A dual-chirped pulse based fiber distributed acoustic sensing device and method

Abstract

The application discloses a kind of based on double-chirp pulse's fiber distributed acoustic wave sensing device, it is related to distributed optical fiber sensing technique field, including: laser module (1), chirp pulse modulation module (2), sensing optical fiber module (3) and Rayleigh scattering signal separation and receiving module (4);The laser generated by laser module is obtained after passing through chirp pulse modulation module different band double-chirp pulse signal, and excite Rayleigh scattering signal in sensing optical fiber module and input to Rayleigh scattering signal separation and receiving module carries out signal separation and demodulation.The application inquires sensing optical fiber by different band double-chirp pulse, excite two kinds of Rayleigh scattering signal respectively realize large range and high sensitivity, use adaptive filtering algorithm to combine the advantages of both, the dynamic range of distributed acoustic wave detection is improved one order of magnitude, to realize the acquisition of large chirp bandwidth Rayleigh scattering signal with low sampling rate, realize the distributed acoustic wave sensing technology of large dynamic range.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of distributed optical fiber sensing technology, and more particularly to a distributed acoustic sensing device and method based on double-chirp pulse. BACKGROUND

[0002] Distributed Fiber Optic Sensing (DFOS) system is an advanced sensing technology that uses optical fiber as a sensor to detect various physical quantities along the length of the optical fiber, such as temperature, strain, pressure, vibration, etc. This technology is valued for its ability to provide continuous, long-distance and high spatial resolution measurements, and distributed fiber optic sensing systems have a wide range of applications in many fields, such as civil engineering, environmental monitoring, oil exploration, aerospace, etc. They can be used to monitor the health of large structures such as bridges, dams, tunnels, oil and gas pipelines, and for environmental temperature and disaster warning systems.

[0003] The principle of distributed fiber optic sensing system is to use the light scattering effect in the optical fiber. When light propagates in the optical fiber, it will scatter due to external environmental changes such as temperature changes or mechanical stress. By measuring the characteristics of these scattered light, the physical quantity changes along the line of the optical fiber can be inferred. In the distributed fiber optic sensing system, Rayleigh scattering technology is mainly used for vibration and sound signal detection, and there is a linear mapping relationship between the external small disturbance quantity and the phase transformation. Based on the distributed acoustic sensing (DAS) technology of Rayleigh scattering, the phase change of Rayleigh scattering light signal can be detected to complete the high-precision real-time reconstruction of external disturbance, restore the measured vibration or sound signal, and realize distributed sensing.

[0004] The patent with publication number CN113810098A discloses an optical time domain reflectometer based on double sideband chirp pulse modulation. The scheme uses EOM to modulate symmetric double sideband chirp pulses, uses two reflective FBGs to extract the upper and lower sidebands, and increases the adjustable optical fiber delay unit to provide time delay on the lower sideband. The probe light pulse is a pair of symmetric double sideband chirp pulses, i.e. a pair of pulses with the same pulse width, the same chirp spectrum content and opposite slopes. The opposite slopes can achieve a doubled strain measurement range, but the hardware cost will increase sharply with the increase of chirp bandwidth.

[0005] Unlike this scheme, the present application uses a combination of different frequency band size chirp pulses, i.e. uses a partial sampling method to collect large chirp signals, retains a large range of measurement range, and reduces the hardware requirements of the acquisition end. In addition, in the present application, small chirp signals are used to compensate for the strain measurement resolution of the system, and the dynamic range of the system is improved.

[0006] The patent with the publication number CN113810099A discloses an optical time domain reflectometer based on asymmetric double sideband chirp pulse modulation. The scheme detects a time-sharing asymmetric double sideband chirp pulse pair, that is, a pulse pair with the same pulse width and different chirp spectrum contents. An IQ modulator is used to modulate sideband lights with different chirp bandwidths in sequence, and an SOA is used to modulate the asymmetric two sidebands into pulses. Compared with the symmetric double sideband chirp pulse, two measurements are performed in sequence in time, the measurement resolution and the measurement range of the to-be-measured are adjusted by adjusting the bandwidth range of the chirp pulse light, and the functions of a large range measurement range and a high measurement resolution are realized. However, the scheme does not solve the problem that a large acquisition bandwidth and a high sampling rate caused by a large chirp bandwidth signal may require increasing the hardware complexity of the system and the requirements of the signal acquisition end, and the time division multiplexing of the double sideband pulse reduces the maximum detection bandwidth of the system by half.

[0007] Unlike the scheme, the present application concentrates two sweep lights with different chirp bandwidths and different frequency bands into one optical pulse. Two groups of signals can be obtained from the sensing optical fiber at each inquiry, avoiding the problem of detection bandwidth loss caused by time division multiplexing, and eliminating the phase deviation of the two chirp pulse signals caused by time division multiplexing.

[0008] In summary, the prior art has the technical problem that the acoustic wave measurement range of the chirp pulse type DAS is limited, it is difficult to realize a large acoustic wave detection range and high detection sensitivity at the same time, and when the chirp bandwidth of the detection signal is increased, the hardware complexity of the system and the requirements of the signal acquisition end are increased, and the hardware cost is high. SUMMARY

[0009] The purpose of the present application is to overcome the technical problem that the acoustic wave measurement range of the chirp pulse type DAS is limited in the prior art, it is difficult to realize a large acoustic wave detection range and high detection sensitivity at the same time, and when the chirp bandwidth of the detection signal is increased, the hardware complexity of the system and the requirements of the signal acquisition end are increased, and the hardware cost is high. The present application provides a fiber distributed acoustic wave sensing device based on double chirp pulses.

[0010] To achieve the above-mentioned purpose, the technical scheme of the present application is as follows:

[0011] The present application provides a fiber distributed acoustic wave sensing device based on double chirp pulses, comprising:

[0012] A laser module, a chirp pulse modulation module, a sensing optical fiber module, and a Rayleigh scattering signal separation and receiving module;

[0013] The laser module generates laser light, which, after passing through the chirp pulse modulation module, becomes a double-chirp pulse signal with different center frequencies and bandwidths after modulation and amplification, the double-chirp pulse signal excites a Rayleigh scattering signal in the sensing fiber module and inputs the Rayleigh scattering signal to the Rayleigh scattering signal separation and receiving module;

[0014] The Rayleigh scattering signal separation and receiving module includes an optical amplifier 2, an optical filter 3, an optical filter 4, an optical detector 1, an optical detector 2, an electrical filter, a collection card, and a signal separation and demodulation unit.

[0015] The Rayleigh scattering signal is amplified by the optical amplifier 2 and then input to the optical filter 3 to extract a swept-frequency Rayleigh scattering light signal 1 with a center frequency f1 and a bandwidth B1, and the Rayleigh scattering signal after passing through the optical filter 3 is input to the optical filter 4 to extract a swept-frequency Rayleigh scattering light signal 2 with a frequency f2 and a bandwidth B2, and the bandwidth B1 is greater than the bandwidth B2.

[0016] The swept-frequency Rayleigh scattering light signal is input to the optical detector 1 for detection and then input to the electrical filter for partial bandwidth sampling of the large-chirp signal, and then the signal is collected by the collection card.

[0017] The swept-frequency Rayleigh scattering light signal is input to the optical detector 2 for detection and then directly collected by the collection card.

[0018] After the collection card collects the signal, the collected signal is input to the signal separation and demodulation unit for signal demodulation processing and adaptive filtering correction of the signal demodulation processing result.

[0019] As a preferred solution, the chirp pulse modulation module includes a coupler 1, an optical modulator 1, an optical modulator 2, a signal generator, an optical filter 1, an optical filter 2, and a coupler 2.

[0020] The laser emitted by the laser module is split into two paths by the coupler 1, and the two paths of laser are injected into the optical modulator 1 and the optical modulator 2, respectively, the first output channel of the signal generator outputs a chirp pulse electrical signal with a center frequency f1-f0 and a chirp bandwidth B1 to drive the optical modulator 1 to generate a swept-frequency light signal 1, the second output channel of the signal generator outputs a chirp pulse electrical signal with a center frequency f0-f2 and a chirp bandwidth B2 to drive the optical modulator 2 to generate a swept-frequency light signal 2, and the chirp bandwidth B1 is greater than the chirp bandwidth B2.

[0021] The optical filter 1 is used to extract the swept sideband light 1 with the center frequency f1 and the bandwidth B1 from the swept optical signal 1 and input the swept sideband light 1 to the coupler 2, the optical filter 2 is used to extract the swept sideband light 2 with the center frequency f2 and the bandwidth B2 from the swept optical signal 2 and input the swept sideband light 2 to the coupler 2, and the two beams of the swept sideband light signals are coupled into a double-chirp pulse signal containing large and small chirp bands in the spectrum in the coupler 2.

[0022] As a preferred solution, the chirp modulation module further comprises a pulse modulator and an optical amplifier 1.

[0023] The pulse modulator modulates the double-chirp pulse signal containing large and small chirp bands in the spectrum again, the third output channel of the signal generator generates an electrical pulse signal and inputs it into the electrical modulation signal input end of the pulse modulator, the double-chirp pulse signal is intercepted to improve the extinction ratio of the double-chirp pulse signal, and then the double-chirp pulse signal is amplified by the optical amplifier 1 to improve the peak power of the double-chirp pulse signal.

[0024] As a preferred solution, the sensing fiber module comprises a circulator and a sensing fiber, the modulated and amplified double-chirp pulse signal with different center frequencies and bandwidths is input into the circulator, the circulator inputs the modulated and amplified double-chirp pulse signal with different center frequencies and bandwidths into the sensing fiber to excite a Rayleigh scattering signal, the sensing fiber inputs the Rayleigh scattering signal into the circulator, and the circulator outputs the Rayleigh scattering signal to the optical amplifier 2.

[0025] As a preferred solution, the electric field expressions of the Rayleigh scattering signals excited by the double-chirp pulses are respectively:

[0026]

[0027] Wherein, E1(t) is the electric field expression of the large-chirp bandwidth Rayleigh scattering signal with the chirp bandwidth B1, E2(t) is the electric field expression of the small-chirp bandwidth Rayleigh scattering signal with the chirp bandwidth B2, L is the total number of scattering points on the whole sensing fiber, rect(·) represents a rectangular window function, τ p is the pulse width, R(τ i ) is the Rayleigh scattering coefficient of the i th scattering point, τ i is the time delay required for receiving the scattered light of the i th scattering point, V1 is the initial frequency of the large-chirp bandwidth pulse, V2 is the initial frequency of the small-chirp bandwidth pulse, k1 and k2 are the swept rates of the two beams of Rayleigh scattering signals respectively, and E0 is the amplitude of the scattering electric field expression at time 0.

[0028] As a preferred solution, the frequency components of Rayleigh scattering signals excited by the double-chirp pulse signals with different center frequencies and bandwidths are separated in the optical frequency domain, and there is no aliasing.

[0029] As a preferred solution, the detection sensitivity of the double-chirp pulse based optical fiber distributed acoustic wave sensing device is defined by the Cramer-Rao Lower Bound (CRLB), and the conventional CRLB is only applicable to signals sampled with full bandwidth. Since the Rayleigh scattering signal with large chirp bandwidth is sampled with partial bandwidth, an improved CRLB analysis is required, and its expression is as follows:

[0030]

[0031] where d 2 is the power per unit time, β 2 is the effective bandwidth of the Rayleigh scattering signal, SNR is the signal-to-noise ratio of the Rayleigh scattering signal, V0 is the center frequency of the laser, δv is the sweep range of the chirp pulse, B1 is the lower limit frequency of the passband of the electrical filter, B2 is the upper limit frequency of the passband of the electrical filter, and B2-B1 is the passband bandwidth of the electrical filter.

[0032] As a preferred solution, the signals collected by the collector (409) are transmitted to the signal separation and demodulation unit (410) for signal cutting separation and time delay information extraction. The spectral correlation demodulation method is used to extract the time delay information between the Rayleigh scattering signals in the same group. The mapping relationship between the time delay information Δt and the temperature and strain disturbance is as follows:

[0033]

[0034] where Δε is the strain, ΔT is the temperature change information, is the sweep rate, k1 and k2 have the same physical meaning as k1 and k2, and V0 is the center frequency of the laser;

[0035] After being detected and received by the optical detector 1 (406) and the optical detector 2 (407), the AC term spectrum expression of the Rayleigh scattering signal is as follows:

[0036]

[0037] where f is the frequency, I0 is the detector output current amplitude at time 0, Δt is the time delay caused by the disturbance, and R0 is the frequency domain normalized Rayleigh scattering coefficient;

[0038] The expression of the strain disturbance result of the two Rayleigh scattering signals, i.e., the demodulation result, is as follows:

[0039] S1 = Δε + n0

[0040]

[0041] Wherein, S1 is the demodulation result of large-chirp bandwidth Rayleigh scattering signal, S2 is the demodulation result of small-chirp bandwidth Rayleigh scattering signal, n0, n1 are system noise, mainly wideband white noise, 1 / f noise term, mainly concentrated in low frequency band, N is the number of reference curve replacement.

[0042] As a preferred solution, the signal demodulation result needs to be adaptively filtered and corrected in the signal separation and demodulation unit, the demodulation signal S2 with lower system noise is taken as the reference signal d(n) in the adaptive filtering algorithm, and S1 is taken as the signal to be filtered x(n), and the error expression of adaptive filtering correction is:

[0043] e(n) = d(n) - y(n) = d(n) - W(n)x(n) T

[0044] Wherein, W(n) is the filter weight matrix, and y(n) is the result of adaptive filtering correction.

[0045] The cost function is the expectation of the square of the error, that is:

[0046] J(n) = E(e 2 (n))

[0047] The update criterion of the filter weight matrix W(n) is to minimize the cost function J(n), and the fastest descending gradient algorithm is adopted:

[0048]

[0049] The update expression of the adjusted filter weight vector is obtained:

[0050]

[0051] The expression of the final output result of adaptive filtering correction is as follows:

[0052]

[0053] Another aspect of the present application also provides a kind of based on double-chirp pulse optical fiber distributed acoustic wave sensing method, it is applied to the above technical solution in a kind of based on double-chirp pulse optical fiber distributed acoustic wave sensing device, the method steps are as follows:

[0054] Laser module outputs laser to the coupler 1 of chirp pulse modulation module, is divided into two-way signal after being coupled, is injected into light modulator 1 and light modulator 2 respectively;

[0055] The first output channel of the signal generator outputs a large-chirp pulse electrical signal to drive the optical modulator 1 to generate a large-chirp swept-frequency optical signal 1, and the second output channel of the signal generator outputs a small-chirp pulse electrical signal to drive the optical modulator 4 to output a small-chirp swept-frequency optical signal 2;

[0056] The large-chirp swept-frequency optical signal 1 and the small-chirp swept-frequency optical signal 2 are respectively input into the optical filter 1 and the optical filter 2 to filter out high-order sidebands and carrier signals generated in the process of outputting the swept-frequency optical signal by the optical modulator, and the two chirped optical pulse signals after filtering are simultaneously input into the coupler 2 for coupling to obtain a double-chirp pulse signal combining the large-chirp frequency bandwidth and the small-chirp frequency bandwidth;

[0057] The third output channel of the signal generator outputs an electrical pulse signal to control the switching timing of the pulse modulator to intercept and modulate the double-chirp pulse signal to improve the extinction ratio of the double-chirp pulse signal, and then the modulated double-chirp pulse signal is input into the optical amplifier 1 for amplification;

[0058] The amplified double-chirp pulse signal is input into the sensing optical fiber through the circulator in the sensing optical fiber module to excite a Rayleigh scattering signal, and the excited Rayleigh scattering signal is input into the optical amplifier 2 in the Rayleigh scattering signal separation and receiving module through the circulator;

[0059] The Rayleigh scattering signal amplified by the optical amplifier 2 is input into the optical filter 3, the optical filter 3 extracts a large-chirp bandwidth Rayleigh scattering signal, the large-chirp bandwidth Rayleigh scattering signal is output to the optical detector 1 through the reflected light output end of the optical filter 3, and the remaining signal components after extracting the large-chirp bandwidth Rayleigh scattering signal are output to the optical filter 4 through the transmitted light output end of the optical filter 3 to extract a small-chirp bandwidth Rayleigh scattering signal, and the small-chirp bandwidth Rayleigh scattering signal is output to the optical detector 2 through the reflected light output end of the optical filter 4;

[0060] When the large-chirp bandwidth Rayleigh scattering signal and the small-chirp bandwidth Rayleigh scattering signal are respectively input into the optical detector 1 and the optical detector 2, signal self-beat interference occurs and is converted into an analog electrical signal, and the large-chirp bandwidth Rayleigh scattering analog electrical signal is partially bandwidth sampled and filtered by the electrical filter, and then the large-chirp bandwidth Rayleigh scattering analog electrical signal and the small-chirp bandwidth Rayleigh scattering analog electrical signal are respectively input into the collector for signal collection;

[0061] The signal collected by the collector is transmitted to a signal separation and demodulation unit for signal demodulation processing, the Rayleigh scattering signals of each group are demodulated respectively, the time offset between different Rayleigh scattering curves is extracted by using a spectrum correlation algorithm, then disturbance information is quantitatively recovered according to the mapping relationship between the time delay information Δt and the temperature and strain disturbance, and the demodulation result is obtained, an adaptive filtering algorithm is used for adaptive filtering correction of the large and small chirp pulse signals, and the demodulation result of the small chirp bandwidth Rayleigh scattering signal is used as a reference signal to correct the sensitivity degradation of the demodulation result of the large chirp bandwidth Rayleigh scattering signal caused by partial bandwidth sampling.

[0062] Compared with the prior art, the beneficial effects of the present application are as follows:

[0063] The present application uses two different frequency bands and different chirp rate pulse interrogation sensing optical fibers, two kinds of Rayleigh scattering signals excited can realize large range and high sensitivity respectively, the adaptive filtering algorithm is used to combine the advantages of the two, which can improve the dynamic range of distributed acoustic wave detection by an order of magnitude, and only in the way of low sampling rate realizes the collection of large chirp bandwidth Rayleigh scattering signal, without the need to increase the hardware complexity of the system and the requirements of the signal acquisition end, has advantages in acquisition cost and signal real-time processing, realizes the distributed acoustic wave sensing technology with large dynamic range and low cost. BRIEF DESCRIPTION OF DRAWINGS

[0064] Figure 1 It is a structure schematic diagram of a distributed acoustic wave sensing device based on double-chirp pulse of the present application.

[0065] Figure 2 It is a schematic diagram of the time-frequency mapping relationship of the double-chirp pulse signal of the present application.

[0066] Figure 3 It is a schematic diagram of the working principle of the double-chirp pulse signal of the embodiment of the present application.

[0067] Figure 4 It is a separation schematic diagram of the double-chirp pulse Rayleigh scattering signal of the embodiment of the present application.

[0068] Figure 5 It is a schematic diagram of the adaptive filtering correction method of the present application.

[0069] Figure 6 It is a schematic diagram of the adaptive filtering correction demodulation result of the embodiment of the present application.

[0070] BRIEF DESCRIPTION OF DRAWINGS:

[0071] 1-laser module, 101-narrow linewidth laser, 101a-output end of narrow linewidth laser; 2-chirped pulse modulation module, 201-coupler 1, 201a-input end of coupler 1, 202-optical modulator 1, 202a-input end of optical modulator 1, 202b-signal light output end of optical modulator 1, 202c-radio frequency input end of optical modulator 1, 203-optical modulator 2, 203a-input end of optical modulator 2, 203b-signal light output end of optical modulator 2, 203c-radio frequency input end of optical modulator 2, 204-signal generator, 204a-first output channel of signal generator, 204b-second output channel of signal generator, 204c-third output channel of signal generator, 205-swept frequency optical signal 1, 206-swept frequency optical signal 2, 207-optical filter 1, 207a-input end of optical filter 1, 207b-output end of optical filter 1, 208-optical filter 2, 208a-input end of optical filter 2, 208b-output end of optical filter 2, 209-coupler 2, 210-pulse modulator, 210a-input end of pulse modulator, 210b-output end of pulse modulator, 210c-electric modulation signal input end of pulse modulator, 211-dual-chirped pulse signal containing large and small chirp bands in spectrum, 212-optical amplifier 1, 212a-input end of optical amplifier 1, 212b-output end of optical amplifier 1; 3-sensing fiber module, 301-circulator, 301a-input end of circulator, 301b-output end of circulator and Rayleigh scattering signal return end, 301c-Rayleigh scattering signal output end of circulator; 4-Rayleigh scattering signal separation and receiving module, 401-optical amplifier 2, 401a-input end of optical amplifier, 401b-output end of optical amplifier, 402-optical filter 3, 402a-input end of optical filter 3, 402b-reflected light output end of optical filter 3, 402c-transmitted light output end of optical filter 3, 403-optical filter 4, 403a-input end of optical filter 4, 403b-reflected light output end of optical filter 4, 404-swept frequency Rayleigh scattering optical signal 1, 405-swept frequency Rayleigh scattering optical signal 2, 406-optical detector 1, 406a-input end of optical detector 1, 406b-radio frequency signal output end of optical detector 1, 407-optical detector 2, 407a-input end of optical detector 2, 407b-radio frequency signal output end of optical detector 2, 408-electric filter, 408a-input end of electric filter, 408b-output end of electric filter, 409-acquisition card, 409a-ADC acquisition channel 1 of acquisition card, 409b-ADC acquisition channel 2 of acquisition card, 409c-output end of acquisition card, 410-signal separation and demodulation unit, 410a-input end of signal separation and demodulation unit. DETAILED DESCRIPTION

[0072] In order to enable a more complete understanding of the above-mentioned objects, features and advantages of the present application, the application will be described in further detail below in conjunction with the accompanying drawings and specific embodiments. It should be noted that the embodiments of the present application and the features in the embodiments can be combined with each other without conflict.

[0073] In the following description, a large number of specific details are set forth in order to facilitate a thorough understanding of the present application, however, the present application can also be implemented in other ways different from those described herein, and therefore, the scope of protection of the present application is not limited by the specific embodiments disclosed below.

[0074] Embodiment 1

[0075] Please refer to Figure 1 Embodiment 1 of the present application provides a kind of based on double-chirp pulse's optical fiber distributed acoustic wave sensing device, comprising:

[0076] Laser module 1, chirp pulse modulation module 2, sensing optical fiber module 3 and Rayleigh scattering signal separation and receiving module 4.

[0077] In the present embodiment 1, laser module 1 includes narrow line width fiber laser 101, wherein the center wavelength 1550nm, line width 100Hz, output power is 16dBm.

[0078] Chirp pulse modulation module 2 includes: 3dB coupler 1 (201), optical modulator 1 (202), electro-optic modulator 2 (203), signal generator 204, optical filter 1 (207), optical filter 2 (208), 3dB coupler 2 (209), pulse modulator 210 and optical amplifier 1 (212).

[0079] Wherein, optical modulator 1 (202) and optical modulator 2 (203) are single sideband IQ modulators, and the operating wavelength is 1550nm, and the bandwidth is 20GHz.Both optical filter 1 (207) and optical filter 2 (208) are adjustable FBG with 8GHz bandwidth, and the operating center wavelength is 1550nm, and the insertion loss is ≤1dB, and they are mainly used to filter out additional band signals.In the present embodiment 1, acoustooptic modulator AOM is used as pulse modulator 210, and the insertion loss is 3dB, and the switch extinction ratio is 50dB. Optical amplifier 1 (212) is an erbium-doped fiber amplifier.

[0080] Sensing optical fiber module 3 includes circulator 301 and sensing optical fiber 302, wherein the sensing optical fiber 302 is 25km long, and the corresponding light flight time in the optical fiber is 250us.

[0081] The Rayleigh scattering signal separation and receiving module 4 comprises an optical amplifier 2 (401), an optical filter 3 (402), an optical filter 4 (403), an optical detector 1 (406), an optical detector 2 (407), an electrical filter 408, a collection card 409, and a signal separation and demodulation unit 410.

[0082] The optical amplifier 2 (401) is an erbium-doped optical fiber amplifier. The optical filter 3 (402) and the optical filter 4 (403) are both 8GHz bandwidth adjustable FBGs with a working center wavelength of 1550nm and an insertion loss of ≤1dB, and are mainly used for filtering out extra-band signals. The bandwidths of the optical detector 1 (406) and the optical detector 2 (407) are both 1GHz. The electrical filter 408 is a low-pass electrical filter with a bandwidth of 500MHz. The sampling rate of the collection card 409 is 1G Sa / s, and the collection bandwidth is 500MHz.

[0083] The output end 101a of the narrow linewidth laser 101 is connected to the input end 201a of the 3dB coupler 201, and the outputs of the 3dB coupler are connected to the input end 202a of the optical modulator 1 and the input end 203a of the optical modulator 2, respectively.

[0084] The first and second output channels 204a, 204b of the signal generator 204 are connected to the radio frequency input end 202c of the optical modulator 1 and the radio frequency input end 203c of the optical modulator 2, respectively. The signal light output end 202b of the optical modulator 1 is connected to the input end 207a of the optical filter 1, and the signal light output end 203b of the optical modulator 2 is connected to the input end 208a of the optical filter 2. The output ends 207b, 208b of the optical filter 1 and the optical filter 2 are connected to the 3dB coupler 209 for beam combination, and the output thereof is connected to the input end 210a of the pulse modulator 210. The third output channel 204c of the signal generator 204 is connected to the electrical modulation signal input end 210c of the pulse modulator 210. The output end 210b of the pulse modulator is connected to the input end 212a of the optical amplifier 1, and the output end 212b of the optical amplifier 1 is connected to the input port 301a of the circulator 301.

[0085] The port 301b of the circulator is connected to the port 302a of the sensing optical fiber. The port 301b of the circulator injects the double-chirped pulse signal to the port 302a of the sensing optical fiber, and the port 302a of the sensing optical fiber returns the Rayleigh scattering to the circulator. The Rayleigh scattering signal is finally injected to the input port 401a of the optical amplifier 2 through the port 301c of the circulator.

[0086] The output port 401b of the optical amplifier 2 is connected to the input port 402a of the optical filter 3, the reflected light output port 402b is connected to the input port 406a of the photodetector 1, the radio frequency signal output port 406b of the photodetector 1 is connected to the input port 408a of the electrical filter 408, and the output port 408b of the electrical filter is connected to the acquisition channel 1 409a of the acquisition card ADC. The transmitted light output port 402c of the optical filter 3 is connected to the input port 403a of the optical filter 4. The reflected light output port 403b of the optical filter 4 is connected to the input port 407a of the photodetector 2, and the radio frequency signal output port 407b of the photodetector 2 is connected to the acquisition channel 2 409b of the acquisition card ADC. The digital signal collected by the acquisition card is transmitted to the signal separation and demodulation unit 410 through the link composed of the output port 409c of the acquisition card and the input port 410a of the signal separation and demodulation unit.

[0087] The working process of the double-chirp pulse-based optical fiber distributed acoustic wave sensing device of the embodiment 1 is as follows:

[0088] The narrow linewidth laser 1 outputs laser with a wavelength of 1550 nm, a linewidth of 100 Hz, and an amplitude of 16 dBm. The laser is split into two paths by the 3dB coupler 202 and injected into the optical modulator 1 and the optical modulator 2.

[0089] The signal generator outputs a chirp pulse electrical signal with a center frequency of (f1-f0) and a chirp bandwidth of B1 from the port 204a to drive the optical modulator 1 to generate a swept frequency optical signal 205. The signal generator outputs a chirp pulse electrical signal with a center frequency of (f0-f2) and a chirp bandwidth of B2 from the port 204b to drive the optical modulator 2 to generate a swept frequency optical signal 206 (B1>B2). The optical filter 1 is used to extract the swept frequency sideband light with a center frequency of f1 and a bandwidth of B1, and the extracted light is connected to the 3dB coupler 209 through the reflected light signal output port 207b. The optical filter 2 is used to extract the swept frequency sideband light with a center frequency of f2 and a bandwidth of B2, and the extracted light is connected to the 3dB coupler 209 through the reflected light signal output port 208b. The two beams of swept frequency sideband light signals are coupled into one optical pulse in the coupler 209, and the frequency spectrum of the optical pulse is shown in 211. Please refer to Figure 2 and Figure 3 , Figure 2 is a schematic diagram of the time-frequency mapping relationship of the coupled double-chirp pulse signal, and one pulse contains two chirp bands with different center frequencies and bandwidths, Figure 3is the working principle diagram of the double-chirp pulse signal. The pulse modulator 210 modulates the coupled double-chirp pulse light again. The output port 204c of the signal generator 204 generates an electrical pulse signal, which is injected into the modulation control port 210c of the pulse modulator, and the double-chirp pulse signal is intercepted to improve the extinction ratio of the double-chirp pulse signal. The double-chirp pulse signal intercepted by the pulse modulator is amplified by the optical amplifier 1 to improve the pulse peak power.

[0090] The output end 212b of the optical amplifier 1 is connected to the 301a port of the circulator, and the double-chirp pulse signal is injected into the sensing optical fiber by the 301b port after passing through the circulator. The double-chirp pulse signal excites a double-chirp pulse Rayleigh scattering light signal, and the double-chirp pulse Rayleigh scattering signal returns to the circulator and is injected into the optical amplifier 2 through the 301c port of the circulator for gain amplification.

[0091] Please refer to Figure 4 , Figure 4 is the separation diagram of the double-chirp pulse Rayleigh scattering signal. The Rayleigh scattering light signals excited by the two chirp pulses with different sweep bandwidths are separated in the optical frequency domain, and there is no aliasing. Therefore, the two chirp Rayleigh scattering lights can be separated by using an optical filter. After amplification, the Rayleigh scattering light signal enters the optical filter 3 to extract the sweep Rayleigh scattering light signal 1 with a central frequency f1 and a bandwidth B1. The sweep Rayleigh scattering light signal 1 output by the reflection light output port 402b of the optical filter 3 is the extracted frequency band component, and is connected to the optical detector 1. The output port 406b of the optical detector 1 is connected to the input end 408a of the low-pass electrical filter 408, realizing partial bandwidth sampling extraction of the large-chirp signal. The output end 408b of the electrical filter is connected to the ADC channel 409a of the acquisition card. The transmission light output end 402c of the optical filter 3 is connected to the input port 403a of the optical filter 4, and the sweep Rayleigh scattering light signal 2 with a frequency f2 and a bandwidth B2 is extracted. The sweep Rayleigh scattering light signal 2 output by the reflection light output port 403b of the optical filter 4 is connected to the optical detector 2. Since the Rayleigh scattering signal is a small-chirp bandwidth signal, full-bandwidth sampling is required, and the output end 407b of the optical detector 2 does not need to be additionally connected to a low-pass electrical filter. The radio frequency signal output end 407b of the optical detector 2 is connected to the ADC acquisition channel 409b of the acquisition card. The acquisition card transmits digital signals to the signal separation and demodulation unit through the link formed by the 409c port and the 410a port. In this device, the measurement range of the system is improved by combining two chirp bandwidth pulse interrogation sensing optical fibers, and the hardware pressure of the system acquisition end is not increased too much.

[0092] The frequency components of the Rayleigh scattering signals excited by the double-chirp pulse signals with different center frequencies and bandwidths are separated in the optical frequency domain, and there is no aliasing. The expressions of the Rayleigh scattering electric fields excited by the double-chirp pulses of the device are respectively:

[0093]

[0094] wherein E1(t) is the electric field expression of the Rayleigh scattering signal with a large chirp bandwidth B1, E2(t) is the electric field expression of the Rayleigh scattering signal with a small chirp bandwidth B2, L is the total number of scattering points on the whole sensing optical fiber, rect(·) represents a rectangular window function, τ p is a pulse width, R(τ i ) is the Rayleigh scattering coefficient of the i-th scattering point, τ i is the time delay required for receiving the scattered light of the i-th scattering point, V1 is the initial frequency of the large chirp bandwidth pulse, V2 is the initial frequency of the small chirp bandwidth pulse, k1 and k2 are the sweep rates of the two Rayleigh scattering signals respectively, and E0 is the amplitude of the scattering electric field expression at time 0.

[0095] Since the starting frequencies of the two chirp pulse lights have a certain frequency interval and the frequency bandwidths occupied have no overlapping part, the backward Rayleigh scattering signals excited by the large and small chirp pulses can be separated by an optical filter. The optical filter 3 is used to extract the large chirp bandwidth Rayleigh scattering light signal, and the optical filter is essentially composed of a fiber grating, which reflects light of a specific wavelength to complete the extraction of the target frequency band light signal. After the returned mixed Rayleigh scattering light is extracted by the large chirp bandwidth component of the optical filter 3, the transmitted light is connected to the optical filter 4 for extracting the small chirp bandwidth Rayleigh scattering light signal. In addition to extracting the signal light of a specific frequency band, the other function of the optical filter 3 and the optical filter 4 is to filter out the ASE noise introduced during the amplification of the optical amplifier 2, thereby improving the signal quality. After being collected by the collection card 409, the signal is transmitted to the signal separation and demodulation unit 410 for signal cutting separation and time delay information extraction. The time delay information is extracted by the spectral correlation demodulation method to obtain the time delay information between the Rayleigh scattering signals in the same group. The mapping relationship between the time delay information and the temperature and strain disturbance is:

[0096]

[0097] wherein Δε is the strain, ΔT is the temperature change information, k is the sweep rate, which is consistent with the physical meaning of k1 and k2, and V0 is the center frequency of the laser.

[0098] After being received by the optical detector 403, the spectral alternating term expression of the Rayleigh scattering signal in the device is:

[0099]

[0100] wherein f is frequency, I0 is the amplitude of the detector output current at time 0, Δt is the time delay caused by the disturbance, and R0 is the Rayleigh scattering coefficient normalized in the frequency domain. As can be seen from equation (3), the time delay Δt caused by the disturbance has no mutual influence with the frequency f, and therefore, the external low-pass electrical filter 408 connected to the optical detector 406 does not change the disturbance information, which can be used as a basis to reduce the bandwidth and sampling rate required for signal acquisition, and to achieve low-cost improvement of the upper limit of system detection.

[0101] The detection sensitivity of the device is defined by the Cramer-Rao Lower Bound (CRLB). The conventional CRLB is only applicable to signals collected in the full bandwidth, and for the above-mentioned partial bandwidth collection, an improved CRLB needs to be analyzed, and its expression is as follows:

[0102]

[0103] wherein d 2 is the power per unit time, β 2 is the effective bandwidth of the Rayleigh scattering signal, SNR is the signal-to-noise ratio of the Rayleigh scattering signal, V0 is the center frequency of the laser, δv is the sweep range of the chirp pulse, B1 is the lower limit frequency of the passband of the electrical filter, B2 is the upper limit frequency of the passband of the electrical filter, and B2-B1 is the passband bandwidth of the electrical filter. As can be seen from equation (4), after partial bandwidth energy collection of the large-chirp bandwidth signal, the sensitivity is lost, and the scheme of using a small-chirp signal with a bandwidth equal to that of the low-pass electrical filter for frequency band multiplexing is equivalent to adding a set of Rayleigh scattering light signals collected in the full bandwidth in the system, and this signal does not have a loss in sensitivity, which can make up for the insufficient sensitivity of the large-chirp bandwidth signal after low-bandwidth and low-sampling-rate collection, and make the system have stronger ability to detect small disturbance signals. Due to the different advantages of the two signals, in the overlapping area of the detection ranges of the two chirp signals, an adaptive filtering method can be used to combine the advantages of the two signals, so that the system can not only measure large strains, but also has high detection sensitivity.

[0104] The strain disturbance results of the two Rayleigh scattering signals are as follows:

[0105]

[0106] wherein S1 is the demodulation result of the large-chirp bandwidth Rayleigh scattering signal, S2 is the demodulation result of the small-chirp bandwidth Rayleigh scattering signal, n0 and n1 are system noise, mainly wideband white noise, is a 1 / f noise term, mainly concentrated in the low frequency band, and N is the number of times of replacing the reference curve. Due to its limited range, the use of a replacement reference curve when demodulating large strains will introduce a 1 / f noise term Mainly concentrated in the low frequency band. Please refer to Figure 5 , Figure 5 Figure 1 is a schematic diagram of the adaptive filter correction method of the present embodiment 1. When performing adaptive filter correction, the demodulation signal S2 with low system noise is used as the reference signal d(n) in the adaptive filter algorithm, and S1 is used as the signal to be filtered x(n). The error expression of adaptive filter correction is:

[0107] e(n) = d(n) - y(n) = d(n) - W(n)x(n) T (6)

[0108] Where W(n) is the filter weight matrix, and y(n) is the result of adaptive filter correction.

[0109] The cost function is the expectation of the square of the error, defined as:

[0110] J(n) = E(e 2 (n)) (7)

[0111] The update criterion of the filter weight matrix W(n) is to minimize the cost function J(n), and the fastest descent gradient algorithm is adopted:

[0112]

[0113] The update expression of the adjusted filter weight vector is obtained:

[0114]

[0115] The expression of the final output result of adaptive filter correction is as follows:

[0116]

[0117] Since the two groups of signals are measured for the same vibration, Δε is correlated, and the noise is independent. When the system converges, as shown in equation (10), the output y(n) will be more close to the true strain change Δε, so as to achieve the effect of noise removal and sensitivity correction. By using the above technical solutions, the modulation mode of double-chirp pulse is adopted, without the need to increase the complexity of the system too much, so as to realize the improvement of the detection dynamic range of the system.

[0118] Embodiment 2

[0119] The present embodiment 2 provides a kind of optical fiber distributed acoustic wave sensing method based on double chirp pulse, applied to the optical fiber distributed acoustic wave sensing device based on double chirp pulse in embodiment 1, it include the following steps:

[0120] S1: the single frequency continuous light output by the narrow line width fiber laser 101 is divided into two single frequency light signals by the 3dB coupler 202 in module 2, and is injected into the optical modulator 202 and the optical modulator 203 respectively.

[0121] S2: the first output channel 204a of the signal generator 204 outputs a large chirp pulse electrical signal with a pulse width of 100ns, a sweep starting frequency of 10GHz and a sweep bandwidth of 8GHz, and drives the optical modulator 1 to generate a large chirp sweep optical signal 205; the second output channel 204b of the signal generator 204 outputs a small chirp pulse electrical signal with a pulse width of 100ns, a sweep starting frequency of 10GHz and a sweep bandwidth of 500MHz, and drives the optical modulator 2 to output a small chirp sweep optical signal 206; in order to facilitate subsequent separation, the optical modulator 1 is set to a lower sideband modulation mode, and the optical modulator 2 is set to an upper sideband modulation mode, so that the frequency bands of the two chirp pulse lights have a large interval, avoiding the aliasing between the chirp pulses.

[0122] S3: the large chirp sweep optical signal 205 and the small chirp sweep optical signal 206 are input into the optical filter 1 and the optical filter 2 respectively to filter out the high-order sidebands and carrier signals generated in the process of outputting the sweep optical signal by the optical modulator, and the two chirp optical pulse signals after filtering are coupled into the coupler 209 at the same time to obtain a double chirp pulse signal 211 combining the large and small chirp frequency bandwidths together, please refer to Figure 2 and Figure 3 , Figure 2 is a time-frequency mapping relationship diagram of the coupled double chirp pulse signal, and one pulse contains two chirp frequency bands with different center frequencies and bandwidths, Figure 3 is a schematic diagram of the working principle of the double chirp pulse signal.

[0123] S4: the third output channel 204c of the signal generator outputs an electrical pulse signal with a repetition frequency of 1kHz and a pulse width of 100ns to control the switching timing of the pulse modulator AOM to intercept and modulate the double chirp pulse signal to improve the extinction ratio of the double chirp pulse signal, and then the modulated double chirp pulse signal is input into the optical amplifier 1 for amplification, and the peak power of the chirp pulse is amplified to 150mW.

[0124] S5: the amplified double chirp pulse signal is input into the 25km sensing optical fiber 302 through the circulator 301 in the sensing optical fiber module 3 to excite Rayleigh scattering signals, and the excited Rayleigh scattering signals are input into the optical amplifier 2 in the Rayleigh scattering signal separation and receiving module 4 through the circulator 301.

[0125] S6: The Rayleigh scattering signal amplified by the optical amplifier 2 is input into the optical filter 3, the optical filter 3 extracts the large chirp bandwidth Rayleigh scattering signal, the large chirp bandwidth Rayleigh scattering signal is output to the optical detector 1 through the reflected light output end 402b of the optical filter 3, and the remaining signal components after the large chirp bandwidth Rayleigh scattering signal is extracted are output to the optical filter 4 through the transmitted light output end 402c of the optical filter 3 to extract the small chirp bandwidth Rayleigh scattering signal, the small chirp bandwidth Rayleigh scattering signal is output to the optical detector 2 through the reflected light output end 403b of the optical filter 4, and the main functions of the optical filters 3 and 4 are to extract the large and small chirp bandwidth Rayleigh scattering signals and suppress the ASE noise.

[0126] S7: When the large and small chirp bandwidth Rayleigh scattering signals are input into the optical detectors 1 and 2 respectively, signal self-beat frequency interference occurs and is converted into analog electrical signals, the Rayleigh scattering signals detected by the optical detectors will present a right-angled triangle form in the frequency spectrum, the starting frequency is 0 Hz, the terminal frequency point is the same as the bandwidth of the chirp pulse, and the analog electrical signals of the large chirp bandwidth Rayleigh scattering signal are filtered by the 500MHz bandwidth low-pass electrical filter to perform partial bandwidth sampling, so that the large chirp bandwidth Rayleigh scattering signal loses part of the energy of the frequency band, while the small chirp bandwidth Rayleigh scattering signal does not cause signal frequency spectrum aliasing and energy loss under the condition of sampling at 500MHz bandwidth, and finally the two analog electrical signals are collected by the collection card 409 at a sampling rate of 500MHz bandwidth and 1GSa / s.

[0127] S8: The signals collected by the collection card 409 are transmitted to the signal separation and demodulation unit 410 for signal demodulation processing:

[0128] Please refer to Figure 4 , the Rayleigh scattering signals excited by the double-chirp pulses have been separated by using the optical filters 3 and 4 before entering the optical detectors 1 and 2, the collection card 409 uses two ADC channels to collect and extract the large and small chirp bandwidth Rayleigh scattering signals excited by the two chirp pulses, the large chirp bandwidth Rayleigh scattering signal with a chirp bandwidth of 8GHz is trace1_1... trace1_n, and the small chirp bandwidth Rayleigh scattering signal with a chirp bandwidth of 500MHz is trace2_1... trace2_n;

[0129] Each group of Rayleigh scattering signals is demodulated, the spectral correlation algorithm is used to extract the time offset between different Rayleigh scattering curves, and then the disturbance information is quantitatively recovered according to the mapping relationship between the time delay information Δt and the temperature and strain disturbance to obtain the demodulation result;

[0130] Please refer to Figure 5, the adaptive filtering algorithm is used for adaptive filtering correction of the large and small chirp pulse signals, and the Rayleigh scattering signal demodulation result with a chirp bandwidth of 500 MHz is taken as a reference signal for correcting the sensitivity degradation of the Rayleigh scattering signal demodulation result with a chirp bandwidth of 8 GHz caused by partial bandwidth sampling. The filter output result is shown in Figure 6

[0131] The other technical implementation details of this embodiment 2 are the same as those of embodiment 1, and will not be described here.

[0132] Embodiment 3

[0133] This embodiment 3 is based on embodiment 1, and part of the hardware structure of the fiber distributed acoustic sensing device based on double-chirp pulses is replaced so as to be applicable to different environments or change the hardware cost. Specifically as follows:

[0134] The narrow linewidth fiber laser is replaced by a narrow linewidth semiconductor laser, the single sideband IQ modulator is replaced by a double sideband intensity modulator, the signal generator is replaced by a combination of a voltage-controlled oscillator and a controllable voltage source, the pulse modulator is replaced by a semiconductor optical amplifier, and the detector is replaced by a low-bandwidth low-noise detector, and the acquisition card is replaced by an oscilloscope.

[0135] The technical implementation details of this embodiment 3 are the same as those of embodiment 1, and will not be described here.

[0136] It should be noted that each replacement hardware structure in embodiment 3 can also be combined with other original hardware structures in embodiment 1 for use.

[0137] In summary of the above embodiments, the embodiments of the present application propose to interrogate the sensing fiber by double-chirp pulses, combine the sensitivity advantage of the small chirp bandwidth range full-bandwidth signal and the detection upper limit increase caused by the large chirp bandwidth range partial-bandwidth signal, realize the dynamic range improvement of the system, and can be adjusted according to the requirements, use multiple chirp pulses and flexibly adjust the bandwidth of the chirp pulse, further expand the dynamic range of the system, at the same time, the scheme of externally connecting a low-pass filter to the back end of the optical detector for partial bandwidth sampling realizes the low-cost detection of the large chirp signal, and when the chirp bandwidth of the detected signal is increased, the hardware complexity of the system and the requirements of the signal acquisition end do not need to be increased, and the detection upper limit of the distributed fiber acoustic sensing device can be flexibly expanded.

[0138] ​The components in the above embodiments can be commercial products, and the connection relationship and implementation principle are protected by the present application, and thus the model of each product is not limited. The position relationship described in the drawings is only used for illustrative purposes, and should not be construed as a limitation of the present application. The above embodiments of the present application are only examples for clearly illustrating the present application, and should not be construed as a limitation of the embodiments of the present application. Based on the above description, other different forms of changes or variations can be made by those skilled in the art. All the embodiments should not be enumerated here. Any modification, equivalent replacement, improvement, etc. within the spirit and principle of the present application should be included in the protection scope of the claims of the present application.

Claims

1. A dual-chirped pulse based optical fiber distributed acoustic sensing apparatus, characterized in that, The laser module (1), the chirped pulse modulation module (2), the sensing fiber module (3) and the Rayleigh scattering signal separation and receiving module (4) are included. The laser generated by the laser module (1) is modulated and amplified by the chirped pulse modulation module (2) to obtain a double-chirped pulse signal with different center frequencies and bandwidths, the double-chirped pulse signal excites a Rayleigh scattering signal in the sensing fiber module (3) and inputs the Rayleigh scattering signal into the Rayleigh scattering signal separation and receiving module (4); The Rayleigh scattering signal separation and receiving module (4) includes an optical amplifier two (401), an optical filter three (402), an optical filter four (403), an optical detector one (406), an optical detector two (407), an electrical filter (408), a collection card (409) and a signal separation and demodulation unit (410); The Rayleigh scattering signal is input into the optical filter three (402) after being amplified by the optical amplifier two (401) to extract the center frequency f1 and the bandwidth B1 of the swept Rayleigh scattering light signal one (404) B The Rayleigh scattering signal is input into the optical filter four (403) after the swept Rayleigh scattering light signal one (404) is extracted by the optical filter three (402) to extract the frequency f2 and the bandwidth B2 of the swept Rayleigh scattering light signal two (405), and the bandwidth B1 is greater than the bandwidth B2. The swept frequency Rayleigh scattering optical signal one (404) is input into the optical detector one (406) for detection, then input into the electrical filter (408) for partial bandwidth sampling of the large-chirp bandwidth signal, and then the signal is collected by the collection card (409); The swept frequency Rayleigh scattering optical signal two (405) is input into the optical detector two (407) for detection, and then the signal is collected by the collection card (409); After the signal is collected by the collection card (409), the collected signal is input into the signal separation and demodulation unit (410) for signal demodulation processing, and the result of the signal demodulation processing is adaptively filtered and corrected; The chirped pulse modulation module (2) includes a coupler one (201), an optical modulator one (202), an optical modulator two (203), a signal generator (204), an optical filter one (207), an optical filter two (208) and a coupler two (209); The laser emitted by the laser module (1) is split into two lasers by the coupler one (201), and the two lasers are injected into the optical modulator one (202) and the optical modulator two (203) respectively, the first output channel (204a) of the signal generator outputs a chirped pulse electrical signal with a center frequency of ƒ1-ƒ0 and a chirp bandwidth of B1 to drive the optical modulator one (202) to generate a swept frequency optical signal one (205), the second output channel (204b) of the signal generator outputs a chirped pulse electrical signal with a center frequency of ƒ0-ƒ2 and a chirp bandwidth of B2 to drive the optical modulator two (203) to generate a swept frequency optical signal two (206), and the chirp bandwidth B1 is greater than the chirp bandwidth B2. The optical filter one (207) is used for extracting a swept sideband light one with a center frequency of f1 and a bandwidth of B1 from the swept optical signal one (205) and inputting the swept sideband light one into the coupler two (209), the optical filter two (208) is used for extracting a swept sideband light two with a center frequency of f2 and a bandwidth of B2 from the swept optical signal two (206) and inputting the swept sideband light two into the coupler two (209), and the two swept sideband light signals are coupled into a double-chirp pulse signal (211) containing large and small chirp bands in a spectrum in the coupler two (209); The chirp pulse modulation module (2) further comprises a pulse modulator (210) and an optical amplifier one (212); The pulse modulator (210) modulates the double-chirp pulse signal containing large and small chirp bands in a spectrum again, the third output channel (204c) of the signal generator (204) generates an electrical pulse signal and inputs the electrical pulse signal into an electrical modulation signal input end (210c) of the pulse modulator (210), the double-chirp pulse signal is intercepted to improve the extinction ratio of the double-chirp pulse signal, and then the double-chirp pulse signal is amplified by the optical amplifier one (212) to improve the peak power of the double-chirp pulse signal.

2. The dual-chirped pulse based optical fiber distributed acoustic sensing apparatus according to claim 1, wherein, The sensing optical fiber module (3) comprises a circulator (301) and a sensing optical fiber (302), the optical amplifier one (212) inputs the modulated and amplified double-chirp pulse signal with different center frequencies and bandwidths into the circulator (301), the circulator (301) inputs the modulated and amplified double-chirp pulse signal with different center frequencies and bandwidths into the sensing optical fiber (302) to excite a Rayleigh scattering signal, the sensing optical fiber (302) inputs the Rayleigh scattering signal into the circulator (301), and the circulator (301) outputs the Rayleigh scattering signal to the optical amplifier two (401).

3. The dual-chirped pulse based optical fiber distributed acoustic sensing apparatus of claim 1, wherein, The electric field expressions of the Rayleigh scattering signals excited by the double-chirp pulse are respectively: where, is the electric field expression of the large-chirp bandwidth Rayleigh scattering signal with the chirp bandwidth B1, is the electric field expression of the small-chirp bandwidth Rayleigh scattering signal with the chirp bandwidth B2, is the total number of scattering points on the whole sensing fiber, represents the rectangular window function, is the pulse width, is the Rayleigh scattering coefficient of the ith scattering point, is the time delay required for receiving the scattered light from the ith scattering point, is the initial frequency of the large-chirp bandwidth pulse, is the initial frequency of the small-chirp bandwidth pulse, and are the sweep rates of the two Rayleigh scattering signals, respectively, is the amplitude of the scattering electric field expression at time 0.

4. The dual-chirped pulse based optical fiber distributed acoustic sensing apparatus of claim 1, wherein, The frequency components of the Rayleigh scattering signals excited by the double-chirp pulse with different center frequencies and bandwidths are separated in the optical frequency domain, and there is no aliasing.

5. The dual-chirped pulse based optical fiber distributed acoustic sensing apparatus of claim 3, wherein, The detection sensitivity of the double-chirp pulse-based optical fiber distributed acoustic sensing device is defined by the Cramer-Rao lower bound (CRLB), and the conventional CRLB is only applicable to signals sampled with full bandwidth. Since the large-chirp bandwidth Rayleigh scattering signal is sampled with partial bandwidth, an improved CRLB analysis is required, and the expression is: wherein, P is the power in a unit of time, B is the effective bandwidth of the Rayleigh scattering signal, S is the signal-to-noise ratio of the Rayleigh scattering signal, f is the center frequency of the laser, f is the sweep range of the chirped pulse, B 1 is the lower limit frequency of the passband of the electrical filter, B 2 is the upper limit frequency of the passband of the electrical filter, B 2- B 1 is the passband bandwidth of the electrical filter.

6. The dual-chirped pulse based optical fiber distributed acoustic sensing apparatus of claim 5, wherein, The signal collected by the collection card (409) is transmitted to the signal separation and demodulation unit (410) to perform signal cutting separation and time delay information extraction. The spectrum correlation demodulation method is used to extract the time delay information between the Rayleigh scattering signals in the same group. The time delay information The mapping relationship between the temperature and the strain disturbance is: wherein, is a strain, is temperature change information, is a sweep rate, and and are consistent with the physical meaning of is the center frequency of the laser; After being detected and received by the optical detector one (406) and the optical detector two (407), the spectral expression of the Rayleigh scattering signal is: wherein, is the frequency, is the amplitude of the detector output current at time 0, is the time delay caused by the perturbation, is the Rayleigh scattering coefficient normalized in the frequency domain; The expression of the strain disturbance result, i.e., the demodulation result, of the two Rayleigh scattering signals is as follows: wherein, is the demodulation result of the large chirp bandwidth Rayleigh scattering signal, is the demodulation result of the small chirp bandwidth Rayleigh scattering signal, , is the system floor, i.e. a wideband white noise, is the 1 / f noise term, concentrated in the low frequency band, N is the number of times of replacing the reference curve.

7. The dual-chirped pulse based optical fiber distributed acoustic sensing apparatus of claim 6, wherein, The signal demodulation result needs to be adaptively filtered and corrected in the signal separation and demodulation unit, and the demodulation signal with lower system noise as a reference signal in the adaptive filtering algorithm , and as a signal to be filtered The error expression of adaptive filtering correction is: wherein is a filter weight matrix, is the result of the adaptive filter correction; The cost function is the expectation of the square of the error, i.e., Filter weight matrix The update rule is to minimize a cost function The steepest descent gradient algorithm is used: The update expression of the adjusted filter weight vector is obtained as follows: The final output result of the adaptive filter correction is as follows: 。 8. A method for dual-chirped pulse based fiber-optic distributed acoustic sensing applied to a dual-chirped pulse based fiber-optic distributed acoustic sensing apparatus according to any one of claims 1-7, characterized in that, The method comprises the following steps: The method comprises the following steps: The laser module (1) outputs laser to the coupler one (201) of the chirped pulse modulation module (2), and the laser is divided into two signals after the coupler one (201) and is injected into the light modulator one (202) and the light modulator two (203) respectively; The first output channel (204a) of the signal generator outputs a large chirp pulse electrical signal, drives the light modulator one to generate a large chirp swept optical signal one (205), and the second output channel (204b) of the signal generator outputs a small chirp pulse electrical signal, drives the light modulator two to output a small chirp swept optical signal two (206); The large chirp swept optical signal one (205) and the small chirp swept optical signal two (206) are input into the optical filter one (207) and the optical filter two (208) respectively to filter out the high-order sidebands and carrier signals generated in the process of outputting the swept optical signal by the light modulator, and the two chirped optical pulse signals filtered respectively are input into the coupler two (209) to be coupled, so that the double chirp pulse signal (211) combining the large and small chirp frequency bandwidths is obtained; The third output channel (204c) of the signal generator outputs an electrical pulse signal to control the switching time sequence of the pulse modulator to intercept and modulate the double chirp pulse signal to improve the extinction ratio of the double chirp pulse signal, and then the modulated double chirp pulse signal is input into the optical amplifier one (212) to be amplified; The amplified double chirp pulse signal is input into the sensing optical fiber (302) through the circulator (301) in the sensing optical fiber module (3) to excite the Rayleigh scattering signal, and the excited Rayleigh scattering signal is input into the optical amplifier two (401) in the Rayleigh scattering signal separation and receiving module (4) through the circulator (301); The Rayleigh scattering signal amplified by the optical amplifier two (401) is input into the optical filter three (402), the optical filter three (402) extracts the large chirp bandwidth Rayleigh scattering signal, the large chirp bandwidth Rayleigh scattering signal is output to the optical detector one (406) through the reflected light output end (402b) of the optical filter three, and the remaining signal components after extracting the large chirp bandwidth Rayleigh scattering signal are output to the optical filter four (403) through the transmitted light output end (402c) of the optical filter three to extract the small chirp bandwidth Rayleigh scattering signal, and the small chirp bandwidth Rayleigh scattering signal is output to the optical detector two (407) through the reflected light output end (403b) of the optical filter four; The large chirp bandwidth Rayleigh scattering signal and the small chirp bandwidth Rayleigh scattering signal are input into the optical detector one (406) and the optical detector two (407) respectively to occur signal self-beat interference and be converted into analog electrical signals, the large chirp bandwidth Rayleigh scattering analog electrical signal is filtered by the electrical filter (408) to be partially sampled, and the small chirp bandwidth Rayleigh scattering analog electrical signal is input into the acquisition card (409) to be collected. The signal collected by the acquisition card (409) is transmitted to a signal separation and demodulation unit (410) for signal demodulation processing. The Rayleigh scattering signals of each group are respectively demodulated, the time offset between different Rayleigh scattering curves is extracted by using a spectrum correlation algorithm, and then the time delay information is used to calculate the strain information of the object The mapping relationship between the temperature and the strain disturbance is quantitatively restored to obtain the demodulation result. An adaptive filtering algorithm is used to perform adaptive filtering correction on the large-chirp pulse signal. The demodulation result of the small-chirp bandwidth Rayleigh scattering signal is used as a reference signal to correct the sensitivity degradation of the demodulation result of the large-chirp bandwidth Rayleigh scattering signal caused by partial bandwidth sampling.

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