Multi-spatial-resolution low-frequency-response DAS system and working method thereof

Abstract

The invention discloses a multi-spatial-resolution low-frequency-response DAS system and a working method thereof, and relates to the technical field of distributed optical fiber sound wave sensing. The system comprises a narrow linewidth laser, an acoustic optical modulator, a semiconductor optical amplifier, an optical coupler, a first circulator, a second circulator, an arbitrary function generator, a first photoelectric detector, a second photoelectric detector, a fiber bragg grating, a data acquisition card and a computer. According to the invention, by adopting a multi-spatial resolution design, a direct detection mode and broadband AOM and SOA combined modulation, high-precision positioning and signal detection can be realized in an optical fiber sensing system; according to the invention, the complexity of the system is reduced, the signal quality is obviously improved, the stability and accuracy of low-frequency signal detection are ensured, and the system is particularly suitable for long-distance and high-precision monitoring application.

Description

Technical Field

[0001] The present invention relates to the technical field of distributed fiber optic acoustic sensing, and particularly relates to a low-frequency response DAS system with multiple spatial resolutions and its working method. Background Art

[0002] Distributed fiber optic acoustic sensing (DAS) technology is one of the rapidly developing optical sensing technologies in recent years. It realizes continuous monitoring of vibrations or strains along the fiber optic path by utilizing the scattering characteristics of optical fibers, such as Rayleigh scattering. Compared with traditional vibration sensors, the DAS system has the advantages of fast measurement speed, high sensitivity, and distributed detection ability, so it is widely used in many fields such as structural health monitoring, seismic exploration, and transmission line monitoring.

[0003] Although the DAS technology has made remarkable progress in many applications, in practical applications, the existing DAS systems still face some technical bottlenecks, which are mainly reflected in the following aspects: 1) Low-frequency response problem: The DAS system performs well in the monitoring of high-frequency vibrations, and the upper frequency response limit depends on the length of the sensing optical fiber; however, there is a problem of poor response ability to low-frequency vibrations, which is mainly caused by the sensing structure of the DAS system and the phase noise of the laser light source. The traditional DAS system mainly adopts the coherent detection method, and this scheme is easily affected by phase noise. Especially when the length of the sensing optical fiber is long, as the optical signal propagates, the noise accumulation effect makes it difficult to accurately demodulate low-frequency signals. Therefore, how to improve the low-frequency response ability of the DAS system is an urgent problem to be solved. 2) Spatial resolution problem: The spatial resolution of the existing DAS system is mainly determined by the width of the detection pulse light / the linear sweep range of the chirped detection pulse light. Among them, the DAS based on chirped detection pulse light has a higher spatial resolution and can realize the detection of small-scale vibration signals; but when the spatial resolution matches the vibration scale, the system has the maximum detection signal-to-noise ratio, and the conventional DAS system usually has only a single spatial resolution in a single effective measurement. 3) Complexity of the modulation and demodulation process: Currently, many DAS systems adopt complex modulation and demodulation technologies to achieve accurate detection of vibration signals. However, the modulation methods of these systems are relatively complex, and there are certain limitations in aspects such as the frequency selection and modulation method of the modulation signal, which affect the flexibility and adaptability of the system. Summary of the Invention

[0004] In view of the above problems, the present invention proposes a low-frequency response DAS system with multiple spatial resolutions and its working method, in an attempt to solve or alleviate one or more of the above problems.

[0005] According to one aspect of the present invention, a low-frequency response DAS system with multiple spatial resolutions is proposed. The system includes: a narrow-linewidth laser 1, an acousto-optic modulator 2, a semiconductor optical amplifier 3, an optical coupler 4, a first circulator 5, a second circulator 6, an arbitrary function generator 7, a first photodetector 8, a second photodetector 9, a fiber Bragg grating 10, a data acquisition card 11, and a computer 12;

[0006] Wherein, the optical signal output end of the narrow-linewidth laser 1 is connected to the optical signal input end of the acousto-optic modulator 2, the optical signal output end of the acousto-optic modulator 2 is connected to the optical signal input end of the semiconductor optical amplifier 3, the optical signal output end of the semiconductor optical amplifier 3 is connected to the input end of the optical coupler 4, and the output end of the optical coupler 4 is respectively connected to the first port 5-1 of the first circulator 5 and the input end of the first photodetector 8; The second port 5-2 of the first circulator 5 is connected to the fiber Bragg grating 10, the third port 5-3 of the first circulator 5 is connected to the first port 6-1 of the second circulator 6, the second port 6-2 of the second circulator 6 is connected to the sensing optical fiber to be measured, and the third port 6-3 of the second circulator 6 is connected to the input end of the second photodetector 9; The output ends of the first photodetector 8 and the second photodetector 9 are connected to the data acquisition card 11;

[0007] The data acquisition card 11 is connected to the computer 12; The arbitrary function generator 7 is respectively connected to the acousto-optic modulator 2, the semiconductor optical amplifier 3, and the computer 12;

[0008] The computer 12 includes a correction coefficient calculation unit, and the correction coefficient calculation unit is used to calculate a correction coefficient according to the data output by the data acquisition card 11 and transmit the correction coefficient to the arbitrary function generator 7.

[0009] Furthermore, the central wavelength of the light wave emitted by the narrow-linewidth laser 1 is 1550.24 nm, and the power is 100 mW.

[0010] Furthermore, the modulation bandwidth of the acousto-optic modulator 2 is not less than 80 MHz, the insertion loss is less than 3 dB, and the extinction ratio is greater than 40 dB; The gain range of the semiconductor optical amplifier 3 is 10 - 30 dB, and the saturated output power is 15 dBm.

[0011] Furthermore, the optical coupler 4 is a 1×2 coupler, and the splitting ratio is 95:5; The 3 dB operating bandwidth corresponding to the first circulator 5 and the second circulator 6 is 0.08 nm.

[0012] Furthermore, the operating wavelength ranges of the first photodetector 8 and the second photodetector 9 are 900 - 1700 nm, the detection responsivity is 0.9 A / W, the 3 dB bandwidth is 10 GHz, and the dark current is less than 5 nA.

[0013] Furthermore, the reflection bandwidth of the fiber Bragg grating 10 is 0.2 nm, and the insertion loss is less than 0.5 dB.

[0014] According to another aspect of the present invention, a working method for a multi-spatial resolution low-frequency response DAS system is proposed. The working method is implemented based on the above-mentioned multi-spatial resolution low-frequency response DAS system; the working method includes:

[0015] The narrow linewidth laser 1 emits a laser signal to the acousto-optic modulator 2. The acousto-optic modulator 2 modulates the laser signal into an optical pulse train containing multiple different frequency components. The semiconductor optical amplifier 3 amplifies the power of the modulated optical pulse train. The optical coupler 4 divides the power-amplified optical pulse train into two paths. The upper branch is used for detection light, and the lower branch is used for monitoring light. The first circulator 5 reflects and outputs the optical pulse train of the upper branch after removing the spontaneous emission noise through the fiber Bragg grating 10 to the second circulator 6; the second circulator 6 inputs the reflected optical signal into the fiber optic sensing cable to be measured.

[0016] The optical signal scattered back by the fiber optic sensing cable to be measured enters the second photodetector 9. The second photodetector 9 converts the received optical signal into an electrical signal; the optical pulse train of the lower branch divided by the optical coupler 4 enters the first photodetector 8. The first photodetector 8 converts the received optical signal into an electrical signal.

[0017] The electrical signals output by the first photodetector 8 and the second photodetector 9 are received, demodulated, and stored by the data acquisition card 11.

[0018] The computer 12 calculates a correction coefficient according to the data output by the data acquisition card 11 and transmits the correction coefficient to any function generator 7.

[0019] Any function generator 7 generates drive signals CH1 to the acousto-optic modulator 2 and drive signals CH2 to the semiconductor optical amplifier 3 based on the correction coefficient.

[0020] Furthermore, calculating the correction coefficient according to the data output by the data acquisition card 11 includes:

[0021] Detect the amplitude of each pulse output by the data acquisition card 11; calculate the correction coefficient corresponding to each pulse amplitude according to the following formula:

[0022]

[0023] where A i represents the amplitude of each pulse; A0 is a preset target amplitude.

[0024] Further, after the acousto-optic modulator 2 receives the drive signal CH1 generated by the arbitrary function generator 7 based on the correction coefficient, the modulated optical pulse train is expressed as:

[0025]

[0026] where t represents the time; f i is the frequency of each pulse, is the initial phase of each pulse; σ i is the width of the Gaussian envelope, t 0,1 represents the pulse interval t between the first radio frequency pulse and the second radio frequency pulse 0,1 = τ1, t 0,2 represents the pulse interval t between the second radio frequency pulse and the third radio frequency pulse 0,2 = τ1 + τ2, t 0,3 represents the pulse interval t between the first radio frequency pulse and the third radio frequency pulse 0,3 = τ1 + τ2 + T p ; T p represents the pulse width, that is, the duration of each pulse; rect(.) is the rectangular window function.

[0027] The beneficial technical effects of the present invention are:

[0028] By adopting the multi-spatial resolution design, the present invention can achieve high-precision positioning and signal detection in the fiber optic sensing system. The combination of different spatial resolutions enables the system to simultaneously process vibration signals from multiple regions, thereby improving the signal acquisition and demodulation accuracy. In the demodulation of vibration signals with high signal-to-noise ratio, it is particularly suitable for long-distance and high-precision monitoring applications, such as structural health monitoring of power transmission lines, bridges, tunnels, etc. Through this design, the system can provide better signal quality and positioning accuracy than traditional methods.

[0029] The present invention innovatively adopts the direct detection method, utilizes the beat frequency process in the optical fiber, and transfers the information carried by the high-frequency signal to the low-frequency signal. Compared with the traditional coherent detection method, it has significant advantages: coherent detection usually requires a complex optical path design, while the direct detection method adopted by the present invention reduces the complexity of the system. Since the optical path difference between the detection pulse trains is constant, the phase noise will not accumulate with the increase of the sensing distance, thus ensuring the stability and accuracy of the low-frequency signal detection. This innovation is particularly suitable for low-frequency signals that require long-distance and high-precision detection, such as vibration or strain monitoring.

[0030] The present invention adopts the combined modulation technology of broadband AOM and SOA to modulate the pulse train, which not only has high flexibility but also can significantly improve the signal quality. Specifically, the broadband AOM can flexibly generate pulse trains with different carrier frequencies and provide precise time delay control, which is beneficial to realizing multi-spatial resolution detection; while the SOA further modulates the pulse train modulated by the AOM, effectively improving the extinction ratio of the pulse train and preventing crosstalk between pulse trains and deterioration of spatial resolution caused by continuous light leakage. In addition, the SOA can effectively replace the EDFA used in the traditional DAS system during the amplification process, thus avoiding the influence of the continuous ASE noise and transient effects generated during the EDFA amplification on the performance of the sensing system. Description of the Drawings

[0031] By referring to the following detailed description with reference to the accompanying drawings, the above and other objects, features and advantages of the exemplary embodiments of the present invention will become readily understood. In the drawings, several embodiments of the present invention are shown in an exemplary rather than restrictive manner, where:

[0032] Figure 1 is a schematic structural diagram of a multi-spatial resolution low-frequency response DAS system described in an embodiment of the present invention;

[0033] Figure 2 is a schematic diagram before and after the amplitude correction of the pulse train in an embodiment of the present invention;

[0034] Figure 3 is a schematic diagram of an optical pulse in an embodiment of the present invention. Detailed Embodiments

[0035] The principles and spirit of the present invention will be described below with reference to several exemplary embodiments. It should be understood that these embodiments are given only to enable those skilled in the art to better understand and then implement the present invention, and not to limit the scope of the present invention in any way. On the contrary, these embodiments are provided to make the present disclosure more thorough and complete, and to be able to convey the scope of the present disclosure fully to those skilled in the art.

[0036] The present invention proposes a multi-spatial resolution low-frequency response DAS system and its working method, aiming to solve the deficiencies of existing distributed fiber optic acoustic sensing technology in terms of low-frequency response, spatial resolution, signal stability, etc. The present invention effectively improves the detection sensitivity of low-frequency vibration signals, enhances the stability of the system, and provides flexible spatial resolution selection through optimization in multiple aspects such as optical signal modulation, signal processing, and amplitude correction, so as to meet the vibration monitoring requirements in different application scenarios.

[0037] An embodiment of the present invention proposes a multi-spatial resolution low-frequency response DAS system, as Figure 1As shown in the figure, the system realizes the modulation, transmission, detection, and data acquisition of optical signals through a series of optical devices to monitor the vibration information along the optical fiber. The system includes: a narrow linewidth laser 1, an acousto-optic modulator 2, a semiconductor optical amplifier 3, an optical coupler 4, a first circulator 5, a second circulator 6, an arbitrary function generator 7, a first photodetector 8, a second photodetector 9, a fiber Bragg grating 10, a data acquisition card 11, and a computer 12;

[0038] Among them, the optical signal output end of the narrow linewidth laser 1 is connected to the optical signal input end of the acousto-optic modulator 2, the optical signal output end of the acousto-optic modulator 2 is connected to the optical signal input end of the semiconductor optical amplifier 3, the optical signal output end of the semiconductor optical amplifier 3 is connected to the input end of the optical coupler 4, and the output end of the optical coupler 4 is respectively connected to the first port 5-1 of the first circulator 5 and the input end of the first photodetector 8; the second port 5-2 of the first circulator 5 is connected to the fiber Bragg grating 10, the third port 5-3 of the first circulator 5 is connected to the first port 6-1 of the second circulator 6, the second port 6-2 of the second circulator 6 is connected to the sensing optical fiber to be measured, and the third port 6-3 of the second circulator 6 is connected to the input end of the second photodetector 9; the output ends of the first photodetector 8 and the second photodetector 9 are connected to the data acquisition card 11;

[0039] The data acquisition card 11 is connected to the computer 12; the arbitrary function generator 7 is respectively connected to the acousto-optic modulator 2, the semiconductor optical amplifier 3, and the computer 12;

[0040] The computer 12 includes a correction coefficient calculation unit, and the correction coefficient calculation unit is used to calculate the correction coefficient according to the data output by the data acquisition card 11 and transmit the correction coefficient to the arbitrary function generator 7.

[0041] The working method implemented based on the above-mentioned multi-spatial resolution low-frequency response DAS system includes:

[0042] The narrow linewidth laser 1 emits a laser signal to the acousto-optic modulator 2, the acousto-optic modulator 2 modulates the laser signal into an optical pulse train containing multiple different frequency components, the semiconductor optical amplifier 3 amplifies the power of the modulated optical pulse train, the optical coupler 4 divides the power-amplified optical pulse train into two paths, where the upper branch is for detection light and the lower branch is for monitoring light, and the first circulator 5 reflects the optical pulse train of the upper branch after removing the spontaneous emission noise through the fiber Bragg grating 10 and outputs it to the second circulator 6; the second circulator 6 inputs the reflected optical signal into the sensing optical fiber to be measured;

[0043] The optical signal scattered back by the sensing optical fiber to be measured enters the second photodetector 9, and the second photodetector 9 converts the received optical signal into an electrical signal; the lower-branch optical pulse train split by the optical coupler 4 enters the first photodetector 8, and the first photodetector 8 converts the received optical signal into an electrical signal;

[0044] The electrical signals output by the first photodetector 8 and the second photodetector 9 are received, demodulated, and stored by the data acquisition card 11;

[0045] The computer 12 calculates a correction coefficient based on the data output by the data acquisition card 11 and transmits the correction coefficient to an arbitrary function generator 7;

[0046] The arbitrary function generator 7 generates drive signals CH1 to the acousto-optic modulator 2 and generates drive signal CH2 to the semiconductor optical amplifier 3 based on the correction coefficient.

[0047] According to an embodiment of the present invention, for the narrow linewidth laser (NLL) 1: The optical path is provided with a laser light source by a narrow linewidth laser 1; the laser emits light waves with a central wavelength of 1550.24 nm and a power of 100 mW.

[0048] For the acousto-optic modulator (AOM) 2: The laser beam first passes through the acousto-optic modulator 2 to modulate the laser signal into an optical pulse train containing multiple frequency components. Specifically, the function of the AOM is to mix the original optical signal (the frequency of the narrow linewidth laser 1 is f0) with multiple different modulation frequencies (f1, f2, f3) to generate optical pulse trains with multiple different frequency combinations, such as: f0 + f1, f0 + f2, f0 + f3. These optical pulse trains carry information of different frequencies and can perform precise frequency demodulation during the subsequent monitoring process. The broadband AOM has the following two functions: The first is to prevent the extinction ratio of the broadband AOM from being poor and continuous light leakage; the second is to amplify the power of the optical pulse train. The central operating wavelength of the acousto-optic modulator 2 is 1550.24 nm, the modulation bandwidth is not less than 80 MHz, the insertion loss is less than 3 dB, and the extinction ratio is greater than 40 dB. It can flexibly modulate pulse trains of different frequencies and achieve precise time delay control to meet the requirements of multi-spatial resolution and optimize the signal modulation effect.

[0049] For the semiconductor optical amplifier (SOA) 3: The modulated optical pulse train is power-amplified by the semiconductor optical amplifier 3. The function of the SOA is to increase the power of the optical pulse train so that the signal will not attenuate too much during long-distance optical fiber transmission, ensuring the signal intensity and ensuring that the subsequent detection part can receive a signal with sufficient intensity. The gain range of the semiconductor optical amplifier 3 is 10 dB to 30 dB, and the saturated output power is 15 dBm.

[0050] Optical coupler (OC) 4: The optically amplified optical pulse train is divided into two paths by a 95:5 optical coupler: the upper path for probe light and the lower path for monitoring light. The function of the optical coupler is to distribute the optical signal to different paths, ensuring the independent transmission of the two optical signals for different signal processing. Optical coupler 4 is a 1×2 coupler with a splitting ratio of 95:5.

[0051] First circulator (Cir) 5: The optical pulse in the upper path first enters port 5-1 of the first circulator 5. The function of the first circulator 5 is to guide the optical pulse to be reflected after passing through the fiber Bragg grating (FBG) 10; the fiber Bragg grating (FBG) 10 can effectively remove the spontaneous emission noise (ASE noise) generated by the amplification of the semiconductor optical amplifier (SOA) 3 and improve the purity of the optical signal. The reflected optical signal returns through port 5-2 of the first circulator 5 again, and the optical pulse with the noise signal removed finally outputs from port 5-3 of the first circulator 5. The 3dB operating bandwidth corresponding to the first circulator 5 is 0.08nm, and the central wavelength is 1550.24nm. The central reflection wavelength of the fiber Bragg grating 10 is 1550.24nm, the reflection bandwidth is 0.2nm, and the insertion loss is less than 0.5dB.

[0052] Second circulator (Cir) 6: The optical pulse output from port 5-3 of the first circulator 5 enters port 6-1 of the second circulator 6 and enters the fiber under test (FUT) through port 6-2. In the fiber under test, external perturbations will cause changes in the effective length and effective refractive index of the fiber, which will cause changes in the phase difference of the Rayleigh scattered light generated at both ends of the perturbed section of the fiber. The scattered optical signal is input again through port 6-2 of the second circulator 6 and outputs from port 6-3. In this process, the change in the Rayleigh scattered light caused by fiber vibration will cause the returned optical signal to carry vibration information. The 3dB operating bandwidth corresponding to the second circulator 6 is 0.08nm, and the central wavelength is 1550.24nm.

[0053] Arbitrary function generator (AFG): The AFG has two important functions in the optical path structure: the first is to generate an accurate signal CH1 to adjust the amplitude of the optical pulse train to ensure that the acousto-optic modulator 2 obtains a Gaussian pulse train with equal amplitude, thereby optimizing the sensing performance; the second is to generate an optical pulse CH2 to drive the operation of the SOA, precisely control the gain of the SOA, and ensure that the amplified optical pulse has sufficient power to overcome the losses during transmission.

[0054] First photodetector (PD1) 8: In the lower branch, the monitoring light is received by a photodetector (PD1) 8. The main function of the monitoring light is to provide a reference signal for the correction unit to perform amplitude correction. After being received by PD1, the monitoring light is transmitted to the data acquisition card (DAQ) 11 to obtain information on the system performance and status.

[0055] Second photodetector (PD2) 9: The optical signal output from the second circulator 6 is received by the second photodetector (PD2) 9, and the received optical signal is converted into an electrical signal. The electrical signal is further transmitted to the data acquisition card (DAQ) 11 for vibration signal demodulation and analysis. The working wavelength ranges of the first photodetector 8 and the second photodetector 9 are 900 - 1700 nm, the central wavelength is 1550.24 nm, the detection responsivity is 0.9 A / W, the 3 dB bandwidth is 10 GHz, and the dark current is less than 5 nA, ensuring that the system has high detection sensitivity and low noise characteristics to improve signal demodulation accuracy and signal-to-noise ratio.

[0056] Data acquisition card (DAQ) 11: Responsible for receiving the electrical signals converted by PD2 and PD1, and demodulating, analyzing, and storing the vibration signals. The data acquisition card 11 not only provides real-time vibration signal data but also performs further processing such as vibration source localization and frequency analysis through built-in signal processing algorithms.

[0057] Computer (PC) 12: Includes a correction unit; the monitoring light is transmitted to the data acquisition card (DAQ) 11 after being received by PD1 and then sent to the computer (PC) 12; the correction unit corrects the amplitude of the monitoring light signal; and then a signal is sent from the PC side, as shown at P1, to transfer the correction coefficient to any function generator 7. This process can ensure that the acousto-optic modulator 2 obtains a Gaussian pulse train with equal amplitude, achieving the best sensing performance. The corrected optical signal will be further transmitted to the data acquisition card 11 for analysis. Figure 1 As shown at P1, to transfer the correction coefficient to any function generator 7. This process can ensure that the acousto-optic modulator 2 obtains a Gaussian pulse train with equal amplitude, achieving the best sensing performance. The corrected optical signal will be further transmitted to the data acquisition card 11 for analysis.

[0058] Among them, the expression of the pulse train before correction is as follows:

[0059]

[0060] Among them, A i , i = 1, 2, 3 are the amplitudes of each pulse, f i is the frequency of each pulse, is the initial phase of each pulse; σ i is the width of the Gaussian envelope, t 0,1 represents the pulse interval t between the first radio frequency pulse and the second radio frequency pulse 0,1 = τ1, t 0,2 represents the pulse interval t between the second radio frequency pulse and the third radio frequency pulse 0,2= τ1 + τ2, t 0,3 represents the pulse interval t between the first radio frequency pulse and the third radio frequency pulse 0,3 = τ1 + τ2 + T p ; T p represents the pulse width, that is, the duration of each pulse, which is used to ensure that the pulse has obvious start and end boundaries in the time domain. rect(x) is a rectangular window function, and the expression is as follows:

[0061]

[0062] Since the original amplitudes A1, A2, and A3 of the pulses in the actually collected pulse train are inconsistent, in order to eliminate the crosstalk between pulse trains and the degradation of spatial resolution caused thereby, digital signal processing is performed on the collected data at the PC side. First, the peak amplitudes of the pulses are detected, and the correction coefficient k is calculated i :

[0063]

[0064] where A0 is a preset target amplitude. The PC side transmits these correction coefficients to the AFG through a digital interface. The AFG generates corresponding modulation signals according to the received values, and then adjusts the output pulse train by controlling the AOM

[0065] such as Figure 2 shown, after correction, the amplitudes of each pulse are unified to A0, and the expression of the corrected pulse train is:

[0066]

[0067] At the optical fiber to be measured, optical beats will occur for three different frequencies of light. After the beat frequency data passes through band-pass filters with three different parameters for filtering processing at the PC side, three different spatial resolutions can be obtained, which are respectively:

[0068]

[0069] In the formula, c represents the speed of light, and n represents the refractive index in the optical fiber. SR1 corresponds to the spatial resolution of obtaining the frequency information |f2 - f1| after the optical beats occur between the first optical pulse and the second optical pulse in the optical pulse train; SR2 corresponds to the spatial resolution of obtaining the frequency information |f3 - f2| after the optical beats occur between the second optical pulse and the third optical pulse in the optical pulse train; SR3 corresponds to the spatial resolution of obtaining the frequency information |f3 - f1| after the optical beats occur between the first optical pulse and the third optical pulse in the optical pulse train. Three different spatial resolutions can be obtained through these three different beat frequencies, and the most suitable spatial resolution can be selected according to the actual situation to improve the signal accuracy

[0070] Among them, the filtering process on the PC side is as follows:

[0071] (1) FFT transformation: Perform a fast Fourier transform (FFT) on the time-domain signal x(t) collected by the photodetector to obtain the frequency-domain signal X(f):

[0072]

[0073] This step separates different beat-frequency components in the frequency domain.

[0074] (2) Band-pass filtering (window function method): For each required beat-frequency component, design and apply the corresponding window function W i (f) in the frequency domain, which is defined as:

[0075]

[0076] where Δf i is the bandwidth parameter set to ensure smooth filtering. By multiplying the frequency-domain signal X(f) with the window function W i (f), the filtered frequency-domain signal is obtained:

[0077] X filtered,i (f) = X(f) · W i (f)

[0078] This step actually only retains the frequency components near the target beat frequency f b,i , and other components are suppressed or set to zero.

[0079] (3) IFFT reconstruction: Perform an inverse fast Fourier transform (IFFT) on each filtered frequency-domain signal to reconstruct the filtered time-domain signal:

[0080]

[0081] Through these three branches, the system respectively obtains the time-domain signals corresponding to the beat frequencies f b,1 , f b,2 and f b,3 , and then calculates the corresponding spatial resolution.

[0082] After the filtering process on the PC side, three different beat-frequency signals can be obtained respectively, corresponding to different spatial resolutions. The system selects the most suitable spatial resolution according to the actual measurement requirements to achieve the best sensing performance.

[0083] The optical pulse is shown as Figure 3As shown, the optical wave represents the optical wave signal emitted by the narrow linewidth laser 1, and CH1 represents the output signal of the output terminal 1 of the arbitrary function generator 7, which is a radio frequency pulse train composed of three pulses carrying different frequencies, with frequencies f1, f2, and f3 respectively, and the radio frequency pulse width is T p , τ1 represents the pulse interval between the first radio frequency pulse and the second radio frequency pulse, and τ2 represents the pulse interval between the second radio frequency pulse and the third radio frequency pulse; CH2 represents the output signal of the output terminal 2 of the arbitrary function generator 7, and the pulse period τ3 of the optical signal output by CH2 ≥ τ1 + τ2 + T p , which is to prevent continuous light leakage.

[0084] The core technologies of the present invention include the following aspects: 1) Design of multi-spatial resolution: By optimizing the modulation of the optical signal and the signal processing flow, the DAS system is enabled to have the ability of multi-spatial resolution. The system can select the most suitable spatial resolution according to different application scenarios, so as to be flexibly applied in different vibration monitoring tasks. This enables the system to have significant advantages in detail monitoring and vibration source positioning, and is especially suitable for the refined monitoring of the health of structures. 2) Optimization of low-frequency response: In traditional coherent detection, when the sensing distance increases, due to the non-fixed optical path difference between detection pulses, phase noise will gradually accumulate, thus reducing the detection accuracy of low-frequency signals. The present invention uses broadband AOM to modulate different carrier frequency pulse trains and combines direct detection technology. The direct detection technology utilizes the constant optical path difference between detection pulse trains to avoid the problem of phase noise increasing with the increase of the sensing distance, so that the information carried by high-frequency signals can still be stably converted into low-frequency signals in long-distance sensing. Therefore, the present invention overcomes the detection defect caused by phase noise accumulation in traditional coherent detection in long-distance sensing and also significantly improves the detection performance of low-frequency signals. 3) Simplified modulation process: The advantage of using broadband AOM + SOA combined modulation is that: AOM is used to flexibly modulate different carrier frequency pulse trains, with precise time delay control, which is beneficial to the realization of multi-spatial resolution; SOA is used to further modulate the different carrier frequency pulse trains modulated by AOM, effectively improving the extinction ratio of the pulse train, avoiding crosstalk between pulse trains and deterioration of spatial resolution caused by continuous light leakage. At the same time, SOA effectively replaces the amplification of optical pulses by EDFA in the traditional DAS system, avoiding the influence of the continuously generated ASE noise and transient effects during the amplification process of EDFA on the performance of the sensing system.

[0085] Although the spirit and principles of the present invention have been described with reference to several specific embodiments, it should be understood that the present invention is not limited to the specific embodiments disclosed, and the division of each aspect does not mean that the features in these aspects cannot be combined for benefit. This division is only for the convenience of expression. The present invention aims to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

1. A low-frequency response DAS system with multiple spatial resolutions, characterized in that, Including: A narrow linewidth laser (1), an acousto-optic modulator (2), a semiconductor optical amplifier (3), an optical coupler (4), a first circulator (5), a second circulator (6), an arbitrary function generator (7), a first photodetector (8), a second photodetector (9), a fiber Bragg grating (10), a data acquisition card (11), and a computer (12); Wherein, the optical signal output end of the narrow linewidth laser (1) is connected to the optical signal input end of the acousto-optic modulator (2), the optical signal output end of the acousto-optic modulator (2) is connected to the optical signal input end of the semiconductor optical amplifier (3), the optical signal output end of the semiconductor optical amplifier (3) is connected to the input end of the optical coupler (4), and the output end of the optical coupler (4) is respectively connected to the first port (5-1) of the first circulator (5) and the input end of the first photodetector (8); The second port (5-2) of the first circulator (5) is connected to the fiber Bragg grating (10), the third port (5-3) of the first circulator (5) is connected to the first port (6-1) of the second circulator (6), the second port (6-2) of the second circulator (6) is connected to the sensing optical fiber to be measured, and the third port (6-3) of the second circulator (6) is connected to the input end of the second photodetector (9); The output ends of the first photodetector (8) and the second photodetector (9) are connected to the data acquisition card (11); The data acquisition card (11) is connected to the computer (12); The arbitrary function generator (7) is respectively connected to the acousto-optic modulator (2), the semiconductor optical amplifier (3), and the computer (12); The computer (12) includes a correction coefficient calculation unit, and the correction coefficient calculation unit is used to calculate a correction coefficient according to the data output by the data acquisition card (11) and transmit the correction coefficient to the arbitrary function generator (7).

2. The multi - spatial - resolution low - frequency - response DAS system according to claim 1, wherein, The center wavelength of the light wave emitted by the narrow linewidth laser (1) is 1550.24 nm, and the power is 100 mW.

3. The multi - spatial - resolution low - frequency - response DAS system according to claim 1, characterized in that, The modulation bandwidth of the acousto-optic modulator (2) is not less than 80 MHz, the insertion loss is less than 3 dB, and the extinction ratio is greater than 40 dB; The gain range of the semiconductor optical amplifier (3) is 10-30 dB, and the saturated output power is 15 dBm.

4. A multi - spatial - resolution low - frequency - response DAS system according to claim 1, wherein, The optical coupler (4) is a 1×2 coupler, and the splitting ratio is 95:5; The 3 dB operating bandwidth corresponding to the first circulator (5) and the second circulator (6) is 0.08 nm.

5. A multi - spatial - resolution low - frequency - response DAS system according to claim 1, wherein The operating wavelength ranges of the first photodetector (8) and the second photodetector (9) are 900-1700 nm, the detection responsivity is 0.9 A / W, the 3 dB bandwidth is 10 GHz, and the dark current is less than 5 nA.

6. A multi - spatial - resolution low - frequency - response DAS system according to claim 1, characterized in that, The reflection bandwidth of the fiber Bragg grating (10) is 0.2 nm, and the insertion loss is less than 0.5 dB.

7. A working method of a low-frequency response DAS system with multiple spatial resolutions, characterized in that, The working method is implemented based on a multi-spatial resolution low-frequency response DAS system according to any one of claims 1-6; The working method includes: The narrow linewidth laser (1) emits a laser signal to the acousto-optic modulator (2). The acousto-optic modulator (2) modulates the laser signal into an optical pulse train containing multiple different frequency components. The semiconductor optical amplifier (3) amplifies the power of the modulated optical pulse train. The optical coupler (4) divides the optically pulse train with amplified power into two paths, where the upper path is for the probe light and the lower path is for the monitoring light. The first circulator (5) reflects and outputs the optical pulse train in the upper path to the second circulator (6) after removing the spontaneous emission noise through the fiber Bragg grating (10); the second circulator (6) inputs the reflected optical signal into the fiber optic sensing fiber under test; The optical signal scattered back by the fiber optic sensing fiber under test enters the second photodetector (9). The second photodetector (9) converts the received optical signal into an electrical signal; the optical pulse train in the lower path divided by the optical coupler (4) enters the first photodetector (8). The first photodetector (8) converts the received optical signal into an electrical signal; The electrical signals output by the first photodetector (8) and the second photodetector (9) are received, demodulated, and stored by the data acquisition card (11); The computer (12) calculates the correction coefficient according to the data output by the data acquisition card (11), and transmits the correction coefficient to an arbitrary function generator (7); The arbitrary function generator (7) generates a drive signal CH1 to the acousto-optic modulator (2) and a drive signal CH2 to the semiconductor optical amplifier (3) based on the correction coefficient.

8. The working method of a multi - spatial - resolution low - frequency - response DAS system according to claim 7, characterized in that, The calculating the correction coefficient according to the data output by the data acquisition card (11) includes: detecting the amplitude of each pulse output by the data acquisition card (11); calculating the correction coefficient corresponding to each pulse amplitude according to the following formula: where A i represents the amplitude of each pulse; A0 is a preset target amplitude.

9. The working method of a multi - spatial - resolution low - frequency - response DAS system according to claim 8, characterized in that, After receiving the drive signal CH1 generated by the arbitrary function generator (7) based on the correction coefficient, the modulated optical pulse train represented by the acousto-optic modulator (2) is: where t represents the time; f i is the frequency of each pulse, is the initial phase of each pulse; σ i is the width of the Gaussian envelope, t 0,1 represents the pulse interval t between the first radio frequency pulse and the second radio frequency pulse 0,1 = τ1, t 0,2 represents the pulse interval t between the second radio frequency pulse and the third radio frequency pulse 0,2 = τ1 + τ2, t 0,3 represents the pulse interval t between the first radio frequency pulse and the third radio frequency pulse 0,3 = τ1 + τ2 + T p ; T p represents the pulse width, that is, the duration of each pulse; rect(.) is the rectangular window function.

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