4 to obtain the center of mass motion trajectory at each moment, use the center of mass position as the representative value of the phase in the phase curve at that moment, and demodulate the obtained vibration waveform.

[0017] The method for determining the centroid solution area is:

[0018] Establish a new coordinate system with the vibration starting point as the coordinate origin O;

[0019] Determine the data point p when the differential phase curve bulges and then drops in the opposite direction to the phase value equal to that corresponding to the origin of the coordinate system;

[0020] The origin of the coordinate system and the data point p are used as the starting point and the end point of the centroid solution area respectively, and the area enclosed by the differential phase curve and the horizontal axis of the coordinate system between the starting point and the end point is used as the range of the centroid solution area;

[0021] The calculation formula of the centroid coordinates is:

[0022]

[0023] in, Indicates the two-dimensional coordinates of the centroid of the centroid solution area, S n S represents the area of ​​the nth grid obtained by discretizing the centroid solution area. B represents the total area of ​​the centroid solution region, x n Represents the horizontal coordinate corresponding to the nth data point, Represents the differential phase corresponding to the nth data point.

[0024] The method for determining the centroid solution area is:

[0025] Establish a new coordinate system with the vibration starting point as the coordinate origin O;

[0026] Determine the data point r corresponding to the highest phase point after the differential phase curve bulges and before it drops in the opposite direction;

[0027] Determine that the differential phase curve drops in the opposite direction and then rises back to the phase The corresponding data point q at in, It represents the phase difference before and after the vibration event;

[0028] Take data point r and data point q as the starting point and end point of the centroid solution area respectively, and compare the differential phase curve between the starting point and the end point with the ordinate equal to The range of the centroid solution area is enclosed by the horizontal line of

[0029] The calculation formula of the centroid coordinates is:

[0030]

[0031] in, Indicates the two-dimensional coordinates of the centroid of the centroid solution area, S l S represents the area of ​​the lth grid obtained by discretizing the centroid solution area. A represents the total area of ​​the centroid solution region, x l represents the horizontal coordinate corresponding to the lth data point, Represents the differential phase corresponding to the lth data point.

[0032] In step 2, the calculation formula of the instantaneous phase is:

[0033]

[0034] in, Indicates the fiber position z i The instantaneous phase at , H represents the Hilbert transform operation, unwrap represents the phase unwrapping operation, I(τ,z i ) represents the position z i The corresponding light intensity at

[0035] In step 2, the calculation formula of the differential phase curve is:

[0036]

[0037] Among them, mod represents the mod function, represents the fiber position z in the first trace i The instantaneous phase at .

[0038] The step 5 also includes a step of removing coordinate mutation points in the motion trajectory curve by using an adaptive threshold.

[0039] Compared with the prior art, the present invention has the following beneficial effects:

[0040] (1) The present invention calculates the differential phase to obtain its variation curve, and obtains the phase compensation area by effectively eliminating the phase unwinding problem caused by the inverse tangent method. The area can be used to realize rapid positioning of the vibration signal.

[0041] (2) Based on the phase compensation effect, the present invention calculates the centroid position at each moment as the phase representative value at each moment through the threshold centroid algorithm, obtains the motion trajectory curve of the centroid position along the detection cycle axis, which corresponds to the vibration waveform of the vibration signal, and realizes high-fidelity demodulation. At the same time, the method of combining the ordinary centroid algorithm with the adaptive threshold method is adopted to realize the elimination of laser noise and fading noise.

[0042] (3) The present invention uses a threshold centroid algorithm to process the phase compensation region to replace the cross-correlation sliding window algorithm used in traditional demodulation, which can simplify the calculation process, save calculation time, and achieve fast demodulation.

[0043] In summary, the present invention proposes a DAS system based on linear frequency modulation pulse and a phase demodulation method thereof, which eliminates the influence of external noise on the system by processing the phase compensation area, thereby realizing rapid positioning and high-fidelity demodulation of vibration signals. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 A schematic diagram of a DAS system structure based on a linear frequency modulation pulse provided by an example of the present invention;

[0045] In the figure: 10-LFM optical pulse modulation shaping module, 101-narrow linewidth laser, 102-polarization maintaining coupler, 103-arbitrary waveform generator, 104-dual parallel Mach-Zehnder modulator, 105-modulator bias controller, 106-optical splitter, 107-first erbium-doped fiber amplifier, 108-filter;

[0046] 11-fiber sensor module, 111-optical circulator, 112-second erbium-doped fiber amplifier, 113-driving signal generator, 114-power amplifier, 115-single-mode optical fiber, 116-piezoelectric ceramic tube;

[0047] 12-heterodyne interference module, 121-adjustable optical attenuator, 122-polarization controller, 123-interference coupler;

[0048] 13-signal acquisition and processing module, 131-balanced photodetector, 132-high-speed real-time oscilloscope, 133-computer.

[0049] Figure 2 A flow chart of a phase demodulation method based on a linear frequency modulation pulse DAS system adopted in an example of the present invention;

[0050] Figure 3A schematic diagram of the calibration of the phase compensation area at the vibration event coverage area used in an example of the present invention. DETAILED DESCRIPTION

[0051] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all the embodiments; based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0052] like Figure 1 As shown, the first embodiment of the present invention provides a DAS system based on linear frequency modulation pulse, including an LFM optical pulse modulation shaping module 10, wherein the LFM optical pulse modulation shaping module 10 includes a narrow linewidth laser 101, a polarization-maintaining coupler 102, an LFM signal generator 103, a dual parallel Mach-Zehnder modulator 104 and an optical filter 108; the laser emitted by the narrow linewidth laser 101 is divided into two parts by the polarization-maintaining coupler 102, and a part of the light separated by the polarization-maintaining coupler 102 is used as a signal light and modulated into an LF by the dual parallel Mach-Zehnder modulator 104 driven by the LFM signal generator 103. M optical pulses, the LFM optical pulses are output to the sensing optical fiber 115 through the optical circulator 111, the Rayleigh backscattered light generated in the sensing optical fiber 115 is outputted by the optical circulator 111 and then incident on an input end of the coupler 123, and the optical circulator 111 plays a role of directional transmission; another part of the light separated by the polarization-maintaining coupler 102 is incident on the other input end of the interference coupler 123 as the reference light; the Rayleigh backscattered light and the reference light interfere with each other at the interference coupler 123, the interference signal is detected by the balanced photodetector 131, and the detection signal is phase-demodulated by the computer 133 to obtain the vibration signal.

[0053] Specifically, in this embodiment, the LFM signal generator 103 is used to generate a digital single pulse signal with a pulse width of 100ns and a repetition period of 110μs, and the narrow linewidth laser 101 is used to output a laser with a central wavelength of 1550.12nm. The LFM signal generator 103 can be implemented by an arbitrary waveform generator.

[0054] Furthermore, a linear frequency modulated pulse based DAS system of this embodiment further includes a beam splitter 106 and a modulator bias controller 105, wherein the beam splitter 106 is arranged at the output end of the dual parallel Mach-Zehnder modulator 104, and is used to split the output laser of the dual parallel Mach-Zehnder modulator 104 into a part and send it to the modulator bias controller 105, and the modulator bias controller 105 is used to bias the intensity modulator to the zero point and the phase shifter to the orthogonal point according to the output laser of the dual parallel Mach-Zehnder modulator 104.

[0055] Specifically, the polarization-maintaining coupler 102 has a splitting ratio of 90:10, wherein 90% of the light is used as signal light and 10% of the light is used as reference light, and the optical splitter 106 is a 99:1 optical coupler, wherein 1% of the light is connected to the modulator bias controller 105 to provide feedback. The interference coupler 123 is a 50:50 optical coupler.

[0056] Specifically, the first erbium-doped fiber amplifier 107 is a pulse-type erbium-doped fiber amplifier, and the second erbium-doped fiber amplifier 112 is a small-signal erbium-doped fiber amplifier.

[0057] Furthermore, a DAS system based on linear frequency modulation pulses in this embodiment also includes an adjustable optical attenuator 121 and a polarization controller 122. The reference light output by the polarization-maintaining coupler 102 is intensity-adjusted by the adjustable optical attenuator 121, and the polarization controller 122 adjusts the polarization and then enters the polarization controller 122, and then enters the other input end of the interference coupler 123 after passing through the polarization controller 122.

[0058] Furthermore, a linear frequency modulated pulse based DAS system of this embodiment further includes an optical filter 108, a first erbium-doped fiber amplifier 107 and a second erbium-doped fiber amplifier 112, wherein the first erbium-doped fiber amplifier 107 is arranged at the output end of the dual parallel Mach-Zehnder modulator 104, and is used to amplify the LFM optical pulse at the output end of the dual parallel Mach-Zehnder modulator 104, and the optical filter 108 is arranged at the output end of the first erbium-doped fiber amplifier 107, and is used to filter out the spontaneous radiation noise of the first erbium-doped fiber amplifier 107. The second erbium-doped fiber amplifier 112 is arranged between the optical circulator 111 and the interference coupler 123, and is used to amplify the Rayleigh backscattered light output by the optical circulator 111.

[0059] Specifically, in this embodiment, a narrow linewidth laser 101, a polarization-maintaining coupler 102, an LFM signal generator 103, a dual parallel Mach-Zehnder modulator 104, a modulator bias controller 105, a beam splitter 106, a first erbium-doped fiber amplifier 107, and an optical filter 108 constitute an LFM optical pulse modulation shaping module 10; an optical circulator 111, a second erbium-doped fiber amplifier 112, and a sensing fiber 115 form an optical fiber sensing module. An adjustable optical attenuator 121, a polarization controller 122, and an interference coupler 123 constitute an optical fiber sensor module 11, and a balanced photodetector 131 and a computer 133 constitute a signal acquisition and processing module 13.

[0060] Furthermore, the signal acquisition and processing module 13 also includes a high-speed real-time oscilloscope 132 for displaying the acquired light intensity waveform.

[0061] Specifically, the sensing optical fiber 115 is a single-mode optical fiber. In this embodiment, in order to test the demodulation effect of the system, a piezoelectric ceramic tube 116 is arranged on the sensing optical fiber 115. The driving signal generator 113 outputs a signal, and the piezoelectric ceramic tube 116 is driven by the power amplifier 114 to make the sensing optical fiber 115 vibrate, thereby simulating environmental vibration.

[0062] Embodiment 2

[0063] like Figure 2 As shown, the second embodiment of the present invention provides a phase demodulation method of a DAS system based on a linear frequency modulation pulse, comprising the following steps:

[0064] Step 1: Obtain the detected light intensity sensing signal.

[0065] Specifically, the light intensity sensing signal of the interference light can be collected by the balanced photodetector 131 .

[0066] Step 2: Perform Hilbert transform and phase unwinding processing on the light intensity sensing signal to obtain the instantaneous phase at each position of the optical fiber, calculate the differential phase based on the instantaneous phase, and then obtain the differential phase curve;

[0067] Step 3: Extract the differential phase curve of the vibration event coverage position, and determine the centroid solution area based on the reverse phase bulge on the differential phase curve;

[0068] Step 4: Calculate the centroid coordinates of the centroid solution area using the threshold centroid method.

[0069] Step 5: Repeat steps 1 to 4 to obtain the center of mass motion trajectory at each moment, use the center of mass position as the representative value of the phase in the phase curve at that moment, and demodulate the obtained vibration waveform.

[0070] The demodulation principle of the phase demodulation method of this embodiment is introduced below.

[0071] The optical field E of the LFM pulse output by the dual parallel Mach-Zehnder modulator 104 and the first erbium-doped fiber amplifier 107 is out(t) The expression is:

[0072]

[0073] In the formula, E 0 represents the amplitude of the light field, f 0 represents the emission frequency of the narrow linewidth laser 101, f s represents the initial optical frequency of the LFM pulse, k represents the chirp rate of the LFM optical pulse, and k satisfies the relationship k 2 represents the LFM pulse cutoff frequency, k 1 represents the starting frequency of the LFM pulse, and T represents the duration of the LFM pulse, that is, the pulse width.

[0074] The LFM pulse is injected into the common single-mode sensing fiber 115 to generate Rayleigh backscattered light, which is output by the optical circulator 111 and amplified by the second erbium-doped fiber amplifier 112, and then coupled with the local reference light output by the polarization controller 122 in the interference coupler 123. The optical signal output by the interference coupler 123 is detected by the balanced photodetector 131. Any scattering point in the common single-mode sensing fiber 115 will generate a scattering trace. For the trace number τ, the corresponding position z i The light intensity I(τ,z i ) is expressed as:

[0075]

[0076] Then we have:

[0077]

[0078] In the formula, E LO represents the amplitude of the local reference light, T represents the duration of the LFM pulse, c represents the speed of light, n represents the refractive index of light inside the optical fiber, and f s represents the initial optical frequency of the LFM pulse, r(τ,z) and θ(τ,z) represent the reflectivity and phase of the Rayleigh backscattered light at position z, respectively. R(τ,z) represents the integral of the Rayleigh backscattered light reflectivity, Indicates position z i The phase of .

[0079] The instantaneous phase can be extracted through the Hilbert transform and phase unwrapping process, and the obtained fiber position z i The instantaneous phase The expression is:

[0080]

[0081] Where H represents the Hilbert transform operation, unwrap represents the phase unwrapping operation, and I(τ,z i ) represents the position z of the sensing fiber i The corresponding light intensity.

[0082] Fiber position z i The phase Φ(τ,z i ) can be expressed as:

[0083]

[0084] In the formula, the phase change is the instantaneous phase difference between any trace and the first trace Since the maximum phase change between instantaneous phases should be less than 2π, the mod function is introduced to extract the modulus of phase change to reduce the influence of false jumps in phase extraction due to signal strength difference.

[0085] Due to the phase compensation effect of the LFM pulse, a reverse phase bulge appears inside the differential phase curve covered by the vibration event, and the phase change of the reverse bulge Phase difference before and after the vibration event The changes are consistent, that is like Figure 3 The reverse convex phase part caused by the phase compensation effect is calibrated, the differential phase curve of the vibration event coverage position is extracted and the coordinate system is established. Taking a certain moment a as an example, the calibration area is as follows Figure 3 As shown, the two symmetrical regions are respectively recorded as region A and region B. Region AB is symmetrical up and down and has the same area. Both regions can be used as centroid solution regions for centroid solution.

[0086] Specifically, the method for determining area B is:

[0087] The method for determining the centroid solution area is:

[0088] Establish a new coordinate system with the vibration starting point as the coordinate origin O;

[0089] Determine the data point p when the differential phase curve bulges and then drops in the opposite direction to the phase value equal to that corresponding to the origin of the coordinate system;

[0090] The origin of the coordinate system and the data point p are used as the starting point and the end point of the centroid solution area, respectively, to determine the range of area B. Specifically, area B is the area between the differential phase curve and the horizontal axis of the coordinate system (i.e., the straight line ) Figure 3 As shown, the phase of the differential phase curve bulges to the rear φ 3 Then it started to descend in the reverse direction.

[0091] Specifically, the threshold centroid algorithm can be used to solve the centroid position of area B. The centroid calculation formula of area B is:

[0092]

[0093] in, Represents the two-dimensional coordinates corresponding to the centroid of region B, m n represents the mass of the nth data point, x n Represents the horizontal coordinate corresponding to the nth data point, represents the differential phase corresponding to the nth data point. Specifically, The value range is arrive n represents the corresponding data point in the centroid solution area B, and the value range of n is from 0 to p. Among them, σ represents the surface density of the area plane B, which can be eliminated because the plane density is evenly distributed. S n Represents the area of ​​the nth grid obtained by grid discretization of region B. This area is used as a weight. When calculating the centroid, a higher weight is given to the grid with a larger area. B The area of ​​region B, that is, the area of ​​the region enclosed by the differential phase curve and the horizontal axis of the coordinate, is given by the Gaussian area formula:

[0094]

[0095] The position of the centroid of region B is used as the representative value of the phase in the phase curve at that moment. After the centroid algorithm is used to calculate each moment along the optical fiber link, the motion trajectory curve of the centroid coordinates along the detection cycle axis can be obtained. In step 4, the step of removing the coordinate mutation points in the motion trajectory curve by adaptive threshold is also included, thereby eliminating the influence of data jump on the result. The obtained centroid motion trajectory curve corresponds to the vibration signal applied to the piezoelectric ceramic tube 116 by the driving signal generator 113 after the power amplifier 114, which can restore the intact vibration waveform and complete the phase demodulation.

[0096] Specifically, the method for determining area A is:

[0097] Establish a new coordinate system with the vibration starting point as the coordinate origin O;

[0098] Determine the data point r corresponding to the highest phase point after the differential phase curve bulges and before it drops in the opposite direction;

[0099] Determine that the differential phase curve drops in the opposite direction and then rises back to the phase The corresponding data point q at in, represents the phase difference before and after the vibration event, Indicates the phase value corresponding to the origin of the new coordinate;

[0100] The data point r and data point q are used as the starting point and end point of the centroid solution area, respectively, to determine the range of area A. Specifically, area A is the area where the differential phase curve and the ordinate are equal to The area enclosed by the horizontal lines.

[0101] In addition, area A can also be processed, and the centroid position of area A can be solved using the threshold centroid algorithm. The expression is:

[0102]

[0103] in, Represents the two-dimensional coordinates corresponding to the centroid of region B, m l represents the mass of the lth data point, x l represents the horizontal coordinate corresponding to the lth data point, represents the differential phase corresponding to the lth data point. Specifically, The value range is arrive l represents the corresponding data point in the centroid solution area A, and the value range of l is from r to q. Among them, σ represents the surface density of the area plane A, which can be negated because the plane density is evenly distributed. S l Represents the area of ​​the lth grid obtained by grid discretization of region A. This area is used as a weight. When calculating the centroid, a higher weight is given to the grid with a larger area. A Represents the total area of ​​region A, that is, the differential phase curve and the ordinate are equal to The area enclosed by the horizontal line of the area. The phase difference between the center of mass coordinates calculated from area A and the center of mass coordinates calculated from area B is The central axis Symmetrical, the centroid coordinates reflect the change in relative phase, and the waveforms calculated and restored in areas A and B are vibration signals that are opposite and symmetrical to each other.

[0104] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein by equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A DAS system based on linear frequency modulation pulse, characterized in that: The invention comprises an LFM optical pulse modulation shaping module (10), wherein the LFM optical pulse modulation shaping module (10) comprises a narrow line width laser (101), a polarization maintaining coupler (102), an LFM signal generator (103), and a dual parallel Mach-Zehnder modulator (104); laser light emitted by the narrow line width laser (101) is divided into two parts by the polarization maintaining coupler (102); a part of the light separated by the polarization maintaining coupler (102) is used as signal light and modulated into an LFM optical pulse by the dual parallel Mach-Zehnder modulator (104) driven by the LFM signal generator (103); the LFM optical pulse is passed through an optical filter (108 ) is filtered and output to a sensing optical fiber (115) through an optical circulator (111); Rayleigh backscattered light generated in the sensing optical fiber (115) is outputted by the optical circulator (111) and incident on an input end of an interference coupler (123); another part of light separated by a polarization-maintaining coupler (102) is incident on another input end of the interference coupler (123) as reference light; the Rayleigh backscattered light and the reference light interfere with each other at the interference coupler (123); an interference signal outputted by the interference coupler (123) is detected by a balanced photodetector (131); and the detection signal is phase-demodulated by a computer (133) to obtain a vibration signal.

2. A linear frequency modulation pulse based DAS system according to claim 1, characterized in that: The LFM signal generator (103) is used to generate a digital single pulse signal with a pulse width of 100 ns and a repetition period of 110 μs, and the narrow linewidth laser (101) is used to output laser light with a central wavelength of 1550.12 nm.

3. A linear frequency modulation pulse based DAS system according to claim 1, characterized in that: The invention also comprises an optical splitter (106) and a modulator bias controller (105). The optical splitter (106) is arranged at the output end of the dual parallel Mach-Zehnder modulator (104) and is used to split the output laser of the dual parallel Mach-Zehnder modulator (104) into a part and send it to the modulator bias controller (105). The modulator bias controller (105) is used to bias the intensity modulator to the zero point and the phase shifter to the orthogonal point according to the output laser of the dual parallel Mach-Zehnder modulator (104).

4. A linear frequency modulation pulse based DAS system according to claim 1, characterized in that: It also includes an adjustable optical attenuator (121) and a polarization controller (122). The reference light output by the polarization-maintaining coupler (102) has its light intensity adjusted by the adjustable optical attenuator (121). The polarization controller (122) adjusts the polarization and then enters the polarization controller (122). After passing through the polarization controller (122), the reference light enters the other input end of the interference coupler (123).

5. A linear frequency modulation pulse based DAS system according to claim 1, characterized in that: The invention also comprises an optical filter (108), a first erbium-doped fiber amplifier (107) and a second erbium-doped fiber amplifier (112), wherein the first erbium-doped fiber amplifier (107) is used to amplify the LFM optical pulse at the output end of the dual-parallel Mach-Zehnder modulator (104), and the optical filter (108) is used to filter out the spontaneous radiation noise of the first erbium-doped fiber amplifier (107); and the second erbium-doped fiber amplifier (112) is used to amplify the Rayleigh backscattered light output by the optical circulator (111).

6. The phase demodulation method of a DAS system based on linear frequency modulation pulse according to claim 1, characterized in that: The following steps are involved: Step 1: Obtain the detected light intensity sensing signal; Step 2: Perform Hilbert transform and phase unwinding processing on the light intensity sensing signal to obtain the instantaneous phase at each position of the optical fiber, calculate the differential phase based on the instantaneous phase, and then obtain the differential phase curve; Step 3: Extract the differential phase curve of the vibration event coverage position, and determine the centroid solution area based on the reverse phase bulge on the differential phase curve; Step 4: Calculate the centroid coordinates of the centroid solution area according to the centroid method; Step 5: Repeat steps 1 to 4 to obtain the center of mass motion trajectory at each moment, use the center of mass position as the representative value of the phase in the phase curve at that moment, and demodulate the obtained vibration waveform.

7. The phase demodulation method of a DAS system based on linear frequency modulation pulse according to claim 6, characterized in that: The method for determining the centroid solution area is: Establish a new coordinate system with the vibration starting point as the coordinate origin O; Determine the data point p when the differential phase curve bulges and then drops in the opposite direction to the phase value equal to that corresponding to the origin of the coordinate system; The origin of the coordinate system and the data point p are used as the starting point and the end point of the centroid solution area respectively, and the area enclosed by the differential phase curve and the horizontal axis of the coordinate system between the starting point and the end point is used as the range of the centroid solution area; The calculation formula of the centroid coordinates is: in, Indicates the two-dimensional coordinates of the centroid of the centroid solution area, S n S represents the area of ​​the nth grid obtained by discretizing the centroid solution area. B represents the total area of ​​the centroid solution region, x n Represents the horizontal coordinate corresponding to the nth data point, Represents the differential phase corresponding to the nth data point.

8. The phase demodulation method of a DAS system based on linear frequency modulation pulse according to claim 6, characterized in that: The method for determining the centroid solution area is: Establish a new coordinate system with the vibration starting point as the coordinate origin O; Determine the data point r corresponding to the highest phase point after the differential phase curve bulges and before it drops in the opposite direction; Determine that the differential phase curve drops in the opposite direction and then rises back to the phase The corresponding data point q at in, It represents the phase difference before and after the vibration event; Take data point r and data point q as the starting point and end point of the centroid solution area respectively, and compare the differential phase curve between the starting point and the end point with the ordinate equal to The range of the centroid solution area is enclosed by the horizontal line of The calculation formula of the centroid coordinates is: in, Indicates the two-dimensional coordinates of the centroid of the centroid solution area, S l S represents the area of ​​the lth grid obtained by discretizing the centroid solution area. A represents the total area of ​​the centroid solution region, x l represents the horizontal coordinate corresponding to the lth data point, Represents the differential phase corresponding to the lth data point.

9. The phase demodulation method of a DAS system based on linear frequency modulation pulse according to claim 6, characterized in that: In step 2, the calculation formula of the instantaneous phase is: in, Indicates the fiber position z i The instantaneous phase at , H represents the Hilbert transform operation, unwrap represents the phase unwrapping operation, I(τ,z i ) represents the position z i The corresponding light intensity at In step 2, the calculation formula of the differential phase curve is: Among them, mod represents the mod function, represents the fiber position z in the first trace i The instantaneous phase at .

10. The phase demodulation method of a DAS system based on linear frequency modulation pulse according to claim 6, characterized in that: The step 5 also includes a step of removing coordinate mutation points in the motion trajectory curve by using an adaptive threshold.

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