High-precision very-low-frequency optical fiber distributed acoustic wave measurement device and method

Abstract

The invention discloses a high-precision very-low-frequency optical fiber distributed acoustic wave measurement device and method. Comprising a narrow linewidth laser 1, a narrow linewidth laser 2, an arbitrary waveform generator 1, an arbitrary waveform generator 2, an electro-optical modulator 1, an electro-optical modulator 2, an electro-optical modulator 3, an acoustic optical modulator 1, an acoustic optical modulator 2, a signal generator 1, a signal generator 2, a 3dB coupler 2, an optical amplifier 1, an optical amplifier 2, an optical amplifier 3, an optical amplifier 4, an optical amplifier 5, a polarization controller 1 and a polarization controller 2. The device comprises a polarization controller 3, a polarization beam splitter 1, a polarization beam splitter 2, an optical filter 1, an optical filter 2, an optical filter 3, an optical filter 4, an optical filter 5, a voltage-controlled oscillator, a circulator, a sensing optical fiber, an optical detector 1, an optical detector 2, an acquisition card and a signal demodulation and decoupling unit. According to the invention, strain and temperature can be decoupled in a large range, and very-low-frequency distributed acoustic wave measurement is realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of acoustic wave measurement, and particularly relates to a high-precision very low frequency optical fiber distributed acoustic wave measurement device and method. BACKGROUND

[0002] Distributed acoustic sensing (DAS) based on optical fiber can sense the vibration and temperature of the external environment through the optical fiber, and has the advantages of high measurement density, good synchronization, high sensitivity, wide frequency response range, long measurement distance, good weather resistance, and anti-electromagnetic interference, and is also widely used in oil and gas exploration, enhanced geothermal system monitoring, interwell strain measurement, hydraulic fracturing monitoring, long-period seismic monitoring and other geophysical related processes. Low-frequency seismic waves can provide deeper and longer observation information, and therefore are highly concerned in oil and gas exploration and geophysical research. Therefore, high-precision low-frequency seismic wave detection has important research significance and application value. Although the existing DAS system has made significant progress in technology, there is still a technical bottleneck of insufficient sensitivity in dealing with low-frequency seismic signal monitoring.

[0003] In low-frequency seismic wave detection, temperature fluctuation interference in the long-term measurement process is a major problem, which puts higher requirements on the noise suppression of the sensor. At present, the low-frequency noise suppression technology at the end of the distributed acoustic sensing detection system has good research results, but the temperature interference problem at the end of the sensing optical fiber has not been well solved, which seriously limits the application of distributed acoustic sensing in low-frequency seismic wave detection. Therefore, how to suppress the temperature noise of the sensing optical fiber in very low frequency acoustic wave detection to realize high-precision distributed acoustic wave detection has become the core problem of the current very low frequency distributed acoustic wave detection technology research.

[0004] Chinese patent CN120934616A discloses a wavelength division multiplexing Φ-OTDR and BOTDR fusion demodulation method, system, device and storage medium. The scheme uses two independent narrow linewidth lasers to output continuous laser, each path is divided into two beams by a coupler, one beam is pulse modulated and the other beam is used as a local oscillator. The pulsed light is modulated into frequency-shifted pulsed light by an acousto-optic modulator, and then injected into a sensing optical fiber after being combined by a light combiner. The backscattered light is separated by a wavelength division multiplexer. The Rayleigh signal is received coherently, and the optical intensity is extracted by quadrature demodulation. The Brillouin signal is amplified and mixed to extract the frequency shift. The optical intensity and frequency shift are combined to decouple the temperature and strain, and the vibration information is demodulated. This scheme needs to use a coherent reception method, and the spatial resolution of the BOTDR is affected by the phonon lifetime. Unlike this scheme, the present scheme uses a scheme of simultaneously measuring Brillouin dynamic gratings and Rayleigh scattering, and uses the different sensitivity coefficients of the two to decouple and suppress the temperature noise influence in very low frequency acoustic wave detection, thereby realizing high-precision very low frequency acoustic wave distributed detection.

[0005] The journal "Distributed phase-matching measurement enabled dynamic temperature-strain discrimination using single chirped pulse probe" discloses a Brillouin-Rayleigh hybrid distributed fiber sensing system based on a single linear chirped probe pulse, which can realize simultaneous distributed measurement of dynamic temperature and dynamic strain in polarization maintaining optical fiber. This technology uses the stress rod structure in polarization maintaining optical fiber, which causes opposite changes in the birefringence of the optical fiber due to temperature and strain, thereby achieving decoupling separation through the differential response of the two physical mechanisms. Due to the small coverage range of the Brillouin dynamic grating frequency, the system sensitivity is limited. Unlike this scheme, the present scheme proposes a method based on multiple sideband optical excitation Brillouin dynamic gratings, which generates multiple Brillouin dynamic gratings in polarization maintaining optical fiber, and expands the effective width of the Brillouin dynamic grating reflection spectrum to achieve temperature / strain measurement range and measurement accuracy improvement, to realize high-precision temperature and strain decoupling.

[0006] Chinese patent CN118603152A discloses a distributed multi-parameter sensing system based on multi-core optical fiber. This scheme uses the principle of space division multiplexing, and each core in the optical cable is used as a separate sensing channel for measurement and signal transmission. The multi-core optical fiber integrates three distributed optical fiber sensing systems together to realize in-situ measurement and mutual compensation decoupling of temperature, strain and vibration, and improves the measurement accuracy of the sensing system. Unlike this scheme, the present scheme uses a single polarization maintaining optical fiber as a sensing optical fiber, and only uses Rayleigh backscattering and multi-sideband Brillouin dynamic gratings for demodulation and decoupling, greatly reducing the system complexity.

[0007] Journal "Ultra-low frequency dynamic strain detection with laser frequency drifting compensation based on a random fiber grating array" discloses a demodulation method based on chirped pulse phase-sensitive OTDR and random fiber grating array. The method is connected in series with a random grating as a reference section at the front end of the optical fiber link, and a random grating array is arranged as a sensing section after it. After the chirped pulse propagates back and forth, the cross-correlation operation is performed on the adjacent echo traces, and the time delay of the reference section and the sensing section is extracted, respectively. The delay of the reference section only reflects the equivalent time delay introduced by the laser frequency drift, while the delay of the sensing section contains both the frequency drift and the external strain information. By subtracting the two, the influence of laser frequency drift and low-frequency phase noise can be effectively eliminated, and the net time delay proportional to the strain is obtained, and then combined with the pre-calibrated strain-time delay coefficient to realize quantitative demodulation. Unlike this scheme, the present scheme proposes a scheme for simultaneously measuring temperature and strain based on polarization-maintaining fiber Brillouin dynamic grating and Rayleigh scattering, and realizes decoupling. The present scheme uses light wave to induce sound wave grating, rather than the method of writing random grating in the scheme. The present scheme realizes high-precision very low frequency distributed sound wave measurement by using high-precision temperature and strain decoupling.

[0008] Journal "Ultra-high resolution strain sensor network assisted with an LS-SVM based hysteresis model" discloses a phase-sensitive optical time domain reflectometer A strain sensing network scheme using a scattering enhancement point (SEP) array fiber: A pre-strained sensing fiber segment and a strain-isolated reference fiber segment are divided within the same fiber with the SEP array. The phase change of each SEP segment is acquired using a φ-OTDR. The sensing channel and the corresponding reference channel form a sensing unit pair. The phase of the reference segment is used to compensate for ultra-low frequency phase noise caused by laser frequency drift and ambient temperature changes. Furthermore, to overcome the thermal hysteresis nonlinear error generated by the sensing fiber and reference fiber under different encapsulations, this scheme introduces a least-squares support vector machine hysteresis model based on the Preisach hysteresis operator to model and compensate for the phase relationship between the two, thereby significantly improving the signal-to-noise ratio of the ultra-low frequency signal while eliminating residual hysteresis error. Unlike this scheme, this scheme proposes a method for simultaneously measuring temperature and strain using a Brillouin dynamic grating and Rayleigh scattering based on polarization-maintaining fiber, achieving decoupling. This scheme uses light-induced acoustic grating generation instead of the weak grating method used in this scheme. This scheme proposes a method based on multiple sideband light-excited Brillouin dynamic gratings. It uses chirped pulses to simultaneously detect the backscattered Rayleigh signal and the reflected signal of the Brillouin dynamic grating. By utilizing the difference in temperature and strain sensing coefficients between the two, temperature and strain decoupling is achieved, thereby improving the temperature noise effect during very low frequency acoustic wave measurement.

[0009] The journal article "Strain measurement range enhanced chirped pulse φ-OTDR for distributed static and dynamic strain measurement based on random fiber grating array" discloses a chirped pulse phase-sensitive optical time-domain reflectometer based on a random fiber grating array. This paper proposes a multi-frequency database demodulation method to address the limited measurement range in distributed strain demodulation using time delay. The method employs linearly chirped pulses generated by a DFB laser and a semiconductor optical amplifier to excite a high-reflectivity, broadband random grating array. RFGA significantly improves the signal-to-noise ratio of the back Rayleigh trace and reduces the uncertainty of time delay measurement. Under strain-free conditions, a set of reference traces is pre-collected at different initial optical frequencies to construct a multi-frequency database. During actual measurement, the strain-encoded trace is cross-correlated with each reference trace in the database. The set with the highest correlation coefficient is selected, and the strain is inversely deduced from the corresponding time delay and the initial optical frequency difference. This achieves both nanostrain-level resolution and a large static and dynamic strain measurement range without relying on interferometric phase deconversion. Unlike this approach, this method does not require a multi-frequency database. Instead, it combines chirped optical time-domain reflectometry (TDR) with Brillouin dynamic grating technology for dual-channel decoupling to suppress temperature-induced noise.

[0010] The journal *Cancellation of reference update-induced 1 / f noise in a chirped-pulse DAS* discloses a demodulation method based on a multi-frequency reference database. Very Low Frequency (VLF) Measurement Scheme: During the initial calibration phase, a high-density database of reference traces covering a 25 GHz frequency range is acquired, corresponding to approximately 165.7 The measurable strain / temperature change range is defined. During the measurement process, each real-time echo trace is first correlated with all reference traces in the database. The reference with the largest correlation coefficient is selected to obtain a rough global frequency shift. Then, the time delay between the reference and the measured trace is estimated to achieve the joint solution of "global frequency shift + local time delay". The absolute strain or temperature change can be obtained directly from the initial state without the need to continuously update the reference and integrate and accumulate in the time series. This significantly improves the availability of the system in monitoring ultra-low frequency processes such as earthquakes and slow changes in ground temperature. However, at the same time, a large-capacity multi-frequency database needs to be established in the early stage, and its maximum measurement range is limited by the reference frequency scanning bandwidth.

[0011] Therefore, in order to solve the above-mentioned problems in the prior art, a device and method that uses Rayleigh and broadband Brillouin dynamic gratings to decouple temperature strain and suppress temperature crosstalk, without the need to establish a database and with a wider range of applications, is a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0012] In view of this, the present invention provides a high-precision very low frequency fiber optic distributed acoustic wave measurement device and method.

[0013] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: A high-precision very low frequency fiber-optic distributed acoustic wave measurement device includes: a narrow linewidth laser 1, a narrow linewidth laser 2, an arbitrary waveform generator 1, an arbitrary waveform generator 2, an electro-optic modulator 1, an electro-optic modulator 2, an electro-optic modulator 3, an acousto-optic modulator 1, an acousto-optic modulator 2, a signal generator 1, a signal generator 2, and a 3dB coupler. 2. Optical amplifier 1, optical amplifier 2, optical amplifier 3, optical amplifier 4, optical amplifier 5, polarization controller 1, polarization controller 2, polarization controller 3, polarization beam splitter 1, polarization beam splitter 2, optical filter 1, optical filter 2, optical filter 3, optical filter 4, optical filter 5, voltage-controlled oscillator, circulator, sensing fiber, photodetector 1, photodetector 2, data acquisition card, and signal demodulation and decoupling unit.

[0014] Preferably, the output terminal 101a of the narrow linewidth laser 1 is connected to the terminal 103a of the electro-optic modulator 1; the port 102a of the arbitrary waveform generator 1 is connected to the port 103b of the electro-optic modulator 1; the output terminal 103c of the electro-optic modulator 1 is connected to the input terminal 115a of the 3dB coupler 115; the output of the 3dB coupler 115 is connected to the input terminal 104a of the electro-optic modulator 2 and the input terminal 110a of the acousto-optic modulator 1, respectively; the output terminal 105a of the arbitrary waveform generator 2 is connected to the input port 104b of the electro-optic modulator 2; the output terminal 104c of the electro-optic modulator 2 is connected to the input terminal 106a of the optical amplifier 1; and the output terminal 107b of the optical amplifier 1 is connected to the input terminal 108a of the polarization controller 1. The output of polarization controller 1 is connected to port 109a of polarization beamsplitter 1, and the output of polarization beamsplitter 1 is connected to input port 114a of sensing fiber. The output of acousto-optic modulator 1 is connected to input port 111a of optical amplifier 2, the output of optical amplifier 2 is connected to input port 112a of polarization controller 2, the output of polarization controller 2 is connected to input port 113a of polarization beamsplitter 2, and the output of polarization beamsplitter 2 is connected to input port 114b of sensing fiber. The three output ports 201a, 201b, and 201c of signal generator 1 are connected to input port 202a of signal generator 2, port 304c of acousto-optic modulator 2, and port 110c of acousto-optic modulator 1, respectively. The output ports 202b and 202c of signal generator 2 are connected to input ports 202a and 202c of signal generator 2. The input terminals 302b and 410c of the voltage-controlled oscillator and the data acquisition card, respectively, are connected. The output terminal 301a of the narrow linewidth laser 2 is connected to the input terminal 303a of the electro-optic modulator 3. The output terminal 303c of the electro-optic modulator 3 is connected to the input terminal 304a of the acousto-optic modulator 2. The output terminal 304b of the acousto-optic modulator 2 is connected to the input terminal 305a of the optical amplifier 3. The output terminal 305b of the optical amplifier 3 is connected to the input terminal 306a of the polarization controller. The output terminal of the polarization controller is connected to the circulator 307a. The port 307b of the circulator is connected to the input terminal 113b of the polarization beam splitter 2. The port 307c of the circulator is connected to the input terminal 401a of the 3dB coupler 401. The output of the 3dB coupler 401 is connected to the optical... Filter 2's 402a and optical filter 3's 406a; the output of optical filter 2 is connected to the input of optical amplifier 4's 403a, the output of optical amplifier 4 is connected to the input of optical filter 4's 404a, the output of optical filter 4 is connected to the input of photodetector 1's 405a, and the output of photodetector 1 is connected to the input of the acquisition card's 410b; the output of optical filter 3 is connected to the input of optical amplifier 5's 407a, the output of optical amplifier 5 is connected to the input of optical filter 5's 408a, the output of optical filter 5 is connected to the input of photodetector 2's 409a, the output of photodetector 2 is connected to the input of the acquisition card's 410a, and the output of the acquisition card's 410d is connected to the input of the signal demodulation and decoupling unit's 411a.

[0015] A high-precision very low frequency fiber-optic distributed acoustic wave measurement method is proposed. A narrow linewidth laser 1 is used as the light source to generate a multi-sideband Brillouin dynamic grating. The output laser 1 has a wavelength of 1550 nm and a power of 30 mW. It is input to an electro-optic modulator 1. An arbitrary waveform generator 1 drives the electro-optic modulator 1 to modulate the laser 1, generating a multi-frequency signal with five sidebands and a sideband spacing of B1. The input 3dB coupler 115 splits the signal into two paths, namely multi-frequency signal 1 and multi-frequency signal 2. Narrow linewidth laser 2 outputs laser 1 with a wavelength of 1550 nm and a power of 30 mW. It generates a chirped signal with a chirped bandwidth of B2 through an electro-optic modulator 3 driven by a voltage-controlled oscillator 302 controlled by a signal generator 2. The pulse electrical signal with a pulse width of τ2 and a repetition rate of k2 output by the signal generator 201b drives the acousto-optic modulator 2 to convert it into chirped pulse light 1. After being amplified by an optical amplifier 3, it is input to a polarization controller 3 to maintain its polarization state. Then, it enters through port 307a and exits through port 307b, and is input to a polarization beam splitter 2 to be injected into the fast axis of the sensing fiber as a probe light. The optical signal output from port 307c of the circulator includes reverse Rayleigh scattering light, polygonal Brillouin dynamic grating reflection light, and leakage light from pump light 1. Optical filters 2 and 3 are used to filter out the reverse Rayleigh scattering light and polygonal Brillouin dynamic grating reflection light, which are channel 1 and channel 2, respectively. The two channels are then passed through optical amplifiers 4 and 5, and then through optical filters 4 and 5, and input to photodetectors 1 and 2 for direct detection. The output of photodetector 1 is connected to port 410b of the acquisition card, and the output of photodetector 2 is connected to port 410a of the acquisition card. The data is then processed by the input signal demodulation and decoupling unit of the acquisition card.

[0016] Preferably, the multi-frequency signal 1 is input to the arbitrary waveform generator 2, and the output electrical signal with a frequency shift of ∆f drives the electro-optic modulator 2 to generate a multi-frequency signal 3. This signal is then amplified by the optical amplifier 1 and filtered by the optical filter 1 to remove spontaneous emission noise. The signal is then input to the polarization controller 1 to maintain its polarization state, and finally injected into the slow axis of the sensing fiber as pump light 1 by the polarization beam splitter 1. The multi-frequency signal 2, output by the signal generator 201c with a pulse width of τ1 and a repetition frequency of k1, drives the acousto-optic modulator 1 to convert it into pulse light 1. This pulse is then amplified by the optical amplifier 2, and its polarization state is maintained by the polarization controller 2. The pulse is then input to the polarization beam splitter 2 and injected into the slow axis of the sensing fiber as pump light 2, which, together with the pump light 1, excites a multi-sideband Brillouin dynamic grating.

[0017] The present invention achieves the following technical effects compared to the prior art: (1) This invention can decouple strain and temperature over a wide range, realize the separation of vibration signal and temperature with a period of 1000 seconds, achieve temperature noise suppression effect of 17dB, and realize very low frequency distributed sound wave measurement. (2) This invention expands the spectral width and spectral density of the dynamic grating by synchronously exciting it with multi-frequency pump light, thereby achieving high-precision temperature and strain decoupling; this method can solve the problem of insufficient measurement range and measurement accuracy of a single Brillouin dynamic grating. (3) The present invention has made innovative improvements to the pump structure of the decoupling system. An acousto-optic modulator 1 is added to the transmission path of the pump light 2. By converting the continuous pump light 2 into a pulse form, two key advantages are achieved: First, the pulsed pump light 2 can effectively slow down the energy transfer rate of the pump light 1, thereby forming a more uniform Brillouin dynamic grating distribution along the fiber axis; Second, by dynamically adjusting the pulse parameters, the spatial distribution characteristics of the stimulated Brillouin effect along the fiber can be precisely controlled, greatly eliminating the pump evacuation phenomenon and solving the problem of limited measurement length in the traditional scheme. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the time-frequency mapping sensing principle of the chirped pulse optical time-domain reflectometer of the present invention; Figure 2 This is a schematic diagram illustrating the generation and detection principle of multiple Brillouin dynamic gratings in this invention; Figure 3 The diagram shows (a) the temperature and strain decoupling measurement results and (b) the improvement effect of very low frequency sound wave measurement in this invention. Figure 4 This is a diagram of the high-precision fiber optic distributed acoustic wave sensing device for temperature and strain decoupling according to the present invention. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] This invention discloses a high-precision very low frequency fiber-optic distributed acoustic wave measurement device, comprising: a narrow linewidth laser 1, a narrow linewidth laser 2, an arbitrary waveform generator 1, an arbitrary waveform generator 2, an electro-optic modulator 1, an electro-optic modulator 2, an electro-optic modulator 3, an acousto-optic modulator 1, an acousto-optic modulator 2, a signal generator 1, a signal generator 2, and a 3dB coupler. 2. Optical amplifier 1, optical amplifier 2, optical amplifier 3, optical amplifier 4, optical amplifier 5, polarization controller 1, polarization controller 2, polarization controller 3, polarization beam splitter 1, polarization beam splitter 2, optical filter 1, optical filter 2, optical filter 3, optical filter 4, optical filter 5, voltage-controlled oscillator, circulator, sensing fiber, photodetector 1, photodetector 2, data acquisition card, and signal demodulation and decoupling unit.

[0021] The output terminal 101a of the narrow linewidth laser 1 is connected to the terminal 103a of the electro-optic modulator 1. The port 102a of the arbitrary waveform generator 1 is connected to the port 103b of the electro-optic modulator 1. The output terminal 103c of the electro-optic modulator 1 is connected to the input terminal 115a of the 3dB coupler 115. The output of the 3dB coupler 115 is connected to the input terminal 104a of the electro-optic modulator 2 and the input terminal 110a of the acousto-optic modulator 1, respectively. The output terminal 105a of the arbitrary waveform generator 2 is connected to the input port 104b of the electro-optic modulator 2. The output terminal 104c of the electro-optic modulator 2 is connected to the input terminal 106a of the optical amplifier 1. The output terminal 107b of the optical amplifier 1 is connected to the input terminal 108a of the polarization controller 1. The output of device 1 is connected to port 109a of polarization beamsplitter 1, and the output of polarization beamsplitter 1 is connected to the input port 114a of sensing fiber. The output of acousto-optic modulator 1 is connected to the input port 111a of optical amplifier 2, the output of optical amplifier 2 is connected to the input port 112a of polarization controller 2, the output of polarization controller 2 is connected to the input port 113a of polarization beamsplitter 2, and the output of polarization beamsplitter 2 is connected to the input port 114b of sensing fiber. The three output ports 201a, 201b, and 201c of signal generator 1 are respectively connected to the input port 202a of signal generator 2, port 304c of acousto-optic modulator 2, and port 110c of acousto-optic modulator 1. The output ports 202b and 202c of signal generator 2 are respectively connected to... The input terminal 302b of the voltage-controlled oscillator is connected to the input terminal 410c of the data acquisition card; the output terminal 301a of the narrow linewidth laser 2 is connected to the input terminal 303a of the electro-optic modulator 3, the output terminal 303c of the electro-optic modulator 3 is connected to the input terminal 304a of the acousto-optic modulator 2, the output terminal 304b of the acousto-optic modulator 2 is connected to the input terminal 305a of the optical amplifier 3, the output terminal 305b of the optical amplifier 3 is connected to the input terminal 306a of the polarization controller, the output terminal of the polarization controller is connected to the circulator 307a, the circulator port 307b is connected to the input terminal 113b of the polarization beam splitter 2, the circulator 307c is connected to the input terminal 401a of the 3dB coupler 401, and the outputs of the 3dB coupler 401 are connected to the optical filters. Optical filter 2 has a 402a input and optical filter 3 has a 406a input. The output of optical filter 2 is connected to the input of optical amplifier 4 (403a), the output of optical amplifier 4 is connected to the input of optical filter 4 (404a), the output of optical filter 4 is connected to the input of photodetector 1 (405a), and the output of photodetector 1 is connected to the input of acquisition card (410b). The output of optical filter 3 is connected to the input of optical amplifier 5 (407a), the output of optical amplifier 5 is connected to the input of optical filter 5 (408a), the output of optical filter 5 is connected to the input of photodetector 2 (409a), the output of photodetector 2 is connected to the input of acquisition card (410a), and the output of acquisition card (410d) is connected to the input of signal demodulation and decoupling unit (411a).

[0022] This invention also discloses a high-precision very low frequency fiber-optic distributed acoustic wave measurement method. A narrow linewidth laser 1 is used as the light source to generate a multi-sideband Brillouin dynamic grating. Its output laser 1 has a wavelength of 1550 nm and a power of 30 mW. It is input to an electro-optic modulator 1. An arbitrary waveform generator 1 drives the electro-optic modulator 1 to modulate the laser 1, generating a multi-frequency signal with five sidebands and a sideband spacing of B1. The input is a 3 dB coupler 115, which splits the signal into two paths, namely multi-frequency signal 1 and multi-frequency signal 2. Narrow linewidth laser 2 outputs laser 1 with a wavelength of 1550 nm and a power of 30 mW. It generates a chirped signal with a chirped bandwidth of B2 through an electro-optic modulator 3 driven by a voltage-controlled oscillator 302 controlled by a signal generator 2. The pulse electrical signal with a pulse width of τ2 and a repetition rate of k2 output by the signal generator 201b drives the acousto-optic modulator 2 to convert it into chirped pulse light 1. After being amplified by an optical amplifier 3, it is input to a polarization controller 3 to maintain its polarization state. Then, it enters through port 307a and exits through port 307b, and is input to a polarization beam splitter 2 to be injected into the fast axis of the sensing fiber as a probe light. The optical signal output from port 307c of the circulator includes reverse Rayleigh scattering light, polygonal Brillouin dynamic grating reflection light, and leakage light from pump light 1. Optical filters 2 and 3 are used to filter out the reverse Rayleigh scattering light and polygonal Brillouin dynamic grating reflection light, which are channel 1 and channel 2, respectively. The two channels are then passed through optical amplifiers 4 and 5, and then through optical filters 4 and 5, and input to photodetectors 1 and 2 for direct detection. The output of photodetector 1 is connected to port 410b of the acquisition card, and the output of photodetector 2 is connected to port 410a of the acquisition card. The data is then processed by the input signal demodulation and decoupling unit of the acquisition card.

[0023] Multi-frequency signal 1 is input to arbitrary waveform generator 2, and the output electrical signal with a frequency shift of ∆f drives electro-optic modulator 2 to generate multi-frequency signal 3. This signal is amplified by optical amplifier 1 and filtered by optical filter 1 to remove spontaneous emission noise. It is then input to polarization controller 1 to maintain its polarization state, and finally injected into the slow axis of the sensing fiber as pump light 1 by polarization beam splitter 1. Multi-frequency signal 2, output by signal generator 201c with a pulse width of τ1 and a repetition rate of k1, drives acousto-optic modulator 1 to convert it into pulse light 1. This pulse is then amplified by optical amplifier 2 and its polarization state is maintained by polarization controller 2. It is then input to polarization beam splitter 2 and injected into the slow axis of the sensing fiber as pump light 2, which, together with pump light 1, excites a multi-sideband Brillouin dynamic grating.

[0024] Among them, the Rayleigh scattering intensity at input terminal 409 Represented as (1) in, I Rayleigh scattering intensity in the optical fiber λ The incident light wavelength,a Where is the particle radius, r The distance between the particle and the observation point. n For the refractive index of the material, θ and φ The incident light angle parameter.

[0025] From formula (1), we can see that the Rayleigh scattering intensity in the optical fiber is... I With wavelength λ It is inversely proportional to the fourth power, and its... L Backscattered Rayleigh power formed by position P r for: (2) in, P in The optical power of the incident light pulse. α s The Rayleigh scattering coefficient of the optical fiber. ν g The group velocity of light propagating in an optical fiber. W The incident light pulse width, α The fiber loss factor is... S The trapping factor for scattered light power, which is related to the mode field radius of the optical fiber, is expressed as: (3) In fiber optic sensing based on Rayleigh scattering, such as Figure 1 As shown, sensing information can be obtained by demodulating the backscattering Rayleigh scattering in the optical fiber.

[0026] Optical filter 3 separates the excited backscattered Rayleigh signal from the dynamic grating reflection signal. The optical filter, essentially composed of a fiber optic grating, reflects light of a specific wavelength to extract the target frequency band optical signal. Optical filter 5 filters out ASE noise introduced during amplification by optical amplifier 5, improving signal quality.

[0027] The signals acquired by the acquisition card 410 are transmitted to the signal demodulation and decoupling unit 411 for time delay information extraction. The time delay information is extracted from Rayleigh scattering signals within the same group using the spectral correlation demodulation method.

[0028] Delay information The mapping relationship between temperature and strain disturbance is as follows: ,in In response, Information on temperature changes This is the sweep rate term.

[0029] The excitation and detection process of a polygonal Brillouin dynamic grating is as follows: The continuous light output from the narrow linewidth laser 1 is modulated by the electro-optic modulator 1 to generate a multi-frequency laser signal with five sidebands. The pump light 1 and pump light 2, after being modulated by the electro-optic modulator 2, satisfy the frequency difference of the Brillouin frequency shift of the sensing fiber. These two pump lights will generate stimulated Brillouin scattering, which excites an acoustic field grating on the fiber core. This acoustic field grating is a moving grating and is called a Brillouin dynamic grating.

[0030] Let the sound wave field it excites be ν B The two pump lights are pump lights. ν 1. Stokes Light ν 2, when ν 1- ν 2= ν B When the phase matching condition is satisfied, we have: (4) In this formula n x The slow-axis refractive index of the optical fiber. V a This represents the speed of sound in the optical fiber, and this value does not change with the fast or slow axis of the fiber. λ Where is the wavelength of light. Because PMF exhibits birefringence, therefore: (5) in, n y For the fast axis refractive index of the optical fiber, λ x and λ y These are polarization-maintaining fibers. x shaft and y The wavelength of light along the axis.

[0031] Transforming equation (5) yields: (6) In polarization-maintaining fiber, the phase difference between the fast and slow axes is much less than 1, and Δ ν = ν x - ν y << ν Therefore, we can simplify to obtain: (7) Where, Δ ν The frequency difference caused by birefringence, Δ n The refractive index difference between the two axes of a polarization-maintaining fiber is Δ. This formula means that the reflected wavelength of the BDG generated on the slow axis of the polarization-maintaining fiber is inconsistent between the fast and slow axes, and the difference between the two is Δ. νIt is determined by the birefringence of PMF.

[0032] For a typical PMF with a length of 2.5mm shot at 1550nm, using a pump light of 1550nm, the birefringence of the PMF is as follows: (8) The frequency difference between the reflected light from the fast axis and the slow axis is: (9) That is, for the PMF of this parameter, the input frequency on its slow axis is ν x The pump light, on the fast axis, needs to use a frequency of [frequency value missing]. ν x The detection was performed using a probe light at +82.67 GHz.

[0033] Chirped pulses are used as detection pulses. Frequency changes are mapped to time delays through time-frequency mapping. The frequency shift is calculated through the time delay to achieve sensing.

[0034] Its specific principles are as follows: Figure 2 As shown, when external environmental disturbances cause changes in birefringence, the frequency difference between the pump light and the probe light caused by birefringence also changes. Therefore, the timing of reflection changes due to the matching of the chirped pulse and the BDG reflection wavelength at that position. This is reflected in the obtained reflection curve as a time delay generated by the signal peak at the corresponding position. The specific time delay depends on the chirping rate of the chirped pulse and the change in the BDG reflection wavelength.

[0035] Because this scheme uses chirped pulses for detection, and a single pulse contains multiple frequency components, it eliminates the need for frequency scanning and provides dynamic measurement capabilities. Furthermore, by utilizing the continuous frequency variation of chirped pulses, this scheme overcomes the limitation of phonon lifetime in the acoustic field, thus achieving higher spatial resolution than conventional schemes.

[0036] Before the optical fiber is disturbed, at any position z Detecting frequency components in pulses ν 1. Reflection occurs when the phase-matching condition is met, generating a peak reflected signal. Upon disturbance, the birefringence at that location changes, affecting the frequency components of the probe pulse. ν 2. The phase-matching condition is met, resulting in a delay in the peak value of the generated reflected signal. The delay amount is Δ. t for: (10) Because of their different mechanisms of action, the coefficients of the frequency shift of the sensed quantity after being affected by strain and temperature are different. The frequency shifts of the reflected light and reverse Rayleigh scattering light from the two channels can be demodulated separately, and then their sensing coefficient equations can be combined to decouple temperature and strain. Let the temperature coefficient of channel 1 be... C T ch1 The strain coefficient is C ε ch1 The frequency shift is Δ f ch1 The temperature coefficient of channel 2 is C T ch2 The strain coefficient is C ε ch2 The frequency shift is Δ f ch2 The change in external temperature is Δ T The applied strain is Δ ε Then we have: (11) After transforming formula (11), we obtain: (12) Using this equation, temperature strain decoupling can be achieved.

[0037] The above description is merely a preferred embodiment of the present invention and does not constitute any limitation on the technical scope of the present invention. Therefore, any minor modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A high-precision very low frequency fiber-optic distributed acoustic wave measurement device, characterized in that, include: Narrow linewidth laser 1, Narrow linewidth laser 2, Arbitrary waveform generator 1, Arbitrary waveform generator 2, Electro-optic modulator 1, Electro-optic modulator 2, Electro-optic modulator 3, Acousto-optic modulator 1, Acousto-optic modulator 2, Signal generator 1, Signal generator 2, 3dB coupler 2. Optical amplifier 1, optical amplifier 2, optical amplifier 3, optical amplifier 4, optical amplifier 5, polarization controller 1, polarization controller 2, polarization controller 3, polarization beam splitter 1, polarization beam splitter 2, optical filter 1, optical filter 2, optical filter 3, optical filter 4, optical filter 5, voltage-controlled oscillator, circulator, sensing fiber, photodetector 1, photodetector 2, data acquisition card, and signal demodulation and decoupling unit.

2. The high-precision very low frequency fiber-optic distributed acoustic wave measurement device according to claim 1, characterized in that, The output terminal 101a of the narrow linewidth laser 1 is connected to the terminal 103a of the electro-optic modulator 1, the port 102a of the arbitrary waveform generator 1 is connected to the port 103b of the electro-optic modulator 1, the output terminal 103c of the electro-optic modulator 1 is connected to the input terminal 115a of the 3dB coupler 115, and the output of the 3dB coupler 115 is connected to the input terminal 104a of the electro-optic modulator 2 and the input terminal 110a of the acousto-optic modulator 1, respectively. The output terminal 105a of the arbitrary waveform generator 2 is connected to the input port 104b of the electro-optic modulator 2; the output terminal 104c of the electro-optic modulator 2 is connected to the input terminal 106a of the optical amplifier 1; the output terminal 107b of the optical amplifier 1 is connected to the input terminal 108a of the polarization controller 1; the output terminal of the polarization controller 1 is connected to port 109a of the polarization beam splitter 1; and the output terminal of the polarization beam splitter 1 is connected to the input terminal 114a of the sensing fiber. The output terminal of the acousto-optic modulator 1 is connected to the input terminal 111a of the optical amplifier 2; the output terminal of the optical amplifier 2 is connected to the input terminal 112a of the polarization controller 2; and the polarization controller 2... The output terminal of the signal generator 1 is connected to the input terminal 113a of the polarization beam splitter 2, and the output terminal of the polarization beam splitter 2 is connected to the input terminal 114b of the sensing fiber. The three output ports 201a, 201b, and 201c of the signal generator 1 are connected to the input terminal 202a of the signal generator 2, the input terminal 304c of the acousto-optic modulator 2, and the input terminal 110c of the acousto-optic modulator 1, respectively. The output terminals 202b and 202c of the signal generator 2 are connected to the input terminal 302b of the voltage-controlled oscillator and the input terminal 410c of the data acquisition card, respectively. The output terminal 301a of the narrow linewidth laser 2 is connected to the input terminal 303a of the electro-optic modulator 3, and the output terminal 301a of the electro-optic modulator 3 is connected to the input terminal 303a of the electro-optic modulator 3. 3c is connected to the input terminal 304a of the acousto-optic modulator 2. The output terminal 304b of the acousto-optic modulator 2 is connected to the input terminal 305a of the optical amplifier 3. The output terminal 305b of the optical amplifier 3 is connected to the input terminal 306a of the polarization controller. The output terminal of the polarization controller is connected to the circulator 307a. The circulator port 307b is connected to the input terminal 113b of the polarization beam splitter 2. The circulator 307c is connected to the input terminal 401a of the 3dB coupler 401. The output of the 3dB coupler 401 is connected to 402a of the optical filter 2 and 406a of the optical filter 3, respectively. The output terminal of the optical filter 2 is connected to the input terminal 402a of the optical amplifier 406a. The input terminal 403a is connected to the optical amplifier 4, the output terminal of the optical amplifier 4 is connected to the input terminal 404a of the optical filter 4, the output of the optical filter 4 is connected to the input terminal 405a of the photodetector 1, and the output of the photodetector 1 is connected to the input terminal 410b of the acquisition card; the output terminal of the optical filter 3 is connected to the input terminal 407a of the optical amplifier 5, the output terminal of the optical amplifier 5 is connected to the input terminal 408a of the optical filter 5, the output of the optical filter 5 is connected to the input terminal 409a of the photodetector 2, the output of the photodetector 2 is connected to the input terminal 410a of the acquisition card, and the output terminal 410d of the acquisition card is connected to the input terminal 411a of the signal demodulation and decoupling unit.

3. A high-precision very low frequency fiber-optic distributed acoustic wave measurement method, characterized in that, Narrow linewidth laser 1 serves as the light source for generating multi-sideband Brillouin dynamic gratings. Its output laser 1 has a wavelength of 1550 nm and a power of 30 mW. It is input to electro-optic modulator 1, and arbitrary waveform generator 1 drives electro-optic modulator 1 to modulate laser 1, generating a multi-frequency signal with five sidebands and a sideband spacing of B1. The input is split into two paths by 3 dB coupler 115, namely multi-frequency signal 1 and multi-frequency signal 2. Narrow linewidth laser 2 outputs laser 1 with a wavelength of 1550 nm and a power of 30 mW. It generates a chirped signal with a chirped bandwidth of B2 through an electro-optic modulator 3 driven by a voltage-controlled oscillator 302 controlled by a signal generator 2. The pulse electrical signal with a pulse width of τ2 and a repetition rate of k2 output by the signal generator 201b drives the acousto-optic modulator 2 to convert it into chirped pulse light 1. After being amplified by an optical amplifier 3, it is input to a polarization controller 3 to maintain its polarization state. Then, it enters through port 307a and exits through port 307b, and is input to a polarization beam splitter 2 to be injected into the fast axis of the sensing fiber as a probe light. The optical signal output from port 307c of the circulator includes reverse Rayleigh scattering light, polygonal Brillouin dynamic grating reflection light, and leakage light from pump light 1. Optical filters 2 and 3 are used to filter out the reverse Rayleigh scattering light and polygonal Brillouin dynamic grating reflection light, which are channel 1 and channel 2, respectively. The two channels are then passed through optical amplifiers 4 and 5, and then through optical filters 4 and 5, and input to photodetectors 1 and 2 for direct detection. The output of photodetector 1 is connected to port 410b of the acquisition card, and the output of photodetector 2 is connected to port 410a of the acquisition card. The data is then processed by the input signal demodulation and decoupling unit of the acquisition card.

4. The high-precision very low frequency fiber-optic distributed acoustic wave measurement method according to claim 3, characterized in that, Multi-frequency signal 1 is input to arbitrary waveform generator 2, and the output electrical signal with a frequency shift of ∆f drives electro-optic modulator 2 to generate multi-frequency signal 3. This signal is amplified by optical amplifier 1 and filtered by optical filter 1 to remove spontaneous emission noise. It is then input to polarization controller 1 to maintain its polarization state, and finally injected into the slow axis of the sensing fiber as pump light 1 by polarization beam splitter 1. Multi-frequency signal 2, output by signal generator 201c with a pulse width of τ1 and a repetition rate of k1, drives acousto-optic modulator 1 to convert it into pulse light 1. This pulse is then amplified by optical amplifier 2 and its polarization state is maintained by polarization controller 2. It is then input to polarization beam splitter 2 and injected into the slow axis of the sensing fiber as pump light 2, which, together with pump light 1, excites a multi-sideband Brillouin dynamic grating.

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