Distributed optical fiber acoustic sensing device and method based on linear frequency modulation pulse sequence

Abstract

The invention 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, and the device comprises a narrow linewidth laser, a double-parallel Mach-Zehnder modulator, and an arbitrary waveform generator. The arbitrary waveform generator is connected with the double-parallel Mach-Zehnder modulator and is used for generating a linear frequency modulation pulse sequence comprising a plurality of pulses of which the center frequencies are sequentially increased or decreased to drive the double-parallel Mach-Zehnder modulator; laser emitted by the narrow linewidth laser is divided into two parts, one part serves as reference light, the other part serves as signal light and is modulated into a linear frequency modulation pulse sequence through the double-parallel Mach-Zehnder modulator, then the linear frequency modulation pulse sequence is output to the sensing optical fiber, Rayleigh back scattering light generated in the sensing optical fiber interferes with the reference light, and the signal light is output to the optical fiber. An interference signal is detected by a balanced photoelectric detector and then is collected by a collection card. According to the invention, high-resolution and large-measurement-range dynamic strain measurement can be realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical fiber distributed sensing, and specifically relates to a distributed optical fiber acoustic sensing device and method based on a linear frequency modulation pulse sequence. The detection light pulse is modulated into a linear frequency modulation pulse sequence with a center frequency scan through phase modulation, and the frequency shift and time shift characteristics are demodulated using a pulse recombination demodulation algorithm and a cyclic calibration correlation method, so as to realize dynamic strain demodulation with high resolution and a large measurement range. Background Art

[0002] Distributed acoustic sensing (DAS) is an advanced sensing technology that uses backward Rayleigh scattering in optical fibers to locate and recover changes in environmental physical quantities at any location on an 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, since the scattering medium in the optical fiber is easily affected by the environment, the scattered signal of the DAS system will inevitably experience interference fading during the coherent superposition process, resulting in strong signal randomness, insufficient signal-to-noise ratio and sensitivity, making it difficult to achieve high-resolution large-scale measurements.

[0003] In order to eliminate the influence of interference fading on the signal, linear frequency modulation technology has been introduced into the DAS system due to its advantages over continuous waveforms and pulse waveforms. By combining linear frequency modulation pulses with matched filtering algorithms, the problem of the trade-off between sensing distance and spatial resolution can be solved while suppressing signal interference fading (Chinese invention patent, CN107990970 B). Combining linear frequency modulation pulse technology with Rayleigh scattering pattern method and demodulating through cross-correlation operation can achieve strain resolution of nε while suppressing interference fading (Optics Express, 2016, 24(12):13121-13133). However, the maximum value of a single strain measurement of the linear frequency modulation pulse scheme is limited by the modulation bandwidth of the linear frequency modulation pulse, and the device index requirements are too high. In response to this, a frequency scanning scheme has been proposed. At present, the use of frequency scanning coherent optical time domain reflectometry technology has achieved 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 system must reduce the frequency scanning step, which will extend the measurement cycle and reduce 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 system's strain measurement range can be effectively increased by performing differential incremental accumulation of the cross-correlation results through long-term continuous measurement, but the limitation of the maximum value of a single strain measurement still exists. For traditional linear frequency modulation pulse DAS systems, the maximum value of a single strain measurement and the strain resolution are mutually restricted. Increasing the modulation bandwidth of the linear frequency modulation pulse will lead to a decrease in strain sensitivity. This limitation makes it impossible to give full play to the performance of the system hardware. Therefore, solving the contradiction between the maximum value of a single strain measurement and the strain resolution is the key to improving the performance of DAS systems based on linear frequency modulation pulses. Summary of the invention

[0005] In view of the contradiction between the maximum value of a single strain measurement and the strain resolution in the existing DAS system based on linear frequency modulation pulses, the present invention proposes a distributed fiber optic acoustic sensing device and method based on a linear frequency modulation pulse sequence, which maintains a high strain sensitivity and improves the measurement capability of large strains without increasing the system hardware configuration.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is: a distributed optical fiber acoustic sensor device based on a linear frequency modulation pulse sequence, comprising: a narrow line width laser, a dual 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 to the dual parallel Mach-Zehnder modulator to generate a linear frequency modulation pulse sequence to drive the dual parallel Mach-Zehnder modulator; the linear frequency modulation pulse sequence output by the arbitrary waveform generator includes a plurality of pulses whose center frequencies increase or decrease in sequence; The 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 and modulated into a linear frequency modulation pulse sequence by a dual parallel Mach-Zehnder modulator driven by an arbitrary waveform generator. The linear frequency modulation pulse sequence is output to the sensing optical fiber, and the Rayleigh backscattered light generated in the sensing optical fiber is sent to the × polarization-maintaining coupler together with the reference light to interfere, and the interference signal is detected by a balanced photodetector, and the acquisition card is used to collect the detection signal output by the balanced photodetector.

[0007] The distributed optical fiber acoustic sensor device based on linear frequency modulation pulse sequence also includes: a 1×2 polarization-maintaining coupler, a first erbium-doped optical fiber amplifier, an optical circulator, and a second erbium-doped optical fiber amplifier; 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 sent to a 2×1 polarization-maintaining coupler together with the reference light to interfere.

[0008] The narrow linewidth laser is used to output laser light with a central wavelength of 1550 nm.

[0009] 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.

[0010] 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.

[0011] 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.

[0012] In addition, the present invention also provides a distributed optical fiber acoustic sensing method based on a linear frequency modulation pulse sequence, which is implemented based on the distributed optical fiber acoustic sensing device based on a linear frequency modulation pulse sequence, and includes the following steps: Step 1: Collect data through the acquisition card 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 curves, cross-correlate the last curve in each time period with the reference curve to calculate the relative frequency shift, and obtain the relative frequency shift amount in each time period; calculate the frequency shift amount of each time period according to the relative frequency shift amount, and calculate the equivalent strain value according to the frequency shift amount to obtain the first time-strain curve of each optical fiber position; calculate the average of the strain values ​​at different times as the first average value according to 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 judge whether the scattered signal at each position is distorted according to the distortion judgment function. If there is no distortion, the time shift size of the signal envelope is obtained by local cross-correlation calculation through a sliding window, and the time-strain curve of each optical fiber position is obtained after converting it into a strain value. If distortion occurs, the time-strain curve of the optical fiber position is obtained by interpolation calculation through the first average value obtained in step 2; the average of the strain values ​​of each optical fiber position at different times is calculated as the second average value, and the second average value is subtracted from the obtained time strain curve to obtain the second time-strain curve of each optical fiber position; repeatedly extract the scattered signals 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 as 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.

[0013] 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.

[0014] 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.

[0015] In step 2, the reorganization method is as follows: for the Rayleigh backscattered light intensity curve obtained at any optical fiber position z The pulse sequence is extracted along the frequency offset axis, and the data of different frequency scanning periods are segmented and reorganized to obtain the pulse sequence frequency spectrum.

[0016] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention is based on the phase compensation effect. After strain is applied to the optical fiber, a local time-shift feature is generated on the Rayleigh backscattering space-time distribution diagram of the linear frequency-modulated pulse. High-resolution strain measurement can be achieved by performing cross-correlation operations on the local time-shift feature, thus overcoming the interference fading problem of traditional phase-sensitive optical time-domain reflectometry technology.

[0017] (2) The present invention proposes a dynamic strain measurement method based on a linear frequency modulation pulse sequence. This technology combines the advantages of frequency scanning and linear frequency modulation pulses, fully utilizes the flexibility of digital modulation, and can simultaneously achieve sensing measurements with a large dynamic strain range and high strain resolution.

[0018] (3) The present invention proposes a dynamic strain demodulation method based on a linear frequency modulation pulse sequence, which can effectively screen the linear frequency modulation pulse distortion value and use the frequency scanning matrix calculation results for numerical prediction. The strain measurement range of the system is expanded through the cyclic calibration correlation method, and finally a large-scale and high-resolution dynamic strain measurement is achieved.

[0019] In summary, the present invention proposes a distributed optical fiber dynamic strain sensing method based on a linear frequency modulation pulse sequence, in which the detection light pulse is modulated into a linear frequency modulation pulse sequence with a center frequency scan through phase modulation, and the frequency shift characteristics generated on the scanning pulse sequence spectrum diagram and the time shift characteristics generated on the RBS space-time distribution diagram are demodulated using a pulse recombination demodulation algorithm and a cyclic calibration correlation method, thereby achieving high-resolution and large-measurement-range dynamic strain demodulation, solving the problem of interference fading and the contradiction between the strain resolution and the strain measurement distance of the linear frequency modulation DAS system, thereby achieving high-resolution and large-measurement-range dynamic strain demodulation. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a schematic structural diagram of a distributed optical fiber acoustic sensor device based on a linear frequency modulation pulse sequence proposed in Embodiment 1 of the present invention; Figure 2 This is a flow chart of a distributed optical fiber acoustic sensing method based on a linear frequency modulation pulse sequence proposed in the second embodiment of the present invention; Figure 3 The original data graph and pulse reorganization graph extracted in the second embodiment of the present invention; Figure 4 The reconstructed demodulation results of the dynamic strain with amplitudes of 80με and 420με by the method in the second embodiment of the present invention; 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 a 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

[0021] 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.

[0022] Embodiment 1 like Figure 1 As shown, embodiment 1 of the present invention provides a distributed optical fiber acoustic sensing device based on a linear frequency modulation 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 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.

[0023] The laser emitted by the narrow linewidth laser 1 is divided into two parts by a polarization-maintaining coupler 2, 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 then 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 then 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.

[0024] 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 optical fiber 6 is a single-mode optical fiber.

[0025] Specifically, in this embodiment, 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. In this embodiment, the modulation frequency of the first pulse of the linear frequency modulation 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.

[0026] Specifically, in this embodiment, the arbitrary waveform generator 7 is also connected to the acquisition card 12 to generate a trigger signal to drive the acquisition card 12 to collect data when each group of linear frequency modulation pulse sequences are transmitted, so that the acquisition card can determine the center frequency of each linear frequency modulation pulse.

[0027] 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.

[0028] Specifically, in this embodiment, a polarization controller 8 is also included. The polarization state of the reference light is controlled by the polarization controller 8 to achieve the best interference effect. The Rayleigh backscattering 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. The sampling rate is 1 GSa / s.

[0029] Embodiment 2 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 modulation pulse sequence, which is implemented based on a distributed optical fiber acoustic sensing device based on a linear frequency modulation pulse sequence described in the first embodiment, and includes 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.

[0030] In this embodiment, the linear frequency modulation pulse sequence output by the dual parallel Mach-Zehnder modulator 3 is amplified by the first erbium-doped fiber amplifier 4 and then injected into the sensing fiber 6 with a length of 10.45 km through the optical circulator 5. The strain is applied at the position of 10385 m to 10395 m of the optical fiber through a piezoelectric ceramic tube or a mechanical stretching device. When the sinusoidal dynamic strain with an amplitude of 80 με and 420 με is applied to the position, the measurement is performed respectively. The sensing fiber 6 is a single-mode fiber.

[0031] 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 curves, cross-correlate the last curve in each time period with the reference curve to calculate the relative frequency shift, and obtain the relative frequency shift amount in each time period; calculate the frequency shift amount of each time period according to the relative frequency shift amount, and calculate the equivalent strain value according to the frequency shift amount 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; Specifically, 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.

[0032] In step 2, the reorganization method is as follows: for the Rayleigh backscattered light intensity curve obtained at any optical fiber position z The pulse sequence is extracted along the frequency offset axis, and the data of different frequency scanning periods are segmented and reorganized to obtain the pulse sequence frequency spectrum.

[0033] Step 3: extract the scattered signal corresponding to a single linear frequency modulation pulse in the linear frequency modulation pulse time shift matrix, and judge whether the scattered signal at each position is distorted according to the distortion judgment function. If there is no distortion, the time shift size of the signal envelope is obtained by local cross-correlation calculation through a sliding window, and the time-strain curve of each optical fiber position is obtained after converting it into a strain value. If distortion occurs, the time-strain curve of the optical fiber position is obtained by interpolation calculation through the first average value obtained in step 2; the average of the strain values ​​of each optical fiber position at different times is calculated as the second average value, and the second average value is subtracted from the obtained time strain curve to obtain the second time-strain curve of each optical fiber position; repeatedly extract the scattered signals 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; Specifically, in step 3, the distortion judgment function is: ; (1) 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, It is generally set as the spatial resolution. In this embodiment, the distortion judgment function realizes data screening by calculating the temporal similarity of the local Rayleigh backscattered light intensity envelope, and uses interpolation calculation for the distorted data part, which can improve the strain measurement range.

[0034] Specifically, when the distortion parameter R at the position z is greater than 0.5, it is determined that the data at this position is distorted.

[0035] Step 4: Add the first average value to the third time-strain curve obtained in step 3 to obtain a final strain demodulation result.

[0036] In this embodiment, after the linear frequency modulated pulse sequence is injected into the sensing optical fiber through the dual parallel Mach-Zehnder modulator, the raw data collected by the acquisition card is a combination of the pulse sequence frequency shift matrix and the linear frequency modulated pulse time shift matrix.

[0037] In step 2, the original data can be reorganized according to the pulse injection order by using the pulse reorganization algorithm. The Rayleigh backscattered light intensity curve obtained at Frequency deviation along the pulse train Extraction is performed, and the data of different frequency scanning cycles are segmented and reorganized into pulse sequence frequency spectra. Among them, is the fiber location, Is the fiber location The corresponding collection time, is the speed of light in a vacuum, is the refractive index of the optical fiber. When the sensing optical fiber is stressed, the frequency spectrum will shift. The intensity curve of Rayleigh backscattered light at different times The frequency offset can be obtained by cross-correlation calculation of the curve , and then the following formula can be used to convert it into strain value Δ ε : . (2) in, v 0 Represents the initial center optical frequency of the pulse.

[0038] Furthermore, in this embodiment, in order to increase the measurement range, the pulse sequence frequency spectrum is divided into time segments, and each time segment has curves, and compare each curve in the segment with the reference curve (Usually take the first one in the segment) to calculate the relative frequency shift change by cross-correlation operation, and Curve The calculated frequency shift value is used as the calibration value, and the frequency shift calculation result of the next time period is added to the calibration value of the frequency shift of each time period before. After the cyclic calibration, the frequency shift value of each time period is obtained. , which can be expressed as: , (3) in, is a calibration value. This embodiment can reduce the calculation error in the traditional differential increment method to a certain extent through cyclic calibration. At the same time, since the number of frequency scanning points of this system is small, the calculation amount of its cross-correlation operation is relatively small. Furthermore, the calculation results of the frequency shift amount in each time period obtained by the cyclic calibration method are averaged in sections, which can reduce the calculation error in the cross-correlation process.

[0039] In addition, in this embodiment, since the pulses in the linear frequency modulation pulse sequence are modulated to have a center frequency of , the bandwidth is For a linear frequency modulated pulse, for the linear frequency modulated pulse time shift matrix, the Rayleigh backscattering light intensity curve of a certain frequency is shifted along the optical fiber time (distance) axis. Extracted and recorded as When the optical fiber is strained, the local Rayleigh backscattering pattern is along the optical fiber time (distance) axis When an offset occurs, the time (corresponding to the fiber position) offset can be obtained by performing sliding window cross-correlation analysis on the local Rayleigh backscattered light intensity envelope. , which is similar to the equivalent strain value Δ ε The relationship can be expressed as: . (4) in, Indicates the pulse time width.

[0040] In step 3, when the strain applied to the optical fiber is too large, the scattered signal pattern extracted by the linear frequency modulation pulse time-shift matrix will be distorted. In this embodiment, data screening is achieved by calculating the temporal similarity of the local Rayleigh backscattered light intensity envelope, and a distortion judgment function is defined. 0.5 is used as a threshold to screen out the distorted measurement value, and the strain prediction value is obtained by interpolation calculation based on the pulse sequence strain demodulation result at the corresponding moment, which can avoid linear frequency modulation pulse time-shift matrix demodulation distortion caused by excessive strain.

[0041] In step 4, the strain calculation values ​​obtained from a single pulse in the linear frequency modulation pulse time shift matrix are averaged to obtain a second average value, and then the strain calculation value is removed from the second average value to obtain a high-resolution strain change curve. Furthermore, the strain change curves of pulses with different center frequencies in the linear frequency modulation pulse sequence are averaged to further reduce the strain measurement error. The strain size corresponding to the time shift can be calculated according to formula (4), and finally the high-resolution and large-range strain demodulation value of the system is obtained by summing up.

[0042] like Figure 3 As shown in the figure, (a) represents the original data diagram, (b) represents the recombined pulse sequence spectrum at 10390 m of the optical fiber; (c) represents the recombined linear frequency modulation pulse time shift matrix diagram like Figure 4 As shown in the figure, it is a schematic diagram of the comparison of various demodulation methods when sinusoidal dynamic strains with amplitudes of 80με and 420με are applied at the positions of 10385 m~10395 m of the optical fiber through a piezoelectric ceramic tube or a mechanical stretching device, wherein (a) represents the reorganized pulse sequence spectrum at 10390 m of the optical fiber when the sinusoidal dynamic strain with an amplitude of 80με is applied, (b) represents the reorganized linear frequency modulation pulse time-shift matrix diagram when the sinusoidal dynamic strain with an amplitude of 80με is applied, (c) represents the demodulation results of the pulse sequence and the linear frequency modulation pulse and the final combined results when the sinusoidal dynamic strain with an amplitude of 80με is applied; (d) represents the demodulation results of the pulse sequence and the linear frequency modulation pulse when the sinusoidal dynamic strain with an amplitude of 420με is applied at 10390 m of the optical fiber. m; (e) represents the reorganized pulse sequence spectrum when a sinusoidal dynamic strain with an amplitude of 420με is applied, and the reorganized linear frequency modulation pulse time-shift matrix diagram; (f) represents 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. Among them, it can be found that when the strain amplitude is small, the linear frequency modulation pulse demodulation result has a high resolution, and the pulse sequence demodulation result is stepped 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 is generally consistent with the true strain value. It can be seen from Figures (c) and (f) that through the reorganization demodulation algorithm of the present invention, the two measurement results are fused, and the strain measurement curve of the final system can well reflect the dynamic strain change. Among them, Fusion results represents the demodulation result obtained by the demodulation method of this embodiment, Frequency scanning represents the demodulation result obtained by pulse sequence demodulation, and LFM pulse represents the demodulation result obtained by linear frequency modulation pulse demodulation. Experiments have confirmed that the fusion algorithm of the present invention can achieve high-resolution and large-range strain demodulation.

[0043] Embodiment 3 Embodiment 3 of the present invention provides a distributed optical fiber acoustic sensing device based on a linear frequency modulation pulse sequence, and its structure is basically the same as that of embodiment 1, except that the sensing device of this embodiment further includes a computing unit, which is connected to the acquisition card 12 for realizing dynamic strain demodulation.

[0044] Specifically, in this embodiment, the calculation unit demodulates the dynamic strain by using the sensing method in the second embodiment.

[0045] In summary, the present invention discloses a distributed fiber optic acoustic sensing device and method based on a linear frequency modulation pulse sequence. By introducing a pulse reorganization demodulation algorithm and a cyclic calibration correlation method to process the frequency shift and time shift characteristics, the contradiction between the single maximum measurement value of strain and the strain resolution in the traditional linear frequency modulation pulse DAS system is solved, the influence of interference fading is eliminated, and dynamic strain demodulation with high resolution and a large measurement range is achieved.

[0046] 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 with 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 distributed optical fiber acoustic sensor 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 optical 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 of which 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 to interfere 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 sensor 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), one part of which is used as signal light and the other part 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 then output to a sensing fiber (6) through an optical circulator (5); 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 then sent together with the reference light to a 2×1 polarization-maintaining coupler (10) to generate interference.

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

7. A distributed optical fiber acoustic sensing method based on a linear frequency modulation pulse sequence, characterized in that: A distributed optical fiber acoustic sensor 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 an acquisition card (12) to extract a pulse sequence frequency shift matrix and a 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 curves, cross-correlate the last curve in each time period with the reference curve to calculate the relative frequency shift, and obtain the relative frequency shift amount in each time period; calculate the frequency shift amount of each time period according to the relative frequency shift amount, and calculate the equivalent strain value according to the frequency shift amount to obtain the first time-strain curve of each optical fiber position; calculate the average of the strain values ​​at different times as the first average value according to 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 judge whether the scattered signal at each position is distorted according to the distortion judgment function. If there is no distortion, the time shift size of the signal envelope is obtained by local cross-correlation calculation through a sliding window, and the time-strain curve of each optical fiber position is obtained after converting it into a strain value. If distortion occurs, the time-strain curve of the optical fiber position is obtained by interpolation calculation through the first average value obtained in step 2; the average of the strain values ​​of each optical fiber position at different times is calculated as the second average value, and the second average value is subtracted from the obtained time strain curve to obtain the second time-strain curve of each optical fiber position; repeatedly extract the scattered signals 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 as 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 optical fiber 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 optical fiber 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 optical fiber 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: for the Rayleigh backscattered light intensity curve obtained at any optical fiber position z The pulse sequence is extracted along the frequency offset axis, and the data of different frequency scanning periods are segmented and reorganized to obtain the pulse sequence frequency spectrum.

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