Distributed optical fiber acoustic sensing apparatus and method based on chirp pulse sequences

Abstract

The application belongs to the technical field of optical fiber distributed sensing, and discloses a distributed optical fiber acoustic sensing device and method based on a linear frequency modulation pulse sequence, which comprises a narrow linewidth laser, double-parallel Mach-Zehnder modulators and an arbitrary waveform generator; the arbitrary waveform generator is connected with the double-parallel Mach-Zehnder modulators and used for generating a linear frequency modulation pulse sequence including multiple pulses with sequentially increasing or decreasing center frequencies to drive the double-parallel Mach-Zehnder modulators; laser emitted by the narrow linewidth laser is divided into two parts, one part of which is used as reference light, and the other part is used as signal light, which is modulated into a linear frequency modulation pulse sequence by the double-parallel Mach-Zehnder modulators and then output to a sensing optical fiber; Rayleigh backscattering light generated in the sensing optical fiber interferes with the reference light, and an interference signal is detected by a balanced photodetector and then collected by a collection card. The application can realize dynamic strain measurement with high resolution and large measurement range.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of optical fiber distributed sensing technology, and particularly relates to a distributed optical fiber acoustic sensing device and method based on a linear frequency pulse sequence. The device modulates a probe light pulse into a linear frequency pulse sequence with a scanning center frequency through phase modulation, demodulates frequency shift and time shift characteristics using a pulse recombination demodulation algorithm and a cyclic calibration correlation method, and can realize high-resolution and large-measurement-range dynamic strain demodulation. BACKGROUND

[0002] Distributed acoustic sensing (DAS) technology is an advanced sensing technology that uses backscattered Rayleigh scattering in an optical fiber to locate and recover changes in environmental physical quantities at any position on the optical fiber link. It is widely used in health monitoring of large infrastructure such as oil and gas pipelines, bridges and tunnels, and transportation tracks. However, the scattering medium in the optical fiber is easily affected by the environment, and the scattering signal of the DAS system inevitably experiences interference fading during coherent superposition, resulting in strong signal randomness, insufficient signal-to-noise ratio and sensitivity, and difficulty in achieving high-resolution and large-range measurement.

[0003] In order to eliminate the influence of interference fading on the signal, linear frequency modulation technology is introduced into the DAS system due to its advantages of continuous waveforms and pulse waveforms. By combining linear frequency pulse and matched filter algorithm, the problem of mutual restraint between sensing distance and spatial resolution can be solved while suppressing signal interference fading (Chinese invention patent, CN107990970 B). By combining linear frequency pulse technology and Rayleigh scattering pattern method, demodulation is performed through cross-correlation operation, which can achieve nε-level strain resolution while suppressing interference fading (Optics Express, 2016, 24(12): 13121-13133). However, the maximum value of single strain measurement of the linear frequency pulse scheme is limited by the modulation bandwidth of the linear frequency pulse, which requires too high device index. In response to this, a frequency scanning scheme is proposed. Currently, using frequency scanning coherent optical time domain reflectometry can achieve a linear strain response as low as 47.5 pε (Optics Express, 2018, 26(8): 10573-10588.). However, in order to achieve high-resolution strain measurement, the frequency scanning step must be reduced, which will result in an increase in measurement period and a decrease in the frequency response range of dynamic strain. However, increasing the frequency scanning step will further limit the strain resolution.

[0004] When the dynamic strain changes slightly, the correlation results are differentially accumulated by long-time continuous measurement, which can effectively increase the strain measurement range of the system, but the limitation of the maximum single measurement of strain still exists. For the traditional linear frequency pulse DAS system, the maximum single measurement of strain and the strain resolution are mutually restricted. Increasing the modulation bandwidth of the linear frequency pulse will lead to the decrease of the strain sensitivity, and this limitation makes the performance of the system hardware unable to be fully utilized. Therefore, solving the contradiction between the maximum single measurement of strain and the strain resolution is the key to improving the performance of the linear frequency pulse-based DAS system. SUMMARY

[0005] In view of the contradiction between the maximum single measurement of strain and the strain resolution in the existing linear frequency pulse-based DAS system, the application provides a distributed optical fiber acoustic sensing device and method based on a linear frequency pulse sequence, which can maintain high strain sensitivity and improve the measurement capability for large strain without increasing the hardware configuration of the system.

[0006] In order to solve the above technical problems, the technical scheme adopted by the application is as follows: a distributed optical fiber acoustic sensing device based on a linear frequency pulse sequence, comprising: a narrow linewidth laser, a double-parallel Mach-Zehnder modulator, a sensing optical fiber, an arbitrary waveform generator, a balanced photodetector and an acquisition card; the arbitrary waveform generator is connected with the double-parallel Mach-Zehnder modulator and is used to generate a linear frequency pulse sequence to drive the double-parallel Mach-Zehnder modulator; the linear frequency pulse sequence output by the arbitrary waveform generator comprises a plurality of pulses with sequentially increasing or decreasing center frequencies;

[0007] The laser emitted by the narrow linewidth laser is divided into two parts, one part of the light is used as reference light, and the other part is used as signal light and is modulated into a linear frequency pulse sequence by the double-parallel Mach-Zehnder modulator driven by the arbitrary waveform generator, the linear frequency pulse sequence is output to the sensing optical fiber, the Rayleigh backscattering light generated in the sensing optical fiber is sent to the × polarization maintaining coupler together with the reference light and interferes, the interference signal is detected by the balanced photodetector, and the acquisition card is used to collect the detection signal output by the balanced photodetector.

[0008] The distributed optical fiber acoustic sensing device based on the linear frequency pulse sequence further comprises a 1x2 polarization maintaining coupler, a first doped fiber amplifier, an optical ring, and a second doped fiber amplifier.

[0009] The laser emitted by the narrow linewidth laser is divided into two parts by a 1×2 polarization-maintaining coupler, one part of which is used as signal light and the other part is used as reference light; the signal light is modulated into a linear frequency-modulated pulse sequence by a dual-parallel Mach-Zehnder modulator driven by an arbitrary waveform generator, the linear frequency-modulated pulse sequence is amplified by a first erbium-doped fiber amplifier and output to the sensing fiber through an optical circulator, the Rayleigh backscattered light generated in the sensing fiber is output through the optical circulator, the output Rayleigh backscattered light is amplified by a second erbium-doped fiber amplifier, and then sent to a 2×1 polarization-maintaining coupler together with the reference light to generate interference.

[0010] The narrow linewidth laser is used to output laser light with a central wavelength of 1550 nm.

[0011] The linear frequency modulation pulse sequence output by the arbitrary waveform generator includes 20 pulses with a center frequency interval of 10 MHz and a modulation bandwidth of 60 MHz.

[0012] The arbitrary waveform generator is also connected to the acquisition card and is used to generate a trigger signal to drive the acquisition card to perform data acquisition when each group of linear frequency modulation pulse sequences is transmitted.

[0013] The first erbium-doped fiber amplifier is a pulsed erbium-doped fiber amplifier, and the second erbium-doped fiber amplifier is a small-signal erbium-doped fiber amplifier.

[0014] In addition, the present invention also provides a distributed optical fiber acoustic sensing method based on a linear frequency modulated pulse sequence, which is implemented based on the distributed optical fiber acoustic sensing device based on a linear frequency modulated pulse sequence, and includes the following steps:

[0015] Step 1: Collect data through the acquisition card and extract the pulse sequence frequency shift matrix and the linear frequency modulation pulse time shift matrix;

[0016] Step 2: Reorganize the pulse sequence frequency shift matrix according to the pulse injection order through the pulse reorganization algorithm to obtain the pulse sequence frequency spectrum at any optical fiber position; divide the pulse sequence frequency spectrum into multiple segments according to time, and extract the pulse sequence frequency spectrum in each time segment. curves, performing cross-correlation operation on the last curve in each time period and the reference curve to calculate the relative frequency shift, thereby obtaining the relative frequency shift amount in each time period; calculating the frequency shift amount of each time period based on the relative frequency shift amount, and calculating the equivalent strain value based on the frequency shift amount to obtain the first time-strain curve of each optical fiber position; and calculating the average of the strain values ​​at different times as the first average value based on the obtained first time-strain curve;

[0017] Step 3: Extract the scattered signal corresponding to a single linear frequency modulation pulse in the linear frequency modulation pulse time-shift matrix, and determine whether the scattered signal at each position is distorted according to the distortion judgment function. If there is no distortion, perform local cross-correlation calculation on the sliding window to obtain the time shift of the signal envelope, and convert it into a strain value to obtain the time-strain curve of each optical fiber position. If distortion occurs, interpolate the first average value obtained in step 2 to obtain the time-strain curve of the optical fiber position; calculate the average of the strain values ​​of each optical fiber position at different times as the second average value, subtract the second average value from the obtained time strain curve, and obtain the second time-strain curve of each optical fiber position; repeatedly extract the scattered signal corresponding to other single linear frequency modulation pulses, and calculate the second time-strain curve corresponding to each position, and average the second time strain curves obtained by demodulating different linear frequency modulation pulses to obtain the third time-strain curve;

[0018] Step 4: Add the first average value to the third time-strain curve obtained in step 3 to obtain a final strain demodulation result.

[0019] In step 2, a cross-correlation operation is performed between the last curve and the first curve in each time period to calculate the relative frequency shift.

[0020] In step 3, the distortion judgment function is:

[0021] ;

[0022] in, Indicates the z position The distortion parameter at the moment, k is used to specify the length of time involved in the calculation, which can be selected according to the actual amount of calculation. express Time Fiber arrive The light intensity curve of the position, express Time Fiber arrive The light intensity curve of the position, Set to spatial resolution;

[0023] When the distortion parameter R>0.5, distortion is determined.

[0024] In step 2, the reorganization method is as follows: the Rayleigh backscattered light intensity curve obtained at any optical fiber position z Extraction is performed along the frequency offset axis of the pulse sequence, and the data of different frequency scanning periods are segmented and recombined to obtain the pulse sequence frequency spectrum.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] (1) The present application is based on the phase compensation effect, after the strain is applied on the optical fiber, the local time shift characteristics are generated on the Rayleigh backscattering time-space distribution map of the linear frequency modulation pulse, and high-resolution strain measurement can be realized by the cross-correlation operation on the local time shift characteristics, which overcomes the interference fading problem of the traditional phase-sensitive optical time domain reflection technology.

[0027] (2) The present application proposes a dynamic strain measurement method based on linear frequency modulation pulse sequence, which combines the advantages of frequency scanning and linear frequency modulation pulse, and fully utilizes the flexibility of digital modulation, so that the sensing measurement of large dynamic strain range and high strain resolution can be realized at the same time.

[0028] (3) The present application proposes a dynamic strain demodulation method based on linear frequency modulation pulse sequence, which can effectively screen the linear frequency modulation pulse distortion value and use the frequency scanning matrix calculation result for numerical prediction, and the strain measurement range of the system is expanded through the cyclic calibration correlation method, so that the dynamic strain measurement of large range and high resolution is finally realized.

[0029] In summary, the present application proposes a distributed optical fiber dynamic strain sensing method based on linear frequency modulation pulse sequence, the probe light pulse is modulated into a linear frequency modulation pulse sequence with center frequency scanning through phase modulation, the frequency shift characteristics generated on the spectrum map of the scanning pulse sequence and the time shift characteristics generated on the RBS time-space distribution map are demodulated by using the pulse recombination demodulation algorithm and the cyclic calibration correlation method, the dynamic strain demodulation of high resolution and large measurement range is realized, the problems of interference fading and the contradiction between strain resolution and strain measurement distance of the linear frequency modulation DAS system are solved, so that the dynamic strain demodulation of high resolution and large measurement range is realized. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 The structure schematic diagram of the distributed optical fiber acoustic sensing device based on linear frequency modulation pulse sequence proposed in the embodiment one of the present application;

[0031] Figure 2 The flowchart of the distributed optical fiber acoustic sensing method based on linear frequency modulation pulse sequence proposed in the embodiment two of the present application;

[0032] Figure 3 The original data map and the pulse recombination map extracted in the embodiment two of the present application;

[0033] Figure 4 The recombination demodulation results of the dynamic strain with amplitude of 80με and 420με by the method in the embodiment two of the present application;

[0034] Figure 1Middle: 1 is a narrow-linewidth laser, 2 is a 1×2 polarization-maintaining coupler, 3 is a dual-parallel Mach-Zehnder modulator, 4 is the first erbium-doped fiber amplifier, 5 is an optical circulator, 6 is the sensing fiber, 7 is an arbitrary waveform generator, 8 is a polarization controller, 9 is the second erbium-doped fiber amplifier, 10 is a 2×1 polarization-maintaining coupler, 11 is a balanced photodetector, and 12 is an acquisition card. DETAILED DESCRIPTION

[0035] 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, not 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.

[0036] Example 1

[0037] like Figure 1 As shown, embodiment 1 of the present invention provides a distributed optical fiber acoustic sensing device based on a linear frequency modulated pulse sequence, comprising: a narrow linewidth laser 1, a 1×2 polarization-maintaining coupler 2, a dual-parallel Mach-Zehnder modulator 3, a first erbium-doped fiber amplifier 4, an optical circulator 5, a sensing fiber 6, an arbitrary waveform generator 7, a polarization controller 8, a second erbium-doped fiber amplifier 9, a 2×1 polarization-maintaining coupler 10, a balanced photodetector 11, and an acquisition card 12; the arbitrary waveform generator 7 is connected to the dual-parallel Mach-Zehnder modulator 3 to generate a linear frequency modulated pulse sequence to drive the dual-parallel Mach-Zehnder modulator 3; the linear frequency modulated pulse sequence output by the arbitrary waveform generator 7 includes a plurality of pulses whose center frequencies increase or decrease in sequence.

[0038] The laser emitted by the narrow linewidth laser 1 is divided into two parts by a polarization-maintaining coupler 2, one part of which is used as signal light and the other part is used as reference light; the signal light is modulated into a linear frequency-modulated pulse sequence by a dual-parallel Mach-Zehnder modulator 3 driven by an arbitrary waveform generator 7, the linear frequency-modulated pulse sequence is amplified by a first erbium-doped fiber amplifier 4 and output to a sensing fiber 6 through an optical circulator 5, the Rayleigh backscattered light generated in the sensing fiber 6 is output through the optical circulator 5, the output Rayleigh backscattered light is amplified by a second erbium-doped fiber amplifier 9 and sent to a 2×1 polarization-maintaining coupler 10 together with the reference light to interfere, the interference signal is detected by a balanced photodetector 11, and an acquisition card 12 is used to collect the detection signal output by the balanced photodetector 11.

[0039] Specifically, in this embodiment, the narrow linewidth laser 1 is used to output laser light with a central wavelength of 1550 nm and a linewidth of less than 1 kHz. Specifically, its central wavelength is 1550.12 nm. The sensing fiber 6 is a single-mode fiber.

[0040] Specifically, in this embodiment, the linear frequency modulated pulse sequence output by the arbitrary waveform generator 7 includes 20 pulses with a center frequency interval of 10 MHz and a modulation bandwidth of 60 MHz. In this embodiment, the modulation frequency of the first pulse in the linear frequency modulated pulse sequence is 50 MHz to 110 MHz, the modulation frequency of the second pulse is 60 MHz to 120 MHz, and so on. The pulse period is 110 μs, the pulse width is 100 ns, and the total period of the pulse sequence is 2200 μs.

[0041] Specifically, in this embodiment, the arbitrary waveform generator 7 is also connected to the acquisition card 12, and is used to generate a trigger signal to drive the acquisition card 12 to perform data acquisition when each group of linear frequency modulation pulse sequences is transmitted, so that the acquisition card can determine the center frequency of each linear frequency modulation pulse.

[0042] The first erbium-doped fiber amplifier 4 is a pulsed erbium-doped fiber amplifier, and the second erbium-doped fiber amplifier 9 is a small-signal erbium-doped fiber amplifier.

[0043] Specifically, this embodiment also includes a polarization controller 8. The polarization state of the reference light is controlled by the polarization controller 8 to achieve an optimal interference effect. The Rayleigh backscattered signal output by the optical circulator 5 is amplified by the second erbium-doped fiber amplifier 9 and interferes with the reference light in the 2×1 polarization-maintaining coupler 10. The balanced photodetector 11 converts the optical signal output by the coupler into an electrical signal and collects it through the acquisition card 12 with a sampling rate of 1 GSa / s.

[0044] Example 2

[0045] like Figure 2 As shown, the second embodiment of the present invention provides a distributed optical fiber acoustic sensing method based on a linear frequency modulated pulse sequence, which is implemented based on the distributed optical fiber acoustic sensing device based on a linear frequency modulated pulse sequence described in the first embodiment, and includes the following steps:

[0046] Step 1: Collect data through the acquisition card 12 to extract the pulse sequence frequency shift matrix and the linear frequency modulation pulse time shift matrix.

[0047] In this embodiment, the linear frequency modulation pulse sequence output by the double-parallel Mach-Zehnder modulator 3 is amplified by the first doped fiber amplifier 4, then injected into the sensing optical fiber 6 with a length of 10.45 km through the optical circulator 5, and the strain is applied at the position of 10385 m~10395 m of the optical fiber 6 by the piezoelectric ceramic tube or mechanical stretching device. The measurement is carried out when the sinusoidal dynamic strain with an amplitude of 80με and 420με is applied to the position. The sensing optical fiber 6 is a single-mode optical fiber.

[0048] Step 2: recombine the pulse sequence frequency spectrum matrix according to the pulse injection order by the pulse recombination algorithm to obtain the pulse sequence frequency spectrum of any optical fiber position; divide the pulse sequence frequency spectrum into multiple segments according to time, extract the relative frequency shift of each segment, and obtain the relative frequency shift amount of each segment by correlating the last curve in each segment with the reference curve; calculate the frequency shift amount of each segment according to the relative frequency shift amount, and calculate the equivalent strain value to obtain the first time-strain curve of each optical fiber position; calculate the strain average value of each optical fiber position as the first average value according to the obtained first time-strain curve;

[0049] Specifically, in step 2, the relative frequency shift is calculated by correlating the last curve in each segment with the first curve.

[0050] In step 2, the recombination method is as follows: the Rayleigh backscattering light intensity curve obtained at any optical fiber position z is extracted along the frequency shift axis of the pulse sequence, and the data of different frequency scanning periods are segmented and recombined to obtain the pulse sequence frequency spectrum graph.

[0051] Step 3: extract the scattering signal corresponding to a single linear frequency modulation pulse in the linear frequency modulation pulse time shift matrix, judge whether the scattering signal at each position is distorted according to the distortion judgment function, if not, calculate the time shift size of the signal envelope by local cross-correlation calculation through sliding window, and then obtain the time-strain curve of each optical fiber position by converting the strain value, if distortion occurs, the time-strain curve of the optical fiber position is obtained by interpolation calculation according to the first average value obtained in step 2; calculate the average of the strain values at different time instants of each optical fiber position as the second average value, subtract the second average value from the obtained time-strain curve to obtain the second time-strain curve of each optical fiber position; repeat the extraction of the scattering signal corresponding to other single linear frequency modulation pulses, and calculate the second time-strain curve corresponding to each optical fiber position, and average the second time-strain curves obtained by demodulating different linear frequency modulation pulses as the third time-strain curve;

[0052] Specifically, in step 3, the distortion judgment function is: ​​

[0053] (1)

[0054] wherein, denotes the distortion parameter at position z at time t, k is used to define the length of time involved in the calculation, which can be chosen according to the actual calculation amount, denotes the light intensity curve of the optical fiber at position z at time t, denotes the light intensity curve of the optical fiber at position z at time t, denotes the light intensity curve of the optical fiber at position z at time t, denotes the light intensity curve of the optical fiber at position z at time t, denotes the light intensity curve of the optical fiber at position z at time t, denotes the light intensity curve of the optical fiber at position z at time t, denotes the light intensity curve of the optical fiber at position z at time t, denotes the light intensity curve of the optical fiber at position z at time t, denotes the light intensity curve of the optical fiber at position z at time t, is generally set as the spatial resolution. In the embodiment, the distortion judgment function realizes data screening by calculating the similarity of the local Rayleigh backscattering light intensity envelope in time, and uses interpolation calculation for the distorted data part, so that the strain measurement range can be improved.

[0055] Specifically, when the distortion parameter R at position z is greater than 0.5, it is determined that the data at this position is distorted.

[0056] Step 4: Adding the third time-strain curve obtained in step 3 to the first average value as the final strain demodulation result.

[0057] In the embodiment, after the linear frequency pulse sequence is injected into the sensing optical fiber through the double-parallel Mach-Zehnder modulator, the original data collected by the acquisition card is a combination of the pulse sequence frequency shift matrix and the linear frequency pulse time shift matrix.

[0058] In step 2, the original data can be reorganized according to the pulse injection sequence through the pulse reorganization algorithm. The Rayleigh backscattering light intensity curve obtained at any optical fiber position is extracted along the frequency offset of the pulse sequence, and the data of different frequency scanning periods are segmented and reorganized into a pulse sequence frequency spectrum. Wherein, is the optical fiber position, is the optical fiber position corresponding to the acquisition time, is the speed of light in vacuum, is the refractive index of the optical fiber. The frequency spectrum of the sensing optical fiber will produce frequency shift after being stressed, and the frequency offset can be obtained by cross-correlation calculation of the Rayleigh backscattering light intensity curves at different times, and then the strain value Δ can be equivalently converted into the strain value Δ ε using the following formula:

[0059] (2)

[0060] wherein, v 0 represents the initial center optical frequency of the pulse.

[0061] Further, in the embodiment, in order to increase the measurement range, the pulse sequence frequency spectrum is segmented by time, each time segment has curves, and the relative frequency shift change is calculated by cross-correlation operation of each curve in the segment and the reference curve (typically the first curve in the segment), and the frequency shift calculation value of the curve is taken as the calibration value, and the frequency shift calculation result of the next time segment is uniformly added to the frequency shift calibration value of each time before. After the cycle calibration, the frequency shift amount of each time segment is obtained.

[0062] , (3)

[0063] wherein, is the calibration value, and the cycle calibration can reduce the calculation error in the traditional differential increment method to a certain extent, and the calculation amount of the cross-correlation operation is relatively small due to the small number of frequency scanning points of the system. Further, the segmented average of the frequency shift amount calculation result of each time segment obtained by the cycle calibration can reduce the calculation error in the cross-correlation process.

[0064] In addition, in the embodiment, since the pulses in the linear frequency modulation pulse sequence are simultaneously modulated into linear frequency modulation pulses with the center frequency and the bandwidth , for the linear frequency modulation pulse time shift matrix, the Rayleigh backscattering light intensity curve at a certain frequency is extracted along the optical fiber time (distance) axis , denoted as , and when the optical fiber is subjected to strain, the local Rayleigh backscattering pattern is shifted along the optical fiber time (distance) axis , and by performing the sliding window cross-correlation analysis on the local Rayleigh backscattering light intensity envelope, the time (corresponding to the optical fiber position) shift can be obtained, and the relationship between the equivalent strain value Δ ε and the time (corresponding to the optical fiber position) shift can be expressed as:

[0065] . (4)

[0066] wherein, represents the pulse time width.

[0067] When the strain applied to the optical fiber is too large, the scattering signal pattern extracted by the chirp pulse time shift matrix will be distorted in step 3. In the embodiment, data screening is realized by calculating the similarity of the local Rayleigh backscattering light intensity envelope in time, a distortion judgment function is defined, 0.5 is taken as the threshold to screen out the distorted measurement value, and the strain prediction value is obtained by interpolation calculation combined with the strain demodulation result of the pulse sequence at the corresponding moment, which can avoid the distortion of the chirp pulse time shift matrix demodulation caused by too large strain.

[0068] In step 4, the strain calculation values of single pulses in the chirp pulse time shift matrix are averaged to obtain a second average value, and then the strain calculation values after removing the second average value can obtain a high-resolution strain change curve. Further, the strain change curves of pulses with different center frequencies in the chirp pulse sequence are averaged, which can further reduce the strain measurement error. According to formula (4), the strain size corresponding to the time shift can be calculated, and finally the high-resolution, large-range strain demodulation value of the system is obtained by summation operation.

[0069] As shown in Figure 3 , wherein (a) represents the original data graph, (b) represents the recombined pulse sequence spectrum graph at 10390 m of the optical fiber, and (c) represents the recombined chirp pulse time shift matrix graph

[0070] As shown in Figure 4Fig. 3 shows a comparison of various demodulation methods when a sinusoidal dynamic strain with an amplitude of 80με and 420με is applied to the optical fiber at a position of 10385 m~10395 m by a piezoelectric ceramic tube or a mechanical stretching device, wherein (a) shows a recombined pulse sequence spectrogram at the position of 10390 m of the optical fiber when a sinusoidal dynamic strain with an amplitude of 80με is applied, (b) shows a recombined linear frequency modulation pulse time shift matrix when a sinusoidal dynamic strain with an amplitude of 80με is applied, (c) shows the demodulation results of the pulse sequence and the linear frequency modulation pulse and the final combined results when a sinusoidal dynamic strain with an amplitude of 80με is applied, (d) shows a recombined pulse sequence spectrogram at the position of 10390 m of the optical fiber when a sinusoidal dynamic strain with an amplitude of 420με is applied, (e) shows a recombined linear frequency modulation pulse time shift matrix when a sinusoidal dynamic strain with an amplitude of 420με is applied, and (f) shows the demodulation results of the pulse sequence and the linear frequency modulation pulse and the final combined results when a sinusoidal dynamic strain with an amplitude of 420με is applied. It can be found that when the strain amplitude is small, the resolution of the linear frequency modulation pulse demodulation result is high, and the pulse sequence demodulation result shows a ladder shape due to insufficient resolution; when the strain amplitude is large, the linear frequency modulation pulse measurement result is distorted, and the pulse sequence measurement result generally conforms to the true strain value. As can be seen from (c) and (f) in the figure, through the recombination demodulation algorithm of the present application, the two measurement results are fused, and finally the strain measurement curve of the system can well reflect the dynamic strain change applied. Wherein, Fusion results represent the demodulation result obtained by the demodulation method of the present embodiment, Frequency scanning represents the demodulation result obtained by pulse sequence demodulation, LFM pulse represents the demodulation result obtained by linear frequency modulation pulse demodulation, and experiments prove that the fusion algorithm of the present application can realize high-resolution and large-range strain demodulation.

[0071] Embodiment three

[0072] Embodiment three of the present application provides a distributed optical fiber acoustic sensing device based on a linear frequency modulation pulse sequence, which has basically the same structure as embodiment one, except that the sensing device of the present embodiment further comprises a computing unit connected with the acquisition card 12, for realizing dynamic strain demodulation.

[0073] Specifically, in the present embodiment, the computing unit demodulates the dynamic strain by the sensing method in embodiment two.

[0074] In summary, the application discloses a kind of distributed optical fiber acoustic sensing device and method based on linear frequency modulation pulse sequence, by introducing pulse recombination demodulation algorithm and cyclic calibration correlation method to process frequency shift and time shift characteristics, solve the contradiction between the maximum measurement value of strain and strain resolution in the traditional linear frequency modulation pulse DAS system, eliminate the influence of interference fading, realize high resolution, dynamic strain demodulation of large measurement range.

[0075] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, but not limited to them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent substitution for part or all of the technical features; and these modifications or substitutions do not make the essence of the corresponding technical solution deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A distributed fiber optic acoustic sensing device based on a linear frequency modulation pulse sequence, characterized in that: include: A narrow linewidth laser (1), a dual-parallel Mach-Zehnder modulator (3), a sensing fiber (6), an arbitrary waveform generator (7), a balanced photodetector (11), and an acquisition card (12); the arbitrary waveform generator (7) is connected to the dual-parallel Mach-Zehnder modulator (3) and is used to generate a linear frequency modulation pulse sequence to drive the dual-parallel Mach-Zehnder modulator (3); the linear frequency modulation pulse sequence output by the arbitrary waveform generator (7) includes a plurality of pulses whose center frequencies increase or decrease in sequence; The laser light emitted by the narrow linewidth laser (1) is divided into two parts, one part of which is used as reference light, and the other part is used as signal light and modulated into a linear frequency modulation pulse sequence by a dual parallel Mach-Zehnder modulator (3) driven by an arbitrary waveform generator (7). The linear frequency modulation pulse sequence is output to the sensing optical fiber (6). The Rayleigh backscattered light generated in the sensing optical fiber (6) is sent to a 2×1 polarization-maintaining coupler (10) together with the reference light and interferes with each other. The interference signal is detected by a balanced photodetector (11). The acquisition card (12) is used to acquire the detection signal output by the balanced photodetector (11).

2. A distributed optical fiber acoustic sensing device based on a linear frequency modulation pulse sequence according to claim 1, characterized in that: It also includes: a 1×2 polarization-maintaining coupler (2), a first erbium-doped fiber amplifier (4), an optical circulator (5), and a second erbium-doped fiber amplifier (9); The laser light emitted by the narrow linewidth laser (1) is divided into two parts by a 1×2 polarization-maintaining coupler (2), wherein one part of the light is used as signal light and the other part is used as reference light; the signal light is modulated into a linear frequency-modulated pulse sequence by a dual-parallel Mach-Zehnder modulator (3) driven by an arbitrary waveform generator (7); the linear frequency-modulated pulse sequence is amplified by a first erbium-doped fiber amplifier (4) and output to a sensing fiber (6) through an optical circulator (5); the Rayleigh backscattered light generated in the sensing fiber (6) is output through the optical circulator (5); the output Rayleigh backscattered light is amplified by a second erbium-doped fiber amplifier (9) and sent together with the reference light to a 2×1 polarization-maintaining coupler (10) to generate interference.

3. The distributed optical fiber acoustic sensing device based on linear frequency modulation pulse sequence according to claim 1, characterized in that: The narrow linewidth laser (1) is used to output laser light with a central wavelength of 1550 nm.

4. The distributed optical fiber acoustic sensing device based on linear frequency modulation pulse sequence according to claim 1, characterized in that: The linear frequency modulation pulse sequence output by the arbitrary waveform generator (7) includes 20 pulses with a center frequency interval of 10 MHz and a modulation bandwidth of 60 MHz.

5. The distributed optical fiber acoustic sensing device based on linear frequency modulation pulse sequence according to claim 1, characterized in that: The arbitrary waveform generator (7) is also connected to the acquisition card (12) and is used to generate a trigger signal to drive the acquisition card (12) to perform data acquisition when each group of linear frequency modulation pulse sequences is transmitted.

6. The distributed optical fiber acoustic sensing device based on linear frequency modulation pulse sequence according to claim 2, characterized in that: The first erbium-doped fiber amplifier (4) is a pulsed erbium-doped fiber amplifier, and the second erbium-doped fiber amplifier (9) is a small-signal erbium-doped fiber amplifier.

7. A distributed fiber optic acoustic sensing method based on a linear frequency modulation pulse sequence, characterized in that: A distributed optical fiber acoustic sensing device based on a linear frequency modulation pulse sequence according to any one of claims 1 to 6 is implemented, comprising the following steps: Step 1: Collect data through the acquisition card (12) to extract the pulse sequence frequency shift matrix and the linear frequency modulation pulse time shift matrix; Step 2: Reorganize the pulse sequence frequency shift matrix according to the pulse injection order through the pulse reorganization algorithm to obtain the pulse sequence frequency spectrum at any optical fiber position; divide the pulse sequence frequency spectrum into multiple segments according to time, and extract the pulse sequence frequency spectrum in each time segment. curves, performing cross-correlation operation on the last curve in each time period and the reference curve to calculate the relative frequency shift, thereby obtaining the relative frequency shift amount in each time period; calculating the frequency shift amount of each time period based on the relative frequency shift amount, and calculating the equivalent strain value based on the frequency shift amount to obtain the first time-strain curve of each optical fiber position; and calculating the average of the strain values ​​at different times as the first average value based on the obtained first time-strain curve; Step 3: Extract the scattered signal corresponding to a single linear frequency modulation pulse in the linear frequency modulation pulse time-shift matrix, and determine whether the scattered signal at each position is distorted according to the distortion judgment function. If there is no distortion, perform local cross-correlation calculation on the sliding window to obtain the time shift of the signal envelope, and convert it into a strain value to obtain the time-strain curve of each optical fiber position. If distortion occurs, interpolate the first average value obtained in step 2 to obtain the time-strain curve of the optical fiber position; calculate the average of the strain values ​​of each optical fiber position at different times as the second average value, subtract the second average value from the obtained time strain curve, and obtain the second time-strain curve of each optical fiber position; repeatedly extract the scattered signal corresponding to other single linear frequency modulation pulses, and calculate the second time-strain curve corresponding to each position, and average the second time strain curves obtained by demodulating different linear frequency modulation pulses to obtain the third time-strain curve; Step 4: Add the first average value to the third time-strain curve obtained in step 3 to obtain a final strain demodulation result.

8. The distributed fiber optic acoustic sensing method based on linear frequency modulation pulse sequence according to claim 7, characterized in that: In step 2, a cross-correlation operation is performed between the last curve and the first curve in each time period to calculate the relative frequency shift.

9. The distributed fiber optic acoustic sensing method based on linear frequency modulation pulse sequence according to claim 7, characterized in that: In step 3, the distortion judgment function is: ； in, Indicates the z position The distortion parameter at the moment, k is used to specify the length of time involved in the calculation, which can be selected according to the actual amount of calculation. express Time Fiber arrive The light intensity curve of the position, express Time Fiber arrive The light intensity curve of the position, Set to spatial resolution; When the distortion parameter R>0.5, distortion is determined.

10. The distributed fiber optic acoustic sensing method based on linear frequency modulation pulse sequence according to claim 7, characterized in that: In step 2, the reorganization method is as follows: the Rayleigh backscattered light intensity curve obtained at any optical fiber position z Extraction is performed along the frequency offset axis of the pulse sequence, and the data of different frequency scanning periods are segmented and recombined to obtain the pulse sequence frequency spectrum.

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