Optical fiber distributed sound wave sensing device and method based on double chirped pulses

Abstract

The invention discloses an optical fiber distributed sound wave sensing device based on double chirped pulses, which relates to the technical field of distributed optical fiber sensing and comprises a laser module (1), a chirped pulse modulation module (2), a sensing optical fiber module (3) and a Rayleigh scattering signal separation and receiving module (4), laser generated by the laser module passes through the chirped pulse modulation module to obtain double chirped pulse signals with different frequency bands, excites Rayleigh scattering signals in the sensing optical fiber module and inputs the Rayleigh scattering signals to the Rayleigh scattering signal separation and receiving module for signal separation and demodulation. The sensing optical fiber is inquired through double chirped pulses of different frequency bands, two Rayleigh scattering signals are excited to achieve a wide range and high sensitivity, the advantages of the two signals are combined through an adaptive filtering algorithm, the dynamic range of distributed sound wave detection is improved by one order of magnitude, and the detection accuracy is improved. The acquisition of large chirp bandwidth Rayleigh scattering signals is realized at a low sampling rate, and a distributed sound wave sensing technology with a large dynamic range is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of distributed optical fiber sensing, and more specifically, to an optical fiber distributed acoustic wave sensing device and method based on double chirped pulses. Background Art

[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 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. 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, as well as for environmental temperature and disaster warning systems.

[0003] The principle of distributed fiber optic sensing system is to utilize the light scattering effect in optical fiber. When light propagates in optical fiber, it will be scattered due to changes in the external environment (such as temperature changes or mechanical stress). By measuring the characteristics of these scattered lights, the changes in physical quantities along the optical fiber can be inferred. In distributed fiber optic sensing systems, Rayleigh scattering technology is mainly used to detect vibration and sound signals. There is a linear mapping relationship between the external tiny disturbance and the phase change. Distributed fiber acoustic sensing (DAS) technology based on Rayleigh scattering can complete high-precision real-time reconstruction of external disturbances by detecting the phase change of Rayleigh scattered light signals, restore the vibration or sound wave signal to be measured, and realize distributed sensing.

[0004] The patent with publication number CN113810098A discloses an optical time domain reflectometer based on double-sideband chirped pulse modulation. This scheme uses EOM to modulate symmetrical double-sideband chirped pulses, uses two reflective FBGs to extract the upper and lower sidebands respectively, and adds an adjustable fiber delay unit to the lower sideband to provide delay. The detection optical pulse is a time-sharing symmetrical double-sideband chirped pulse pair, that is, a pulse pair with the same pulse width, the same chirp spectrum content, and opposite slopes. The opposite slope can double the strain measurement range, but as the chirp bandwidth increases, the hardware cost will increase sharply.

[0005] Different from this solution, the present invention adopts a combination of chirped pulses of different frequency bands, that is, a partial sampling method is used to collect large chirp signals, retain a large range of measurement, and reduce the hardware requirements on the acquisition end. In addition, in the present invention, small chirp signals are used to compensate for the system strain measurement resolution, thereby improving the system dynamic range.

[0006] The patent with publication number CN113810099A discloses an optical time domain reflectometer based on asymmetric double-sideband chirped pulse modulation. The detection optical pulse of this scheme is a time-sharing asymmetric double-sideband chirped pulse pair, that is, a pulse pair with the same pulse width and different chirp spectrum content. The IQ modulator is used to modulate the sideband light with different chirp bandwidths in turn, and then the two asymmetric sidebands are modulated into pulses using SOA. Compared with the symmetric double-sideband chirped pulse, two measurements are performed in time in turn. By adjusting the bandwidth range of the chirped pulse light, the measurement resolution and measurement range to be measured are adjusted to achieve the function of taking into account a large range of measurement range and high measurement resolution. However, this scheme does not solve the problem that the large acquisition bandwidth and high sampling rate brought by the large chirp bandwidth signal may require an increase in the hardware complexity of the system and the requirements of the signal acquisition end, and the double-sideband pulse adopts time division multiplexing, which will cause the maximum detection bandwidth of the system to be reduced by half.

[0007] Different from this solution, the present invention concentrates two beams of swept-frequency light with different chirp bandwidths and different frequency bands into one optical pulse. Two sets of signals can be obtained each time the sensing optical fiber is interrogated, thus avoiding the problem of detection bandwidth loss caused by time division multiplexing in the system and eliminating the phase deviation of the two chirp pulse signals introduced by time division multiplexing.

[0008] In summary, the prior art has the technical difficulty that the acoustic wave measurement range of the chirped pulse DAS is limited, and it is difficult to achieve a large acoustic wave detection range and high detection sensitivity at the same time. Increasing the chirp bandwidth of the detection signal requires increasing the hardware complexity of the system and the requirements of the signal acquisition end, resulting in high hardware costs. Summary of the invention

[0009] The purpose of the present invention is to overcome the technical difficulties in the prior art that the acoustic wave measurement range of the chirped pulse DAS is limited, it is difficult to achieve a large acoustic wave detection range and high detection sensitivity at the same time, and when increasing the chirp bandwidth of the detection signal, it is necessary to increase the hardware complexity of the system and the requirements of the signal acquisition end, and the hardware cost is high. An optical fiber distributed acoustic wave sensing device based on double chirp pulses is provided.

[0010] To achieve the above object, the technical solution of the present invention is as follows:

[0011] The present invention provides an optical fiber distributed acoustic wave sensing device based on double chirped pulses, comprising:

[0012] Laser module, chirped pulse modulation module, sensing fiber module and Rayleigh scattering signal separation and receiving module;

[0013] The laser generated by the laser module passes through the chirped pulse modulation module to obtain a modulated and amplified double-chirped pulse signal with different center frequencies and bandwidths, and the double-chirped pulse signal excites a Rayleigh scattering signal in the sensing optical fiber module and inputs the Rayleigh scattering signal into 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, a light detector 1, a light detector 2, an electrical filter, a collection card and a signal separation and demodulation unit;

[0015] The Rayleigh scattered signal is amplified by the optical amplifier 2 and then input into the optical filter 3 to extract the swept-frequency Rayleigh scattered light signal 1 with a center frequency of f1 and a bandwidth of B1. After the swept-frequency Rayleigh scattered light signal is extracted by the optical filter 3, the Rayleigh scattered signal is then input into the optical filter 4 to extract the swept-frequency Rayleigh scattered light signal 2 with a frequency of f2 and a bandwidth of B2, wherein the bandwidth B1 is greater than the bandwidth B2;

[0016] The swept-frequency Rayleigh scattered light signal is input into the optical detector 1 for detection and then input into the electrical filter for partial bandwidth sampling of large chirp signals, and then the acquisition card performs signal acquisition;

[0017] The swept-frequency Rayleigh scattered light signal is input into the optical detector 2 for detection and then directly collected by the acquisition card;

[0018] After the acquisition card acquires the signal, the acquired signal is input into the signal separation and demodulation unit for signal demodulation processing and adaptive filtering correction is performed on the result of the signal demodulation processing.

[0019] As a preferred solution, the chirped 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 injected into the coupler 1 and split into two laser beams, and the two laser beams are respectively injected into the optical modulator 1 and the optical modulator 2. The first output channel of the signal generator outputs a chirped pulse electrical signal with a center frequency of f1-f0 and a chirp bandwidth of B1 to drive the optical modulator 1 to generate a swept frequency optical signal 1. The second output channel of the signal generator outputs a chirped pulse electrical signal with a center frequency of f0-f2 and a chirp bandwidth of B2 to drive the optical modulator 2 to generate a swept frequency optical signal 2, and the chirp bandwidth B1 is greater than the chirp bandwidth B2.

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

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

[0023] The pulse modulator re-modulates the double-chirped pulse signal containing large and small chirp frequency bands in the one spectrum, and 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, intercepts the double-chirped pulse signal to improve the extinction ratio of the double-chirped pulse signal, and then amplifies it through the optical amplifier 1 to increase the peak power of the double-chirped pulse signal.

[0024] As a preferred embodiment, the sensing fiber module includes a circulator and a sensing fiber, the optical amplifier 1 inputs the modulated and amplified double-chirped pulse signal with different center frequencies and bandwidths into the circulator, the circulator inputs the modulated and amplified double-chirped pulse signal with different center frequencies and bandwidths into the sensing fiber to excite a Rayleigh scattering signal, the sensing fiber then 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-chirped pulse are respectively:

[0026]

[0027] Where, E1(t) is the electric field expression of the large chirp bandwidth Rayleigh scattering signal with chirp bandwidth B1, E2(t) is the electric field expression of the small chirp bandwidth Rayleigh scattering signal with chirp bandwidth B2, L is the total number of scattering points on the entire sensing fiber, rect(·) represents the 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 to receive the scattered light from 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 scattered electric field expression at time 0.

[0028] As a preferred solution, the frequency components of the Rayleigh scattering signal excited by the double-chirped pulse signal with different central frequencies and bandwidths are separated in the optical frequency domain without aliasing.

[0029] As a preferred solution, the detection sensitivity of the optical fiber distributed acoustic wave sensor device based on double chirped pulses is defined by the Cramer-Rao lower bound CRLB. Conventional CRLB is only applicable to signals sampled with full bandwidth. Since the large chirp bandwidth Rayleigh scattering signal adopts the partial bandwidth sampling, it needs to be analyzed by an improved CRLB, and its expression is:

[0030]

[0031] Among them, 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 frequency range of the chirped pulse, B1 is the passband lower limit frequency of the electric filter, B2 is the passband upper limit frequency of the electric filter, and B2-B1 is the passband bandwidth of the electric filter.

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

[0033]

[0034] Among them, Δε is the strain, ΔT is the temperature change information, is the frequency sweep rate, which is consistent with the physical meaning of 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:

[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 Rayleigh scattering coefficient normalized in the frequency domain;

[0038] The strain perturbation result of the two Rayleigh scattered signals, i.e. the demodulation result, is expressed as follows:

[0039] S1=Δε+n0

[0040]

[0041] Among them, S1 is the demodulation result of the Rayleigh scattering signal with a large chirp bandwidth, S2 is the demodulation result of the Rayleigh scattering signal with a small chirp bandwidth, n0 and n1 are the system background noise, which is mainly broadband white noise. is the 1 / f noise term, which is mainly concentrated in the low frequency band, and N is the number of times the reference curve is changed.

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

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

[0044] Where 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) using the fastest descent gradient algorithm:

[0048]

[0049] Get the update expression for adjusting the filter weight vector:

[0050]

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

[0052]

[0053] On the other hand, the present invention also provides an optical fiber distributed acoustic wave sensing method based on double chirped pulses, which is applied to an optical fiber distributed acoustic wave sensing device based on double chirped pulses in the above technical solution, and the method steps are as follows:

[0054] The laser module outputs laser light to the coupler 1 of the chirped pulse modulation module. After passing through the coupler, the laser light is divided into two signals, which are injected into the optical modulator 1 and the optical modulator 2 respectively.

[0055] The first output channel of the signal generator outputs a large chirp pulse electrical signal, driving 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, driving 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 the high-order sidebands and carrier signals additionally generated in the process of the optical modulator outputting the swept-frequency optical signal. The two chirped optical pulse signals after filtering are simultaneously input into the coupler 2 for coupling, so as to obtain a double-chirp pulse signal combining the large and small chirp frequency bandwidths;

[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-chirped pulse signal to improve the extinction ratio of the double-chirped pulse signal, and then inputs the modulated double-chirped pulse signal into the optical amplifier 1 for amplification;

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

[0059] The optical amplifier 2 amplifies the Rayleigh scattering signal and then inputs it into the optical filter 3. The optical filter 3 extracts the large-chirp-bandwidth Rayleigh scattering signal of the 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. After extracting the large-chirp-bandwidth Rayleigh scattering signal, the remaining signal component is output from the transmitted light output end of the optical filter 3 to the optical filter 4 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 of the optical filter 4.

[0060] When the large and small chirp bandwidth Rayleigh scattering signals are respectively input into the optical detector 1 and the optical detector 2, the self-beating frequency interference of the signals occurs and the signals are converted into analog electrical signals. The large chirp bandwidth Rayleigh scattering analog electrical signals are subjected to partial bandwidth sampling and filtering by an electrical filter and then respectively input into the collector for signal collection together with the small chirp bandwidth Rayleigh scattering analog electrical signals;

[0061] The signals collected by the collector are transmitted to the signal separation and demodulation unit for signal demodulation processing. The Rayleigh scattering signals of each group are demodulated separately. The spectral correlation algorithm is used to extract the time offset between different Rayleigh scattering curves. Then, the disturbance information is quantitatively restored and the demodulation result is obtained according to the mapping relationship between the time delay information Δt and the temperature and strain disturbances. The adaptive filtering algorithm is used to perform adaptive filtering correction on the large and small chirp pulse signals. 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 present invention has the following beneficial effects:

[0063] The present invention adopts two pulse interrogation sensing optical fibers with two different frequency bands and different chirp rates, and the two Rayleigh scattering signals excited can respectively achieve a wide range and high sensitivity. The advantages of the two are combined by using an adaptive filtering algorithm, which can improve the dynamic range of distributed acoustic wave detection by an order of magnitude. At the same time, the collection of large chirp bandwidth Rayleigh scattering signals is achieved only at a low sampling rate, without additionally increasing the hardware complexity of the system and the requirements of the signal collection end. It has advantages in terms of collection cost and real-time signal processing, and realizes a relatively low-cost distributed acoustic wave sensing technology with a large dynamic range. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] Figure 1 It is a structural schematic diagram of an optical fiber distributed acoustic wave sensing device based on double chirped pulses 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 Schematic diagram of the working principle of the double-chirped pulse signal of the embodiment of the present application;

[0067] Figure 4 is a schematic diagram of separation of double-chirped pulse Rayleigh scattering signals according to an embodiment of the present application;

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

[0069] Figure 6 Schematic diagram of adaptive filtering correction demodulation result of an embodiment of the present application

[0070] Description of reference numerals:

[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-RF 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-RF input end of optical modulator 2, 204-signal generator, 204a-first output channel of signal generator, 204b-second output channel of signal generator, 204c-signal The third output channel of the signal generator, 205-sweep frequency optical signal 1, 206-sweep 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-electrical modulation signal input end of pulse modulator, 211-double-chirp pulse signal with large and small chirp frequency bands in the spectrum, 212-optical amplifier 1, 212a-input end of optical amplifier 1, 212b-optical Output end of amplifier 1; 3-sensing optical 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- Sweep-frequency Rayleigh scattered light signal 1, 405-sweep-frequency Rayleigh scattered light signal 2, 406-light detector 1, 406a-input end of light detector 1, 406b-RF signal output end of light detector 1, 407-light detector 2, 407a-input end of light detector 2, 407b-RF signal output end of light 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 more clearly understand the above-mentioned purpose, features and advantages of the present invention, the present invention is further described in 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, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Therefore, the protection scope of the present invention is not limited to the specific embodiments disclosed below.

[0074] Example 1

[0075] See also Figure 1 Embodiment 1 of the present invention provides an optical fiber distributed acoustic wave sensing device based on double chirped pulses, comprising:

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

[0077] In this embodiment 1, the laser module 1 includes a narrow linewidth fiber laser 101 with a central wavelength of 1550 nm, a linewidth of 100 Hz, and an output power of 16 dBm.

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

[0079] Among them, optical modulator 1 (202) and optical modulator 2 (203) are single-sideband IQ modulators, with an operating wavelength of 1550nm and a bandwidth of 20GHz. Optical filter 1 (207) and optical filter 2 (208) are both 8GHz bandwidth adjustable FBGs, with an operating center wavelength of 1550nm and an insertion loss of ≤1dB, and are mainly used to filter out extra frequency band signals. In this embodiment 1, an acousto-optic modulator AOM is used as the pulse modulator 210, with an insertion loss of 3dB and a switch extinction ratio of 50dB. Optical amplifier 1 (212) is an erbium-doped fiber amplifier.

[0080] The sensing optical fiber module 3 includes a circulator 301 and a sensing optical fiber 302 , wherein the length of the sensing optical fiber 302 is 25 km, corresponding to a flight time of light in the optical fiber of 250 us.

[0081] The Rayleigh scattering signal separation and receiving module 4 includes: 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, an acquisition card 409 and a signal separation and demodulation unit 410.

[0082] Among them, optical amplifier 2 (401) is an erbium-doped fiber amplifier. Optical filter 3 (402) and optical filter 4 (403) are both 8GHz bandwidth adjustable FBG, with a working center wavelength of 1550nm and an insertion loss of ≤1dB, mainly used to filter out extra frequency band signals. The bandwidth of optical detector 1 (406) and optical detector 2 (407) are both 1GHz. Electrical filter 408 is a low-pass electrical filter with a bandwidth of 500MHz. The sampling rate of acquisition card 409 is 1G Sa / s, and the acquisition 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 output of the 3dB coupler is 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 and 204b of the signal generator 204 are respectively connected to the RF input terminal 202c of the optical modulator 1 and the RF input terminal 203c of the optical modulator 2. The signal light output terminal 202b of the optoelectronic modulator 1 is connected to the input terminal 207a of the optical filter 1. The signal light output terminal 203b of the optoelectronic modulator 2 is connected to the input terminal 208a of the optical filter 2. The output terminals 207b and 208b of the optical filter 1 and the optical filter 2 are connected to the 3dB coupler 209 for beam combining, and their outputs are connected to the input terminal 210a of the pulse modulator 210. The third output channel 204c of the signal generator 204 is connected to the electrical modulation signal input terminal 210c of the pulse modulator 210. The pulse modulator output terminal 210b is connected to the input terminal 212a of the optical amplifier 1. The output terminal 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 fiber, and the port 301b of the circulator injects a double chirped pulse signal into the port 302a of the sensing fiber, and the port 302a of the sensing fiber returns the Rayleigh scattering from the port 301b of the circulator to the circulator. The Rayleigh scattering signal is finally injected into the input port 401a of the optical amplifier 2 by 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, and its reflected light output port 402b is connected to the input port 406a of the photodetector 1. The RF signal output port 406b of the photodetector 1 is connected to the input port 408a of the electric filter 408, and the output port 408b of the electric filter is connected to the acquisition channel 1 of the acquisition card ADC, i.e., 409a. 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 RF signal output port 407b of the photodetector 2 is connected to the acquisition channel 2 of the acquisition card ADC, i.e., 409b. The digital signal collected by the acquisition card is transmitted to the signal separation and demodulation unit 410 by the link formed by connecting the output port 409c of the acquisition card with the input port 410a of the signal separation and demodulation unit.

[0087] The working process of the optical fiber distributed acoustic wave sensing device based on double chirped pulses in this embodiment 1 is as follows:

[0088] The output end 101a of the narrow linewidth laser 1 emits a laser with a wavelength of 1550nm, a linewidth of 100Hz, and an amplitude of 16dBm. The laser is injected from the end 101a and split into two laser beams by the 3dB coupler 202, which are injected into the optical modulator 1 and the optical modulator 2 respectively.

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

[0090] The output end 212b of the optical amplifier 1 is connected to the port 301a of the circulator. The double-chirped pulse signal passes through the circulator and is injected into the optical fiber to be sensed through the port 301b. The double-chirped pulse signal excites the backward double-chirped pulse Rayleigh scattered light signal. The double-chirped pulse Rayleigh scattered signal returns to the circulator and is injected into the optical amplifier 2 through the port 301c of the circulator for gain amplification.

[0091] See also Figure 4 , Figure 4 It is a schematic diagram of separation of double-chirped pulse Rayleigh scattering signal. The Rayleigh scattering light signal excited by chirped pulses with two different sweep bandwidths has separate frequency components in the optical frequency domain, and there is no aliasing. Therefore, an optical filter can be used to separate the two chirped Rayleigh scattering lights. After the double-chirped pulse Rayleigh scattering signal is amplified, the Rayleigh scattering light signal enters the optical filter 3 to extract the swept Rayleigh scattering light signal 1 with a center frequency of f1 and a bandwidth of B1. The swept Rayleigh scattering light signal 1, i.e., 404, outputted from the reflected 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 electric filter 408 to realize partial bandwidth sampling and extraction of the large chirp signal. The output end 408b of the electric filter is connected to the acquisition card ADC channel 409a. The transmitted light output end 402c of the optical filter 3 is connected to the input port 403a of the optical filter 4, and the swept-frequency Rayleigh scattered light signals 2 and 405 with a frequency of f2 and a bandwidth of B2 are extracted. The reflected light output end 403b of the optical filter 4 outputs the swept-frequency Rayleigh scattered light signal 2, i.e., 405, which is connected to the optical detector 2. Since the Rayleigh scattered signal is a small chirp bandwidth signal, full bandwidth sampling is required. The output end 407b of the optical detector 2 does not need to be additionally connected to a low-pass electric filter. The RF 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 the digital signal to the signal separation and demodulation unit through the link formed by connecting the port 409c with the port 410a. In this device, the pulse interrogation sensing optical fiber with two chirp bandwidths is combined to achieve the improvement of the measurement range of the system without increasing the hardware pressure of the system acquisition end too much.

[0092] The frequency components of the Rayleigh scattering signal excited by the double-chirped pulse signal 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 field excited by the double-chirped pulse of the device are:

[0093]

[0094] Where, E1(t) is the electric field expression of the large chirp bandwidth Rayleigh scattering signal with chirp bandwidth B1, E2(t) is the electric field expression of the small chirp bandwidth Rayleigh scattering signal with chirp bandwidth B2, L is the total number of scattering points on the entire sensing fiber, rect(·) represents the 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 to receive the scattered light from 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 scattered electric field expression at time 0.

[0095] Since the starting frequencies of the two chirped pulse lights have a certain frequency interval, and the occupied frequency bandwidths do not overlap, the backward Rayleigh scattering signals excited by the large and small chirped pulses can be separated by an optical filter. Optical filter 3 is used to extract the large chirp bandwidth Rayleigh scattering light signal. The optical filter is essentially composed of a fiber grating, which reflects light of a specific wavelength and completes the extraction of the target frequency band light signal. After the returned mixed Rayleigh scattered light passes through the optical filter 3 to extract the large chirp bandwidth component, its transmitted light is connected to the optical filter 4 to extract the small chirp bandwidth Rayleigh scattering light signal. In addition to extracting the signal light of a specific frequency band, another function of the optical filter 3 and the optical filter 4 is to filter out the ASE noise introduced by the optical amplifier 2 during amplification to improve the signal quality. The signal collected by the acquisition card 409 is transmitted to the signal separation and demodulation unit 410 for signal cutting and separation and delay information extraction. The delay information uses the spectrum correlation demodulation method to extract the delay information between the Rayleigh scattering signals in the same group. The mapping relationship between the delay information and the temperature and strain disturbance is:

[0096]

[0097] Among them, Δε is the strain, ΔT is the temperature change information, 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] In the device, after being received by the light detector 403, the spectrum AC term expression of the Rayleigh scattered signal is:

[0099]

[0100] 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 Rayleigh scattering coefficient normalized in the frequency domain. It can be seen from formula (3) that the time delay Δt caused by the disturbance does not affect the frequency f, so the external low-pass electric filter 408 after the optical detector 406 does not cause the disturbance information to change. Based on this, the bandwidth and sampling rate requirements required for signal acquisition can be reduced, and the detection upper limit of the system can be improved at a low cost.

[0101] The detection sensitivity of the device is defined by the Cramer-Rao lower bound (CRLB). Conventional CRLB is only applicable to signals acquired with full bandwidth. For the above-mentioned partial bandwidth acquisition situation, an improved CRLB analysis is required, and its expression is:

[0102]

[0103] Among them, 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 chirped pulse, B1 is the lower passband frequency of the electric filter, B2 is the upper passband frequency of the electric filter, and B2-B1 is the passband bandwidth of the electric filter. It can be seen from formula (4) that the large chirp bandwidth signal will lead to sensitivity loss after partial bandwidth energy collection. The scheme of using frequency band multiplexing to collect a small chirp signal with the same bandwidth as the low-pass electric filter is equivalent to adding a set of full-bandwidth collected Rayleigh scattered light signals to the system. This signal does not suffer from sensitivity loss and can make up for the lack of sensitivity of the large chirp bandwidth signal after low bandwidth and low sampling rate collection, making the system more capable of detecting small disturbance signals. Due to the different emphasis of the two beams of signals, in the overlapping area of ​​the detection range of the two chirp signals, the adaptive filtering method can be used to combine the advantages of the two groups of signals, so that the system measurement can maintain large strain measurement and have high detection sensitivity.

[0104] The strain perturbation result expression of two beams of Rayleigh scattered signal light is as follows:

[0105]

[0106] Among them, S1 is the demodulation result of the Rayleigh scattering signal with a large chirp bandwidth, S2 is the demodulation result of the Rayleigh scattering signal with a small chirp bandwidth, n0 and n1 are the system background noise, which is mainly broadband white noise. is the 1 / f noise term, which is mainly concentrated in the low frequency band, and N is the number of times the reference curve is replaced. Due to its limited range, the 1 / f noise term will be introduced when demodulating large strains due to the use of changing reference curves Mainly concentrated in the low frequency band. Figure 5 , Figure 5 Schematic diagram of the adaptive filtering correction method of this embodiment 1. When performing adaptive filtering correction, the demodulated signal S2 with low system noise floor is used as the reference signal d(n) in the adaptive filtering algorithm, and S1 is used as the signal to be filtered x(n). The error expression of the adaptive filtering correction is:

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

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

[0109] Its 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) using the fastest descent gradient algorithm:

[0112]

[0113] Get the update expression for adjusting the filter weight vector:

[0114]

[0115] The final output result of adaptive filtering correction is expressed as follows:

[0116]

[0117] Since the two sets of signals measure the same vibration, Δε is correlated, while the noise is independent. When the system converges, as shown in formula (10), the output y(n) will be closer to the actual strain change Δε, so as to achieve the effect of noise removal and sensitivity correction. Combining the above technical solutions, the dual-chirp pulse modulation method can improve the system detection dynamic range without increasing the complexity of the system too much.

[0118] Example 2

[0119] This embodiment 2 provides an optical fiber distributed acoustic wave sensing method based on double chirped pulses, which is applied to an optical fiber distributed acoustic wave sensing device based on double chirped pulses in embodiment 1, and includes the following steps:

[0120] S1: The single-frequency continuous light output by the narrow-linewidth fiber laser 101 passes through the 3dB coupler 202 in module 2 and is evenly divided into two single-frequency optical signals, which are 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 start frequency of 10GHz, and a sweep bandwidth of 8GHz, driving the optical modulator 1 to generate a large chirp swept frequency 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 start frequency of 10GHz, and a sweep bandwidth of 500MHz, driving the optical modulator 2 to output a small chirp swept frequency optical signal 206. In order to facilitate subsequent separation, the optical modulator 1 is set to the lower sideband modulation mode, and the optical modulator 2 is set to the upper sideband modulation mode, so that the frequency bands of the two chirped pulse lights have a larger interval to avoid aliasing of the chirped pulses.

[0122] S3: Input the large chirp swept frequency optical signal 205 and the small chirp swept frequency optical signal 206 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 the optical modulator outputting the swept frequency optical signal. The two chirped optical pulse signals after filtering are simultaneously input into the coupler 209 for coupling to obtain the double chirp pulse signal 211 combining the large and small chirp frequency bandwidths. Please refer to Figure 2 and Figure 3 , Figure 2 This is a schematic diagram of the time-frequency mapping relationship of the coupled double-chirped pulse signal. One pulse contains two chirped frequency bands with different center frequencies and bandwidths. Figure 3 This is a schematic diagram of the working principle of the double-chirped 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-chirped pulse signal to improve the extinction ratio of the double-chirped pulse signal, and then inputs the modulated double-chirped pulse signal into the optical amplifier 1 for amplification, and amplifies the peak power of the chirped pulse to 150mW.

[0124] S5: The amplified double-chirped pulse signal is input into the 25 km 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 then input into the optical amplifier 2 in the Rayleigh scattering signal separation and receiving module 4 through the circulator 301.

[0125] S6: The optical amplifier 2 amplifies the Rayleigh scattering signal and inputs it into the optical filter 3. The optical filter 3 extracts the large chirp bandwidth Rayleigh scattering signal of the 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. After extracting the large chirp bandwidth Rayleigh scattering signal, the remaining signal component is output from the transmitted light output end 402c of the optical filter 3 to the optical filter 4 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. The main functions of the optical filters 3 and 4 are to extract the large and small chirp bandwidth Rayleigh scattering signals and suppress ASE noise.

[0126] S7: When the Rayleigh scattering signals with large and small chirp bandwidths are input to the optical detector 1 and the optical detector 2 respectively, signal self-frequency interference occurs and the signals are converted into analog electrical signals. The Rayleigh scattering signals detected by the optical detector will appear in the form of a right triangle on the spectrum, with a starting frequency of 0 Hz and an end frequency point that is the same as the bandwidth of the chirp pulse. After the analog electrical signal of the Rayleigh scattering signal with large chirp bandwidth is sampled and filtered with a low-pass electrical filter with a bandwidth of 500 MHz for partial bandwidth, the Rayleigh scattering signal with large chirp bandwidth will lose energy in part of the frequency band, while the Rayleigh scattering signal with small chirp bandwidth will not experience signal spectrum aliasing and energy loss when sampled under the condition of 500 MHz bandwidth. Finally, both analog electrical signals are collected by the acquisition card 409 with a bandwidth of 500 MHz and a sampling rate of 1 GSa / s.

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

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

[0129] The Rayleigh scattering signals of each group are demodulated separately, and the time offset between different Rayleigh scattering curves is extracted using the spectral correlation algorithm. Then, the disturbance information is quantitatively restored and the demodulation result is obtained based on the mapping relationship between the time delay information Δt and the temperature and strain disturbances.

[0130] See also Figure 5, an adaptive filtering algorithm is used to perform adaptive filtering correction on the large and small chirped pulse signals, and the demodulation result of the Rayleigh scattering signal with a chirp bandwidth of 500MHz is used as a reference signal to correct the sensitivity degradation of the demodulation result of the Rayleigh scattering signal with a chirp bandwidth of 8GHz caused by partial bandwidth sampling. The filter output result is as follows Figure 6 As shown in the figure, the sensitivity of the high frequency part is improved by 10dB compared with the unfiltered part. The sensitivity of the low frequency part is determined by the demodulation result of the Rayleigh scattering signal with a chirp bandwidth of 8GHz. Since the 1 / f noise introduced by the reference curve is not replaced, the sensitivity at the low frequency is improved by 4dB compared with the reference signal.

[0131] Other technical implementation details of this embodiment 2 are the same as those of embodiment 1 and will not be repeated here.

[0132] Example 3

[0133] Based on Example 1, this Example 3 replaces part of the hardware structure of the optical fiber distributed acoustic wave sensing device based on double chirped pulses to adapt to different environments or change the hardware cost. The details are as follows:

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

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

[0136] It should be noted that each replacement hardware structure in Example 3 can also be used in combination with other original hardware structures in Example 1.

[0137] In summary of the above embodiments, the embodiments of the present invention propose to interrogate the sensing optical fiber through a double chirp pulse, combining the sensitivity advantage of the full-bandwidth signal in the small chirp bandwidth range and the increase in the detection upper limit brought by the partial-bandwidth signal in the large chirp bandwidth range, so as to realize the improvement of the dynamic range of the system, and can be adjusted according to needs, using multiple chirped pulses and flexibly adjusting the bandwidth of the chirped pulses to further expand the dynamic range of the system. At the same time, a solution of externally connecting a low-pass filter at the rear end of the optical detector to perform partial bandwidth sampling realizes low-cost detection of large chirp signals. When increasing the chirp bandwidth of the detection signal, there is no need to increase the hardware complexity of the system and the requirements of the signal acquisition end, and the detection upper limit of the distributed optical fiber acoustic wave sensing device can be flexibly expanded.

[0138] Each component in the above embodiments can be a commercial product on the market. The present invention is intended to protect their connection relationship and implementation principle, so the model of each product itself is not limited. The description of the positional relationship in the accompanying drawings is only for illustrative purposes and cannot be understood as a limitation of this patent. The above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not limitations on the implementation methods of the present invention. For ordinary technicians in the field, other different forms of changes or modifications can be made on the basis of the above description. It is not necessary and impossible to list all the implementation methods here. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the claims of the present invention.

Claims

1. An optical fiber distributed acoustic wave sensing device based on double chirped pulses, characterized in that: It comprises a laser module (1), a chirped pulse modulation module (2), a sensing optical fiber module (3) and a Rayleigh scattering signal separation and receiving module (4); The laser light generated by the laser module (1) passes through the chirped pulse modulation module (2) to obtain a modulated and amplified double-chirped pulse signal with different central frequencies and bandwidths, and the double-chirped pulse signal excites a Rayleigh scattering signal in the sensing optical 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) 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), an acquisition card (409) and a signal separation and demodulation unit (410); The Rayleigh scattered signal is amplified by the optical amplifier 2 (401) and then input into the optical filter 3 (402) to extract a swept-frequency Rayleigh scattered light signal (404) having a center frequency of f1 and a bandwidth of B1. After the swept-frequency Rayleigh scattered light signal 1 (404) is extracted by the optical filter 3 (402), the Rayleigh scattered signal is input into the optical filter 4 (403) to extract a swept-frequency Rayleigh scattered light signal 2 (405) having a frequency of f2 and a bandwidth of B2, wherein the bandwidth B1 is greater than the bandwidth B2. The swept-frequency Rayleigh scattered light signal (404) is input into the optical detector 1 (406) for detection and then input into the electrical filter (408) for partial bandwidth sampling of the large chirp bandwidth signal, and then the acquisition card (409) performs signal acquisition; The swept-frequency Rayleigh scattered light signal (405) is input into the light detector 2 (407) for detection and then directly collected by the acquisition card (409); After the acquisition card (409) acquires the signal, the acquired signal is input into the signal separation and demodulation unit (410) for signal demodulation processing and adaptive filtering correction is performed on the result of the signal demodulation processing.

2. The optical fiber distributed acoustic wave sensing device based on double chirped pulse according to claim 1, characterized in that: The chirped pulse modulation module (2) comprises a coupler 1 (201), an optical modulator 1 (202), an optical modulator 2 (203), a signal generator (204), an optical filter 1 (207), an optical filter 2 (208) and a coupler 2 (209); The laser emitted by the laser module (1) is injected into the coupler 1 (202) and split into two laser beams. The two laser beams are respectively injected into the optical modulator 1 (202) and the optical modulator 2 (203). The first output channel (204a) of the signal generator outputs a chirped pulse electrical signal with a center frequency of f1-f0 and a chirp bandwidth of B1 to drive the optical modulator 1 (202) to generate a swept frequency optical signal 1 (205). The second output channel (204b) of the signal generator outputs a chirped pulse electrical signal with a center frequency of f0-f2 and a chirp bandwidth of B2 to drive the optical modulator 2 (203) to generate a swept frequency optical signal 2 (206). The chirp bandwidth B1 is greater than the chirp bandwidth B2. The optical filter 1 (207) is used to extract a swept-frequency sideband light 1 having a center frequency of f1 and a bandwidth of B1 from the swept-frequency optical signal 1 (205), and input the swept-frequency sideband light 1 into a coupler 2 (209). The optical filter 2 (208) is used to extract a swept-frequency sideband light 2 having a center frequency of f2 and a bandwidth of B2 from the swept-frequency optical signal 2 (206), and input the swept-frequency sideband light 2 into a coupler 2 (209). The two swept-frequency sideband optical signals are coupled in the coupler 2 (209) into a double-chirped pulse signal (211) having a large chirp frequency band and a small chirp frequency band in a frequency spectrum.

3. The optical fiber distributed acoustic wave sensing device based on double chirped pulse according to claim 2, characterized in that: The chirp modulation module (2) further comprises a pulse modulator (210) and an optical amplifier 1 (212); The pulse modulator (210) re-modulates the double-chirped pulse signal containing large and small chirp frequency bands in the one spectrum, and the third output channel (204c) of the signal generator (204) generates an electrical pulse signal and inputs it into the electrical modulation signal input terminal (210c) of the pulse modulator (210). The double-chirped pulse signal is intercepted to improve the extinction ratio of the double-chirped pulse signal, and then amplified by the optical amplifier 1 (212) to increase the peak power of the double-chirped pulse signal.

4. The optical fiber distributed acoustic wave sensing device based on double chirped pulse according to claim 2, characterized in that: The sensing optical fiber module (3) comprises a circulator (301) and a sensing optical fiber (302); the optical amplifier 1 (212) inputs the modulated and amplified double-chirped pulse signal with different center frequencies and bandwidths into the circulator (301); the circulator (301) inputs the modulated and amplified double-chirped pulse signal with different center frequencies and bandwidths into the sensing optical fiber (302) to stimulate a Rayleigh scattering signal; the sensing optical fiber (302) then inputs the Rayleigh scattering signal into the circulator (301); and the circulator (301) outputs the Rayleigh scattering signal to the optical amplifier 2.

5. The optical fiber distributed acoustic wave sensing device based on double chirped pulse according to claim 1, characterized in that: The electric field expressions of the Rayleigh scattering signals excited by the double chirped pulse are: Where, E1(t) is the electric field expression of the large chirp bandwidth Rayleigh scattering signal with chirp bandwidth B1, E2(t) is the electric field expression of the small chirp bandwidth Rayleigh scattering signal with chirp bandwidth B2, L is the total number of scattering points on the entire sensing fiber, rect(·) represents the 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 to receive the scattered light from 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 scattered electric field expression at time 0.

6. The optical fiber distributed acoustic wave sensing device based on double chirped pulse according to claim 1, characterized in that: The frequency components of the Rayleigh scattering signal excited by the double-chirped pulse signal with different central frequencies and bandwidths are separated in the optical frequency domain, and there is no aliasing.

7. The optical fiber distributed acoustic wave sensing device based on double chirped pulse according to claim 1, characterized in that: The detection sensitivity of the optical fiber distributed acoustic wave sensor device based on double chirped pulses is defined by the Cramer-Rao lower bound CRLB. Conventional CRLB is only applicable to signals sampled with full bandwidth. Since the large chirp bandwidth Rayleigh scattering signal adopts the partial bandwidth sampling, it needs to be analyzed by improved CRLB, and its expression is: Among them, 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 frequency range of the chirped pulse, B1 is the passband lower limit frequency of the electric filter, B2 is the passband upper limit frequency of the electric filter, and B2-B1 is the passband bandwidth of the electric filter.

8. The optical fiber distributed acoustic wave sensing device based on double chirped pulse according to claim 1, characterized in that: The signal collected by the collector (409) is transmitted to the signal separation and demodulation unit (410) for signal cutting and separation and time delay information extraction. The time delay information between Rayleigh scattering signals in the same group is extracted by using a spectrum correlation demodulation method. The mapping relationship between the time delay information Δt and the temperature and strain disturbance is: Among them, Δε is the strain, ΔT is the temperature change information, is the frequency sweep rate, which is consistent with the physical meaning of k1 and k2, and V0 is the center frequency of the laser; After being detected and received by the optical detector 1 (406) and the optical detector 2 (407), the spectrum expression of the Rayleigh scattering signal is: 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 Rayleigh scattering coefficient normalized in the frequency domain; The strain perturbation result of the two Rayleigh scattered signals, i.e. the demodulation result, is expressed as follows: S1=Δε+n0 Among them, S1 is the demodulation result of the Rayleigh scattering signal with a large chirp bandwidth, S2 is the demodulation result of the Rayleigh scattering signal with a small chirp bandwidth, n0 and n1 are the system background noise, which is mainly broadband white noise. is the 1 / f noise term, which is mainly concentrated in the low frequency band, and N is the number of times the reference curve is changed.

9. The optical fiber distributed acoustic wave sensing device based on double chirped pulse according to claim 8, characterized in that: In the signal separation and demodulation unit, the signal demodulation result needs to be adaptively filtered and corrected. The demodulated signal S2 with a lower system noise floor is used as the reference signal d(n) in the adaptive filtering algorithm, and S1 is used as the signal to be filtered x(n). The error expression of the adaptive filtering correction is: e(n)=d(n)-y(n)=d(n)-W(n)x(n) T Where W(n) is the filter weight matrix, and y(n) is the result of adaptive filtering correction; The cost function is the expectation of the square of the error, that is: J(n)=E(e 2 (n)) The update criterion of the filter weight matrix W(n) is to minimize the cost function J(n) using the fastest descent gradient algorithm: Get the update expression for adjusting the filter weight vector: The final output result of adaptive filtering correction is expressed as follows:

10. An optical fiber distributed acoustic wave sensing method based on double chirped pulses, which is applied to an optical fiber distributed acoustic wave sensing device based on double chirped pulses as claimed in any one of claims 1 to 9, characterized in that: The steps include: The laser module (1) outputs laser light to the coupler 1 (201) of the chirped pulse modulation module (2), and after passing through the coupler (201), the laser light is divided into two signals, which are respectively injected into the optical modulator (202) and the optical modulator (203); The first output channel (204a) 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 (205); the second output channel (204b) of the signal generator outputs a small chirp pulse electrical signal to drive the optical modulator 2 to output a small chirp swept frequency optical signal 2 (206); The large-chirp swept-frequency optical signal 1 (205) and the small-chirp swept-frequency optical signal 2 (206) are respectively input into an optical filter 1 (207) and an optical filter 2 (208) to filter out the high-order sidebands and carrier signals that are additionally generated in the process of the optical modulator outputting the swept-frequency optical signal, and the two chirped optical pulse signals after filtering are simultaneously input into a coupler 2 (209) for coupling, so as to obtain a double-chirp pulse signal (211) that combines the large and small chirp frequency bandwidths; The third output channel (204c) of the signal generator outputs an electrical pulse signal to control the switching timing of the pulse modulator to intercept and modulate the double-chirped pulse signal to improve the extinction ratio of the double-chirped pulse signal, and then the modulated double-chirped pulse signal is input into the optical amplifier 1 (212) for amplification; The amplified double-chirped pulse signal is input into the sensing optical fiber (302) through a circulator (301) in the sensing optical fiber module (3) to excite a Rayleigh scattering signal, and the excited Rayleigh scattering signal is then input into the optical amplifier 2 (401) in the Rayleigh scattering signal separation and receiving module (4) through the circulator (301); The optical amplifier 2 (401) amplifies the Rayleigh scattering signal and then inputs it into the optical filter 3 (402). The optical filter 3 (402) extracts a large chirp bandwidth Rayleigh scattering signal of the Rayleigh scattering signal. The large chirp bandwidth Rayleigh scattering signal is output to the optical detector 1 (406) through the reflected light output end (402b) of the optical filter 3. After extracting the large chirp bandwidth Rayleigh scattering signal, the remaining signal component is output from the transmitted light output end (402c) of the optical filter 3 to the optical filter 4 (403) to extract a small chirp bandwidth Rayleigh scattering signal. The small chirp bandwidth Rayleigh scattering signal is output to the optical detector 2 (407) through the reflected light output end (403b) of the optical filter 4. When the large and small chirp bandwidth Rayleigh scattering signals are respectively input into the optical detector 1 (406) and the optical detector 2 (407), the self-beating frequency interference of the signals occurs and the signals are converted into analog electrical signals. The large chirp bandwidth Rayleigh scattering analog electrical signals are subjected to partial bandwidth sampling and filtering by the electrical filter (408) and the small chirp bandwidth Rayleigh scattering analog electrical signals are respectively input into the collector (409) for signal collection. The signal collected by the collector (409) is transmitted to the signal separation and demodulation unit (410) for signal demodulation processing, and each group of Rayleigh scattering signals is demodulated respectively. The time offset between different Rayleigh scattering curves is extracted using a spectral correlation algorithm, and then the disturbance information is quantitatively restored and the demodulation result is obtained based on the mapping relationship between the time delay information Δt and the temperature and strain disturbances. The adaptive filtering algorithm is used to perform adaptive filtering correction on the large and small chirped 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.

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