A chirp-based DAS system and a phase demodulation method thereof

Abstract

The application belongs to the technical field of optical fiber distributed sensing, and particularly relates to a DAS system based on a linear frequency modulation pulse and a phase demodulation method thereof. Laser emitted by a narrow linewidth laser in the device is divided into two parts through a polarization maintaining coupler. A part of light divided through the polarization maintaining coupler is modulated into LFM optical pulses through a double-parallel Mach-Zehnder modulator driven by an LFM signal generator, and the LFM optical pulses are output to a sensing optical fiber through an optical circulator after being filtered through an optical filter. Rayleigh backscattering light generated in the sensing optical fiber is output through the optical circulator and then is incident on an interference coupler. The other part of light divided through the polarization maintaining coupler is incident on the interference coupler as reference light. An interference signal output by the interference coupler is detected by a balanced photodetector and is subjected to phase demodulation by a computer to obtain a vibration signal. The application realizes rapid positioning and high-fidelity demodulation of the vibration signal.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of optical fiber distributed sensing, and particularly relates to a DAS system based on a 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 fast positioning and high-fidelity demodulation of a vibration signal. BACKGROUND

[0002] In recent years, optical fiber distributed acoustic sensing (DAS) technology has significant advantages and application prospects in the fields of seismic wave monitoring, perimeter security, operation and maintenance of large infrastructure, and health monitoring of urban underground space, due to its wide sensing range, high sensing and measuring sensitivity, high time and space resolution accuracy, and less environmental influence. Among them, the phase-sensitive optical time domain reflectometer (Φ-OTDR) is the most widely used DAS system. Based on the theory that the phase of backscattered Rayleigh light after injecting probe pulse light into an optical fiber is linearly related to the strain of the optical fiber, the phase information of the backscattered Rayleigh signal is demodulated to recover the vibration signal. Linear frequency modulation (LFM) has the advantages of continuous waveforms and pulse waveforms, and is also applied in the DAS system. Introducing LFM into the DAS system can break the limitation of pulse width and spatial resolution.

[0003] The phase demodulation methods based on chirp DAS system mainly include two types of interferometer structure and heterodyne coherent structure. The passive demodulation method based on interferometer structure mainly includes 3*3 coupler demodulation method and phase generated carrier demodulation method. Although the demodulation method has a simple structure, the system cost and measurement error are increased, and the environmental stability is deteriorated due to the use of different numbers of photoelectric detectors. The demodulation method based on heterodyne coherent structure can be roughly divided into three categories. The first category is IQ demodulation method, which divides the collected signals into I and Q polarization states through a 90-degree optical mixer, and realizes phase demodulation through arctangent method (Optics express, 2016, 24 (2): 853-858.). Compared with the interferometer structure, the environmental adaptability and detection sensitivity are improved, but the introduction of 90-degree mixer will increase the system cost and cause 90-degree induced inaccuracy. The second category is differential phase demodulation method, which can obtain the phase change along the optical fiber link by point difference operation of the phase before and after the vibration position (IEEE Photonics Journal, 2018, 10 (1): 1-9.). Compared with the IQ demodulation method, the external noise can be eliminated through the differential step, and the vibration waveform recovery effect is better after arithmetic average, but the data volume and calculation amount are very large, the time consumption is longer, and it is difficult to realize real-time demodulation. The third category is 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 is broken through to sub-meter level, the vibration waveform recovery effect after demodulation is poor.

[0004] Due to the real-time requirements of phase information acquisition and processing, it is necessary to design a new type of phase demodulation device and method to break through the existing technical problems, which can realize fast and accurate positioning of vibration signals and reflect external disturbance information, and realize fast and high-fidelity demodulation. SUMMARY

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

[0006] In order to solve the above technical problems, the technical scheme adopted by the present application is: a DAS system based on linear frequency modulation pulse, comprising an LFM optical pulse modulation shaping module, the LFM optical pulse modulation shaping module comprises a narrow linewidth laser, a polarization maintaining coupler, an LFM signal generator and a double-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 divided by the polarization maintaining coupler is modulated into an LFM optical pulse by the double-parallel Mach-Zehnder modulator driven by the LFM signal generator, and the LFM optical pulse is filtered by an optical filter and then output to a sensing optical fiber through an optical circulator; the Rayleigh backscattering light generated in the sensing optical fiber is output through the optical circulator and then incident to one input end of an interference coupler; the other part of the light divided by the polarization maintaining coupler is incident to the other input end of the interference coupler as reference light; the Rayleigh backscattering light and the reference light interfere 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 laser with a center wavelength of nm.

[0008] The DAS system based on linear frequency modulation pulse further comprises a splitter and a modulator bias controller, the splitter is arranged at the output end of the double-parallel Mach-Zehnder modulator and used to divide the output laser of the double-parallel Mach-Zehnder modulator into a part sent to the modulator bias controller, and the modulator bias controller is used to bias the intensity modulator to zero and the phase shifter to a quadrature point according to the output laser of the double-parallel Mach-Zehnder modulator.

[0009] The DAS system based on linear frequency modulation pulse further comprises an adjustable optical attenuator and a polarization controller, the reference light output by the polarization maintaining coupler is adjusted in light intensity by the adjustable optical attenuator, and the light adjusted in polarization is incident to the other input end of the interference coupler through the polarization controller.

[0010] The DAS system based on linear frequency modulation pulse further comprises an optical filter, a first doped fiber amplifier and a second doped fiber amplifier, the first doped fiber amplifier is used to amplify the LFM optical pulse at the output end of the double-parallel Mach-Zehnder modulator, the optical filter is used to filter out the spontaneous emission noise of the first doped fiber amplifier, and the second doped fiber amplifier is used to amplify the Rayleigh backscattering light output through 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: obtaining a light intensity sensing signal obtained by detection;

[0013] Step 2: performing Hilbert transformation and phase unwrapping processing on the light intensity sensing signal to obtain an instantaneous phase at each position of the optical fiber, calculating a differential phase according to the instantaneous phase, and further obtaining a differential phase curve;

[0014] Step 3: extracting the differential phase curve of the vibration event covering position, and determining a centroid solving region according to a reverse phase protrusion on the differential phase curve;

[0015] Step 4: calculating a centroid coordinate of the centroid solving region according to a centroid method;

[0016] Step 5: repeating steps 1-4 to obtain a centroid motion trajectory at each time, taking the centroid position as a representative value of the phase in the phase curve at the time, and demodulating a vibration waveform.

[0017] The determination method of the centroid solving region is:

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

[0019] A data point p is determined when the differential phase curve protrusion reverses and falls to a phase value equal to that corresponding to the coordinate origin;

[0020] The differential phase curve between the origin and the end point of the centroid solving region and the coordinate horizontal axis enclose an area as the range of the centroid solving region;

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

[0022]

[0023] wherein, represents a two-dimensional coordinate corresponding to the centroid of the centroid solving region, S n represents the area of the nth grid obtained by discretizing the centroid solving region with a grid, S B represents the total area of the centroid solving 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 determination method of the centroid solving region is:

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

[0026] A data point r corresponding to the highest point of the phase before the differential phase curve protrusion reverses and falls is determined;

[0027] determining the differential phase curve after the reverse drop rises again to the corresponding data point q of the phase wherein, wherein, represents the phase difference before and after the vibration event;

[0028] Taking the data point r and the data point q as the starting point and the ending point of the centroid solving area respectively, the differential phase curve between the starting point and the ending point and the horizontal line with the longitudinal coordinate equal to surrounded by the range of the centroid solving area;

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

[0030]

[0031] wherein, represents the two-dimensional coordinates corresponding to the centroid of the centroid solving area, S l represents the area of the lth grid obtained by discretizing the centroid solving area with a grid, S A represents the total area of the centroid solving area, x l represents the horizontal coordinate corresponding to the lth data point, represents the differential phase corresponding to the lth data point.

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

[0033]

[0034] wherein, represents the instantaneous phase of the optical fiber at the position z i , H represents the Hilbert transform operation, unwrap represents the phase unwrapping operation, and I(τ,z i ) represents the light intensity corresponding to the position z i .

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

[0036]

[0037] wherein, mod represents the mod function, represents the instantaneous phase of the optical fiber at the position z i in the first trace.

[0038] In the step 5, the method further comprises the step of removing the coordinate mutation point in the motion trajectory curve by an adaptive threshold.

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

[0040] (1) The application calculates the differential phase to obtain its change curve, and through effectively eliminating the phase unwrapping problem caused by the arctangent method, a phase compensation region is obtained, and the region can be used to realize the fast positioning of the vibration signal.

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

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

[0043] In summary, the application provides a DAS system based on a linear frequency modulation pulse and a phase demodulation method thereof, the influence of external noise on the system is eliminated by processing the phase compensation region, so that the fast positioning and high-fidelity demodulation of the vibration signal are realized. BRIEF DESCRIPTION OF DRAWINGS

[0044] Figure 1 A DAS system structure schematic diagram based on a linear frequency modulation pulse is provided for the application example;

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

[0046] 11-optical fiber sensor module, 111-optical circulator, 112-second 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 phase demodulation method flowchart of the linear frequency modulation pulse DAS system used for the application example is provided;

[0050] Figure 3A vibration event overlaying a phase compensation region demarcation diagram used for the embodiment of the present application. DETAILED DESCRIPTION

[0051] In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0052] As shown in Figure 1 The embodiment one of the present application provides a DAS system based on LFM optical pulse, which comprises an LFM optical pulse modulation shaping module 10, the LFM optical pulse modulation shaping module 10 comprises a narrow linewidth laser 101, a polarization maintaining coupler 102, an LFM signal generator 103, a double-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, a part of the light divided by the polarization maintaining coupler 102 is modulated into LFM optical pulse by the double-parallel Mach-Zehnder modulator 104 driven by the LFM signal generator 103 as signal light, the LFM optical pulse is output to a sensing optical fiber 115 through an optical circulator 111, the Rayleigh backscattering light generated in the sensing optical fiber 115 is output after the optical circulator 111 and then is incident to one input end of a coupler 123, the optical circulator 111 plays a role of directional transmission; the other part of the light divided by the polarization maintaining coupler 102 is incident to the other input end of the interference coupler 123 as reference light; the Rayleigh backscattering light and the reference light interfere at the interference coupler 123, the interference signal is detected by a balanced photodetector 131, and the detection signal is phase demodulated by a computer 133 to obtain a vibration signal.

[0053] Specifically, in the embodiment, 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 with a center wavelength of 1550.12 nm. The LFM signal generator 103 can be realized by an arbitrary waveform generator.

[0054] Further, the LFM-based DAS system of the embodiment further 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 sent to the modulator bias controller 105, and the modulator bias controller 105 is used to bias the intensity modulator to zero and the phase shifter to quadrature according to the output laser of the dual-parallel Mach-Zehnder modulator 104.

[0055] Specifically, the splitting ratio of the polarization maintaining coupler 102 is 90:10, in which the 90% part is used as the signal light, and the 10% part is used as the reference light, and the optical splitter 106 is a 99:1 optical coupler, in which the 1% part 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 doped fiber amplifier 107 is a pulse doped fiber amplifier, and the second doped fiber amplifier 112 is a small-signal doped fiber amplifier.

[0057] Further, the LFM-based DAS system of the embodiment further comprises an adjustable optical attenuator 121 and a polarization controller 122, the reference light output by the polarization maintaining coupler 102 is adjusted in light intensity by the adjustable optical attenuator 121, and is incident on the polarization controller 122 after polarization adjustment, and is incident on the other input end of the interference coupler 123 through the polarization controller 122.

[0058] Further, the LFM-based DAS system of the embodiment further comprises an optical filter 108, a first doped fiber amplifier 107 and a second doped fiber amplifier 112, the first 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, the optical filter 108 is arranged at the output end of the first doped fiber amplifier 107, and is used to filter out the spontaneous emission noise of the first doped fiber amplifier 107. The second doped fiber amplifier 112 is arranged between the optical circulator 111 and the interference coupler 123, and is used to amplify the Rayleigh backscattering 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 and 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 fiber 115 is a single-mode fiber. In this embodiment, in order to test the demodulation effect of the system, a piezoelectric ceramic tube 116 is set on the sensing fiber 115. The signal output by the drive signal generator 113 is used to drive the piezoelectric ceramic tube 116 to vibrate the sensing fiber 115 after passing through the power amplifier 114, thereby simulating environmental vibration.

[0062] Example 2

[0063] like Figure 2 As shown, Embodiment 2 of the present invention provides a phase demodulation method for a DAS system based on linear frequency modulated pulses, comprising the following steps:

[0064] Step 1: Acquire 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 transformation and phase unwrapping processing on the light intensity sensing signal to obtain the instantaneous phase at various locations of the optical fiber. Calculate the differential phase based on the instantaneous phase to obtain the differential phase curve.

[0067] Step 3: Extract the differential phase curve of the location covered by the vibration event, and determine the centroid solution region based on the reverse phase bulge on the differential phase curve;

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

[0069] Step 5: Repeat steps 1 to 4 to obtain the trajectory of the center of mass at each moment. Use the position of the center of mass 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 in this embodiment is described below.

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

[0072]

[0073] In the formula, E0 represents the amplitude of the optical field, f0 represents the emission optical 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 k2 represents the cutoff frequency of the LFM pulse, k1 represents the starting frequency of the LFM pulse, and T represents the duration of the LFM pulse, i.e. the pulse width.

[0074] The LFM pulse is injected into the ordinary single-mode sensing optical fiber 115 to generate Rayleigh backscattered light, which is output through the optical circulator 111 and amplified by the second 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 ordinary single-mode sensing optical fiber 115 will generate a scattering trace, and for a trace number τ, the corresponding position z i The optical intensity I(τ, z i ) detected by the balanced photodetector 131 is expressed as:

[0075]

[0076] Then:

[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 the light in the optical fiber, 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 reflectivity of the Rayleigh backscattered light, represents the phase at position z i .

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

[0080]

[0081] Where H represents the Hilbert transform operation, unwrap represents the phase expansion operation, and I(τ,z) i The position z of the sensing fiber is represented by ) i The corresponding light intensity at that location.

[0082] Fiber position z i Phase Φ(τ,z) at the point i This can be represented as:

[0083]

[0084] In the formula, the phase change is the difference between the instantaneous phases of any trace and the first trace. We obtain the result. Since the maximum phase change between instantaneous phases should be less than 2π, a mod function is introduced to extract the modulus of the phase change to mitigate the influence of spurious transitions in phase extraction caused by signal strength differences.

[0085] Because LFM pulses have a phase compensation effect, 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 Consistent changes, that is like Figure 3 As shown. The reverse bulge phase portion caused by the phase compensation effect is calibrated. The differential phase curve of the vibration event coverage location is extracted and a coordinate system is established. Taking a certain time 'a' as an example, the calibration area is as follows: Figure 3 As shown, the two symmetrical regions are denoted as region A and region B, respectively. Regions A and B are vertically symmetrical and have equal areas. Both regions can be used as the centroid solution regions for centroid calculation.

[0086] Specifically, the method for determining region B is as follows:

[0087] The method for determining the centroid solution region is as follows:

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

[0089] Determine the data point p where the differential phase curve rises and then falls back to the point where the phase value corresponding to the origin is equal;

[0090] Using the origin and data point p as the starting and ending points of the centroid solution region, respectively, the range of region B is determined. Specifically, region B is the intersection of the differential phase curve and the horizontal axis (i.e., the straight line). The area enclosed by ( ). For example, Figure 3 As shown, the phase of the differential phase curve bulges to φ3 and then begins to decline in the opposite direction.

[0091] Specifically, the centroid position of region B can be determined using the threshold centroid algorithm. The formula for calculating the centroid of region B is as follows:

[0092]

[0093] wherein, represents the two-dimensional coordinate corresponding to the centroid of region B, m n represents the mass of the nth data point, x n represents the abscissa corresponding to the nth data point, represents the differential phase corresponding to the nth data point, specifically, the value range of is to n represents the data point corresponding to the centroid solving region B, the value range of n is from 0 to p. Wherein, σ represents the area density of region plane B, since the plane density is uniform, it can be ignored. S n represents the area of the nth grid obtained by grid discretization of region B, this area is used as a weight, and the grid with larger area is given higher weight in calculating the centroid. S B represents the area of region B, that is, the area of the region surrounded by the differential phase curve and the coordinate abscissa, which is given by the Gauss 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 fiber link, the motion trajectory curve of the centroid coordinates along the detection period axis can be obtained. In step 4, the step of removing the coordinate mutation point in the motion trajectory curve by an adaptive threshold is further included, so as to eliminate 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 through the power amplifier 114, which can restore the perfect vibration waveform and complete phase demodulation.

[0096] Specifically, the determination method of region A is:

[0097] A new coordinate system is established with the vibration starting point as the coordinate origin O;

[0098] The data point r corresponding to the highest phase point before the differential phase curve rises and then falls is determined;

[0099] The data point q corresponding to the phase point after the differential phase curve falls and then rises again is determined; wherein, wherein, wherein, represents the phase difference before and after the vibration event, represents the phase value corresponding to the new coordinate origin;

[0100] The range of the region A is determined by taking data point r and data point q as the starting point and the ending point of the centroid solving region respectively. Specifically, the region A is the area enclosed by the differential phase curve and the horizontal line with the ordinate equal to .

[0101] In addition, the region A can also be processed to solve the centroid position of the region A by using the threshold centroid algorithm, and the expression is:

[0102]

[0103] wherein, represents the two-dimensional coordinates corresponding to the centroid of the region B, m l represents the mass of the lth data point particle, 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 of to l represents the corresponding data point in the centroid solving region A, and the value range of l is from r to q. Wherein, σ represents the area density of the region plane A, and since the plane density is uniformly distributed, it can be ignored. S l represents the area of the lth grid obtained by grid discretization of the region A, and this area is used as a weight, giving a higher weight to the grid with a larger area when calculating the centroid. A represents the total area of the region A, that is, the area enclosed by the differential phase curve and the horizontal line with the ordinate equal to . The centroid coordinates calculated according to the region A and the centroid coordinates calculated according to the region B are symmetrical about the median axis of the phase difference , and the centroid coordinates reflect the change of the relative phase. The waveforms recovered by the calculation of the regions A and B are the vibration signals that are mutually and oppositely symmetrical.

[0104] Finally, it should be pointed out that: the above embodiments are only used to illustrate the technical solutions of the present application, but not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A DAS system based on linear frequency modulated pulses, characterized in that, The system includes an LFM optical pulse modulation and shaping module (10), which comprises a narrow-linewidth laser (101), a polarization-maintaining coupler (102), an LFM signal generator (103), and a dual-parallel Mach-Zehnder modulator (104). The laser emitted by the narrow-linewidth laser (101) is split into two parts by the polarization-maintaining coupler (102). One part of the split light is used as a 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 then filtered by an optical filter (108). After filtering, the light is output to the sensing fiber (115) through the optical circulator (111). The Rayleigh backscattered light generated in the sensing fiber (115) is output through the optical circulator (111) and then incident on one input end of the interference coupler (123). The other part of the light split off by the polarization maintaining coupler (102) is used as the reference light and incident on the other input end of the interference coupler (123). The Rayleigh backscattered light and the reference light interfere at the interference coupler (123). The interference signal output by the interference coupler (123) is detected by the balanced photodetector (131). The detected signal is phase demodulated by the computer (133) to obtain the vibration signal.

2. The DAS system based on linear frequency modulated pulses 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 100ns and a repetition period of 110μs, and the narrow linewidth laser (101) is used to output a laser with a center wavelength of 1550.12nm.

3. The DAS system based on linear frequency modulated pulses according to claim 1, characterized in that, It also includes a beam splitter (106) and a modulator bias controller (105). The beam splitter (106) is located at the output end of the dual parallel Mach-Zehnder modulator (104) and is used to split a portion of the output laser of the dual parallel Mach-Zehnder modulator (104) and send it to the modulator bias controller (105). The modulator bias controller (105) is used to bias the intensity modulator to zero and the phase shifter to the quadrature point according to the output laser of the dual parallel Mach-Zehnder modulator (104).

4. A DAS system based on linear frequency modulated pulses according to claim 1, characterized in that, It also includes an adjustable light attenuator (121) and a polarization controller (122). The reference light output from the polarization-maintaining coupler (102) is adjusted in intensity by the adjustable light attenuator (121), and after the polarization is adjusted by the polarization controller (122), it is incident on the other input end of the interference coupler (123).

5. A DAS system based on linear frequency modulated pulses according to claim 1, characterized in that, It also includes an optical filter (108), a first erbium-doped fiber amplifier (107), and a second erbium-doped fiber amplifier (112). The first erbium-doped fiber amplifier (107) is used to amplify the LFM optical pulse at the output of the dual parallel Mach-Zehnder modulator (104). The optical filter (108) is used to filter out the spontaneous emission noise of the first erbium-doped fiber amplifier (107). 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 for a DAS system based on linear frequency modulated pulses according to claim 1, characterized in that, Includes the following steps: Step 1: Acquire the detected light intensity sensing signal; Step 2: Perform Hilbert transformation and phase unwrapping processing on the light intensity sensing signal to obtain the instantaneous phase at various locations of the optical fiber. Calculate the differential phase based on the instantaneous phase to obtain the differential phase curve. Step 3: Extract the differential phase curve of the location covered by the vibration event, and determine the centroid solution region based on the reverse phase bulge on the differential phase curve; Step 4: Calculate the centroid coordinates of the solution region using the centroid method; Step 5: Repeat steps 1 to 4 to obtain the trajectory of the center of mass at each moment. Use the position of the center of mass 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 for a DAS system based on linear frequency modulated pulses according to claim 6, characterized in that, The method for determining the centroid solution region is as follows: Establish a new coordinate system with the vibration origin O as the starting point. Determine the data point p where the differential phase curve rises and then falls back to the point where the phase value corresponding to the origin is equal; The origin of the coordinate system and the data point p are taken as the start and end points of the centroid solution region, respectively. The area enclosed by the differential phase curve between the start and end points and the horizontal axis of the coordinate system is taken as the range of the centroid solution region. The formula for calculating the centroid coordinates is: in, S represents the two-dimensional coordinates corresponding to the centroid of the centroid solution domain. n S represents the area of ​​the nth grid obtained by discretizing the centroid solution domain using a grid. B x represents the total area of ​​the solution region of the centroid. n This represents the x-coordinate of the nth data point. This represents the differential phase corresponding to the nth data point.

8. The phase demodulation method for a DAS system based on linear frequency modulated pulses according to claim 6, characterized in that, The method for determining the centroid solution region is as follows: Establish a new coordinate system with the vibration origin O as the starting point. Determine the data point r corresponding to the highest point of the phase before the reverse descent after the bulge of the differential phase curve; The differential phase curve is determined to have reversed its downward trend and then rebounded to its phase. The corresponding data point q; where, in, This represents the phase difference before and after the vibration event; Using data points r and q as the start and end points of the centroid solution domain, respectively, the differential phase curve between the start and end points is plotted against the ordinate equal to... The area enclosed by the horizontal line serves as the solution region for the centroid; The formula for calculating the centroid coordinates is: in, S represents the two-dimensional coordinates corresponding to the centroid of the centroid solution domain. l S represents the area of ​​the l-th grid obtained by discretizing the centroid solution domain using a grid. A x represents the total area of ​​the solution region of the centroid. l This represents the x-coordinate of the l-th data point. This represents the differential phase corresponding to the l-th data point.

9. The phase demodulation method for a DAS system based on linear frequency modulated pulses according to claim 6, characterized in that, In step 2, the formula for calculating the instantaneous phase is: in, Indicates the fiber position z i The instantaneous phase at point H represents the Hilbert transform operation, unwrap represents the phase expansion operation, and I(τ,z) represents the phase expansion operation. i ) represents the position z i The corresponding light intensity at that location; In step 2, the formula for calculating the differential phase curve is: Here, mod represents the mod function. Indicates the fiber position z in the first trace i The instantaneous phase at that point.

10. A phase demodulation method for a DAS system based on a linear frequency modulated pulse according to claim 6, characterized in that, Step 5 also includes removing coordinate abrupt change points in the motion trajectory curve using an adaptive threshold.

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