Distributed optical fiber sensing system based on four-frequency chirp modulation

Abstract

The invention discloses a distributed optical fiber sensing system based on four-frequency chirp modulation. The distributed optical fiber sensing system comprises a narrow linewidth laser module, a signal modulation module, a filling light modulation module, a coherent detection module and a signal demodulation and noise reduction module. Laser generated by the narrow linewidth laser is divided into probe light and local oscillator light through the polarization-maintaining coupler, the probe light passes through the signal modulation module to obtain a four-frequency multiplexing chirped pulse signal, the signal enters the filling light modulation module to obtain an amplified four-frequency chirped pulse signal, the amplified signal is sent to the sensing fiber to excite backward Rayleigh scattering light, and the laser is sent to the local oscillator light to be subjected to optical fiber laser scanning. The scattered light is sent to a coherent detection module to be subjected to beam combination interference with local oscillation light and then converted into a Rayleigh scattering analog electric signal, and the electric signal is sent to a GPU to be subjected to four-frequency chirp signal phase demodulation and noise reduction processing. According to the invention, pulse square wave distortion suppression in the light amplification process is realized, demodulation and noise reduction processing are carried out based on the GPU while long-distance and high-resolution distributed sensing is ensured, and real-time detection and sensing of high-frequency sound wave vibration signals are realized.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of distributed optical fiber sensing, and particularly relates to a distributed optical fiber sensing system based on four-frequency chirp modulation. BACKGROUND

[0002] The distributed optical fiber sensing technology is widely applied to the fields of structural health monitoring, optical fiber link diagnosis, intrusion monitoring and the like. The distributed acoustic sensing (DAS) technology based on Rayleigh scattering can complete high-precision real-time reconstruction of external vibration signals by detecting Rayleigh scattering light demodulation phase. This real-time distributed sensing technology has high requirements for the length of the distance that can be sensed by the equipment, the spatial resolution and the vibration frequency range that can be detected.

[0003] In the DAS technology, the performance of the distributed optical fiber sensing device is closely related to the modulation technology of the probe light and the laser pulse repetition frequency. According to the Shannon sampling law, the sampling frequency should be greater than twice the signal frequency. The vibration signal frequency range that can be sensed by the distributed optical fiber sensing device is affected by the sampling frequency, that is, the highest vibration signal frequency that can be demodulated by the distributed optical fiber sensing device is limited by the laser pulse repetition frequency in the device. The higher the laser pulse repetition frequency, the higher the vibration signal frequency that can be sensed. However, in order to ensure that the backward Rayleigh scattering of the two laser pulses does not overlap in time, the higher the laser pulse repetition frequency in the device, the shorter the distance of the sensing optical fiber. For example, if a 1kHz acoustic vibration signal needs to be detected, the laser pulse repetition frequency of the distributed optical fiber sensing device should be at least 2kHz. At this time, considering the propagation speed of light in the optical fiber, the maximum optical fiber distance that can be measured is 50km. If a 2kHz acoustic vibration signal needs to be detected, the maximum optical fiber distance that can be measured by the distributed optical fiber sensing device will decrease to 25km.

[0004] In order to ensure long distance, high resolution and wide detectable vibration frequency range, a reliable probe light modulation technology needs to be provided to improve the laser pulse repetition frequency, reduce signal distortion in the optical power amplification process, and improve the real-time detection capability of the equipment. SUMMARY

[0005] The application aims to provide a distributed optical fiber sensing system based on four-frequency chirp modulation, which constructs a four-frequency multiplexing distributed optical fiber sensing signal modulation scheme based on an acousto-optic frequency shifter (AOM), and realizes pulse square wave distortion suppression in the optical amplification process through a filling light modulation module. While ensuring the sensing capability of the distributed sensing device in terms of long distance, high resolution and wide detectable vibration frequency range, real-time demodulation and noise reduction processing are performed based on a GPU, and the real-time detection and sensing capability of high-frequency acoustic vibration signals is realized.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A distributed fiber optic sensing system based on four-frequency chirped modulation includes a narrow-linewidth laser module, a signal modulation module, a filler light modulation module, a coherent detection module, and a signal demodulation and noise reduction module. The narrow-linewidth laser module splits the laser light generated by the narrow-linewidth laser into a probe beam and a local oscillator beam via a 10 / 90 polarization-maintaining coupler. The probe beam passes through the signal modulation module to obtain modulated four-frequency multiplexed chirped pulse signals with different center frequencies. These four-frequency multiplexed chirped pulse signals then enter the filler light modulation module for multiplexing, amplification, and demultiplexing. The amplified four-frequency chirped pulse signal is obtained and sent into the sensing fiber to excite backscattered Rayleigh light. The backscattered Rayleigh light is then sent to the coherent detection module and interfered with the local oscillator light. After being converted into a Rayleigh scattering analog electrical signal by the balanced detector (BPD), the Rayleigh scattering analog electrical signal is input to the signal demodulation and noise reduction module and acquired by the acquisition card. The acquired signal is then sent to the GPU for four-frequency chirped signal phase demodulation and noise reduction processing, and finally sent to the storage hard disk to complete the signal storage.

[0008] Preferably, the signal modulation module includes a chirped modulation board, a synchronization control board, AOM-1, a 1×2 AOM, two 1×2 fiber optic splitters, AOM-2, AOM-3, AOM-4, AOM-5, and a 1×4 coupler; the continuous probe light passes through AOM-1 and is modulated by the chirped modulation board and the synchronization control board to modulate the injected probe light into a chirped pulse signal S; the chirped pulse signal S is frequency-shifted by the 1×2 AOM to output chirped pulse signals S0 and S1 respectively; the chirped pulse signals S0 and S1 are distributed into two optical signals by the two 1×2 fiber optic splitters; the chirped pulse... After signal S0 is split, one path is injected into AOM-2 and modulated into chirped pulse signal S2 under the control of the synchronization control board; the other path is injected into AOM-3 and modulated into chirped pulse signal S3 under the control of the synchronization control board. After chirped pulse signal S1 is split, one path is injected into AOM-4 and modulated into chirped pulse signal S4 under the control of the synchronization control board; the other path is injected into AOM-5 and modulated into chirped pulse signal S5 under the control of the synchronization control board. The four modulated chirped pulse signals S2, S3, S4 and S5 are injected into a 1×4 coupler and combined into a four-frequency multiplexed chirped pulse signal.

[0009] Preferably, all four chirped pulse signals are pulse signals with equal time delay difference, and the time delay difference between adjacent pulses in the combined four-frequency multiplexed chirped pulse signal is T0.

[0010] Preferably, the filling light modulation module includes a filling laser, AOM-6, WDM-1, EDFA-1, and WDM-2 connected in sequence; the filling laser generates a filling light signal with a wavelength different from the probe light, which is injected into AOM-6 and modulated under the control of the synchronization control board to generate a filling light signal with a peak-to-valley value that is exactly opposite to that of the probe light; the modulated filling light signal and the four-frequency multiplexed chirped pulse signal are both injected into WDM-1 for multiplexing, the combined light signal is injected into EDFA-1 for optical amplification, and then injected into WDM-2 for demultiplexing to obtain the amplified four-frequency chirped pulse signal.

[0011] Preferably, the coherent detection module includes EDFA-2, AOM-7, a coupler, and BPD; the amplified four-frequency chirped pulse signal is injected into the sensing fiber through a circulator, and the returned backscattered Rayleigh light is injected into EDFA-2 for optical amplification; the split local oscillator light is injected into AOM-7 and then frequency-shifted; the amplified Rayleigh scattered light and the frequency-shifted local oscillator light are simultaneously injected into the coupler to generate an optical interference signal; the optical interference signal is injected into BPD to be converted into an electrical signal and sent to the signal demodulation and noise reduction module.

[0012] Preferably, the optical interference signal generated by the backscattered Rayleigh light and the frequency-shifted local oscillator light after beam combining and interfering has four frequency bands, the center frequency intervals of the four frequency bands are equal, and their frequency components are separated in the optical domain.

[0013] Preferably, the signal demodulation and noise reduction module includes a data acquisition card, a GPU phase demodulation and noise reduction module, and a storage hard disk; the signal is converted into a Rayleigh scattering analog electrical signal by BPD and sent to the data acquisition card to be converted into a digital signal that the GPU can process, and then sent to the GPU phase demodulation and noise reduction module to realize frequency division phase demodulation, noise reduction processing and data synthesis; the processed digital sensing signal is sent to the storage hard disk.

[0014] Preferably, the processing flow of the signal demodulation and noise reduction module includes the following steps:

[0015] (1) Using a data acquisition card, the electrical signal converted by BPD is sampled at a fixed sampling frequency, converted into a digital signal that can be processed by the GPU, and sent to the GPU phase demodulation and noise reduction module.

[0016] (2) Based on the four-frequency chirped pulse signal, the original data is segmented along the time axis using a sliding time window with a time delay difference of T0;

[0017] (3) Bandpass filtering is performed on the segmented data according to the frequency sweep range of the chirped pulse corresponding to the window to obtain the Rayleigh scattering digital signal of a single chirped pulse;

[0018] (4) Perform matched filtering on the bandpass filtered data based on the center frequency and chirp bandwidth of the chirped pulse corresponding to the window;

[0019] (5) The data after matched filtering is processed by the rotating vector method to suppress coherent fading;

[0020] (6) Based on the Hilbert principle, calculate the vibration phase information of each point on the optical fiber reflected by the Rayleigh scattering signal of a single chirped pulse after suppressing coherent fading of the data.

[0021] (7) Call the denoising algorithm in the denoising algorithm library to filter the demodulated vibration phase data. The denoising algorithm called includes any one of spectral subtraction, wavelet denoising algorithm and denoising graph neural network.

[0022] (8) Based on the timing relationship of the four-frequency chirped pulses, the noise-reduced vibration phase information is combined to construct vibration phase change data of each point on the optical fiber within a certain time period, and then sent to the storage hard disk for data storage.

[0023] Compared with the prior art, the present invention has the following advantages:

[0024] This invention proposes a four-frequency multiplexed distributed fiber optic sensing signal modulation scheme based on an acousto-optic frequency shifter (AOM). While ensuring long-distance, high-resolution distributed sensing, the four-frequency multiplexing technology increases the laser pulse repetition frequency, thereby expanding the frequency range of the acoustic vibration signal that can be demodulated. Simultaneously, by filling the optical modulation module, pulse square wave distortion suppression during optical amplification is effectively achieved. Furthermore, through a combination of hardware and software, real-time demodulation and noise reduction processing based on a GPU is performed, enabling the device to achieve real-time detection and sensing capabilities for long-distance, high-resolution, and high-frequency acoustic vibration signals. Attached Figure Description

[0025] Figure 1 This is a block diagram of the distributed optical fiber sensing system based on four-frequency chirped modulation according to the present invention.

[0026] Figure 2 This is a timing diagram of the four-frequency multiplexed chirped pulse modulation process in the distributed optical fiber sensing system based on four-frequency chirped modulation of the present invention.

[0027] Figure 3 This is an estimated power spectral density diagram of the backscattered Rayleigh light signal and the local oscillator light signal after beam interference in the distributed optical fiber sensing system based on four-frequency chirped modulation according to the present invention.

[0028] Figure 4 This is a flowchart of the signal demodulation and noise reduction module in the distributed optical fiber sensing system based on four-frequency chirped modulation of the present invention. Detailed Implementation

[0029] To make the objectives and advantages of the present invention clearer, the present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0030] like Figure 1 As shown, this invention is a distributed fiber optic sensing system based on four-frequency chirped modulation, including a narrow-linewidth laser module, a signal modulation module, a filler light modulation module, a coherent detection module, and a signal demodulation and noise reduction module. The narrow-linewidth laser module generates a continuous laser with a stable center frequency and injects it into a 10 / 90 polarization-maintaining coupler to split it into a probe beam and a local oscillator beam. The signal modulation module modulates and generates the four-frequency multiplexed chirped pulse signal. The filler light modulation module amplifies the four-frequency chirped pulse signal while suppressing pulse square wave distortion during optical amplification. The coherent detection module performs coherent and photoelectric conversion between the local oscillator beam and the Rayleigh scattering signal. The signal demodulation and noise reduction module acquires, demodulates, reduces noise, and stores the received signal.

[0031] The narrow-linewidth laser module splits the laser generated by the narrow-linewidth laser into a probe beam and a local oscillator beam via a 10 / 90 polarization-maintaining coupler. The probe beam is then passed through the signal modulation module to obtain a modulated four-frequency multiplexed chirped pulse signal with different center frequencies. The four-frequency multiplexed chirped pulse signal enters the filler light modulation module, where it undergoes multiplexing, amplification, and demultiplexing to obtain an amplified four-frequency chirped pulse signal. This amplified four-frequency chirped pulse signal is then sent into the sensing fiber to excite backscattered Rayleigh light. The backscattered Rayleigh light is then sent to the coherent detection module and interfered with the local oscillator beam. After being converted into a Rayleigh scattering analog electrical signal by a balanced detector, the Rayleigh scattering analog electrical signal is input to the signal demodulation and noise reduction module. The signal is then acquired by the acquisition card, and the acquired signal is sent to the GPU for four-frequency chirped signal phase demodulation and noise reduction processing. Finally, the signal is sent to the storage hard disk for storage.

[0032] In a preferred embodiment, the signal modulation module modulates the injected probe laser into a four-frequency multiplexed chirped pulse signal with different center frequencies. The timing diagram of the four-frequency multiplexed chirped pulse modulation process is shown below. Figure 2As shown. Specifically, the signal modulation module includes a chirped modulation board, a synchronization control board, AOM-1, a 1×2 AOM, two 1×2 fiber optic splitters, AOM-2, AOM-3, AOM-4, AOM-5, and a 1×4 coupler. The chirped modulation board is used to modulate a chirped pulse signal with a specified center frequency and frequency sweep range. The synchronization control board is used to time-shift the chirped pulse signal of the AOM. Under the modulation of the chirped modulation board and the synchronization control board, AOM-1 modulates the injected probe light into a chirped pulse signal S with a period of T0, a center frequency of F1, and a chirped bandwidth of ΔF. The 1×2 AOM, under the control of the synchronization control board, frequency-shifts the injected chirped pulses, outputting a chirped pulse signal S0 with a period of 2T0, a center frequency of F1, and a chirped bandwidth of ΔF (0th-order frequency shift) and a chirped pulse signal S1 with a period of 2T0, a center frequency of F1+F2, and a chirped bandwidth of ΔF (1st-order frequency shift). The two 1×2 fiber optic splitters are used to split the injected optical signal. AOM-2, under the control of the synchronization control board, modulates the injected chirped pulse signal S0 after splitting into a chirped pulse signal S2 with a period of 4T0, a center frequency of F1+F2, and a chirped bandwidth of ΔF. AOM-3, under the control of the synchronization control board, modulates the injected chirped pulse signal S0 after splitting into a chirped pulse signal S3 with a period of 4T0, a center frequency of F1-F2, and a chirped bandwidth of ΔF. Under the control of the synchronization control board, AOM-4 modulates the injected chirped pulse signal S1 after splitting into a chirped pulse signal S4 with a period of 4T0, a center frequency of F1+2F2, and a chirped bandwidth of ΔF. AOM-5, under the control of the synchronization control board, modulates the injected chirped pulse signal S1 after splitting into a chirped pulse signal S5 with a period of 4T0, a center frequency of F1, and a chirped bandwidth of ΔF. The 1×4 coupler combines the four injected chirped pulse signals S2, S3, S4, and S5 into a single four-frequency multiplexed chirped pulse signal, with a time delay difference of T0 between adjacent pulses.

[0033] In the signal modulation module: the probe light, after being modulated by the AOM-1 on the chirped modulation board and the synchronization control board, is modulated into a chirped pulse signal S with a period of 0.25ms, a center frequency of 175MHz, and a chirped bandwidth of 20MHz. The modulated chirped pulse signal S is then frequency-shifted by the 1×2 AOM to output a chirped pulse signal S0 with a 0th-order frequency shift of 2T0, a center frequency of 175MHz, and a chirped bandwidth of 20MHz, and a chirped pulse signal S1 with a 1st-order frequency shift of 0.5ms, a center frequency of 275MHz, and a chirped bandwidth of 20MHz. The chirped pulse signals S0 and S1 are then distributed into two optical signals by the two 1×2 fiber optic splitters. After the chirped pulse signal S0 is split, one path is injected into AOM-2, and under the control of the synchronization control board, it is modulated into a chirped pulse signal S2 with a period of 1ms, a center frequency of 275MHz, and a chirped bandwidth of 20MHz; the other path is injected into AOM-3, and under the control of the synchronization control board, it is modulated into a chirped pulse signal S3 with a period of 1ms, a center frequency of 75MHz, and a chirped bandwidth of 20MHz. After the chirped pulse signal S1 is split, one path is injected into AOM-4, and under the control of the synchronization control board, it is modulated into a chirped pulse signal S4 with a period of 1ms, a center frequency of 375MHz, and a chirped bandwidth of 20MHz; the other path is injected into AOM-5, and under the control of the synchronization control board, it is modulated into a chirped pulse signal S5 with a period of 1ms, a center frequency of 175MHz, and a chirped bandwidth of 20MHz. The modulated four-channel chirped pulse signals S2, S3, S4, and S5 are injected into the 1×4 coupler, synthesized into a single four-frequency multiplexed chirped pulse signal. For example... Figure 2 As shown, the four chirped pulse signals are pulse signals with equal time delay difference, and the time delay difference between adjacent pulses in the combined four-frequency multiplexed chirped pulse signal is 0.25ms.

[0034] The fill light modulation module modulates the generated fill light and combines it with a four-frequency multiplexed chirped pulse signal to form continuous light, thus suppressing the distortion of the pulse square wave during optical amplification. After optical amplification and demultiplexing, an amplified four-frequency chirped pulse signal is obtained. Specifically, the fill light modulation module includes a fill laser, AOM-6, WDM-1, EDFA-1, and WDM-2 connected in sequence. The fill laser generates a fill light signal with a wavelength different from the probe light. AOM-6, under the control of the synchronization control board, modulates and generates a fill light signal with a peak-to-valley value exactly opposite to that of the probe laser. WDM-1 is used for combining the fill light and probe light. EDFA-1 is used for amplifying the combined optical signal. WDM-2 is used for demultiplexing the amplified optical signal to obtain the amplified four-frequency chirped pulse signal.

[0035] like Figure 1As shown, the fill laser generates a fill light signal with a wavelength different from the probe light. This fill light signal is injected into the AOM-6 and modulated under the control of the synchronization control board to generate a fill light signal with a peak-to-valley value that is exactly opposite to that of the probe laser. The modulated fill light signal and the four-frequency multiplexed chirped pulse signal are both injected into the WDM-1 for multiplexing. The combined light signal is then injected into the EDFA-1 for optical amplification, and then injected into the WDM-2 for demultiplexing to obtain the amplified four-frequency chirped pulse signal.

[0036] The coherent detection module includes an EDFA-2, an AOM-7, a coupler, and a BPD, which receives backscattered Rayleigh light in the optical fiber. The EDFA-2 amplifies the backscattered Rayleigh light in the fiber. The AOM-7 shifts the injected local oscillator light by 2F². The coupler combines the frequency-shifted local oscillator light with the amplified backscattered Rayleigh light to generate an optical interference signal. The BPD detects the optical interference signal and converts it into an electrical signal, which is then sent to the signal demodulation and noise reduction module. An amplified four-frequency chirped pulse signal is injected into the sensing fiber via a circulator, and the backscattered Rayleigh light is injected into the EDFA-2 for optical amplification. The local oscillator light is injected into the AOM-7 and then frequency-shifted. The amplified Rayleigh scattered light and the frequency-shifted local oscillator light are simultaneously injected into the coupler to generate an optical interference signal. The optical interference signal is injected into the BPD, converted into an electrical signal, and then sent to the signal demodulation and noise reduction module.

[0037] like Figure 3 The diagram shows the power spectral density estimation of the optical interference signal generated after beam combining and interference via a coupler. After the backscattered Rayleigh light signal and the frequency-shifted local oscillator light signal are combined and interfered, four frequency bands still exist. The frequency components of these four frequency bands are separate in the optical domain, and there is no aliasing. Corresponding to the chirped pulse signals S2, S3, S4, and S5, the center frequencies of the four-frequency multiplexed chirped optical interference signal are F1-F2, 3F2-F1, F1, and 2F2-F1, respectively, with a frequency component bandwidth of ΔF. Furthermore, the correlation frequencies satisfy the relationship F1 = 1.75F2 and... F < 0.5F2. In this embodiment, corresponding to the chirped pulse signals S2, S3, S4 and S5, the center frequency intervals of the four-frequency multiplexed chirped optical interference signals are equal at 50MHz, namely 75MHz, 125MHz, 175MHz and 25MHz, and the frequency component bandwidth is 20MHz.

[0038] The signal demodulation and noise reduction module utilizes a hardware-software combination, employing a GPU to perform phase demodulation and noise reduction on the detected interference electrical signals, and then stores the data. Specifically, the signal demodulation and noise reduction module includes a data acquisition card, a GPU phase demodulation and noise reduction module, and a storage hard drive. The data acquisition card is used to acquire and receive the electrical signals after BPD conversion and convert them into digital signals that the GPU can process. The GPU phase demodulation and noise reduction module is used for phase demodulation and noise reduction processing of the acquired digital four-frequency chirped pulse sensing signals. The storage hard drive is used to store the processed digital sensing signals.

[0039] In the signal demodulation and noise reduction module, the signal is converted into a Rayleigh scattering analog electrical signal via BPD, and then input to the signal demodulation and noise reduction module for signal acquisition, processing, and storage. For example... Figure 1 As shown, the Rayleigh scattering analog electrical signal is sent to the acquisition card and converted into a digital signal that can be processed by the GPU. Then, it is sent to the GPU phase demodulation and noise reduction module to realize frequency division phase demodulation, noise reduction processing and data synthesis. The processed digital sensing signal is sent to the storage hard disk.

[0040] The signal demodulation and noise reduction module's processing flow includes the following steps:

[0041] (1) Using a data acquisition card, the electrical signal converted by BPD is sampled at a fixed sampling frequency, converted into a digital signal that can be processed by the GPU, and sent to the GPU phase demodulation and noise reduction module.

[0042] (2) Based on the four-frequency chirped pulse signal, the original data is divided into segments along the time axis with a time delay difference of 0.25ms using a sliding time window. Each time window corresponds to data with a time length of 1ms.

[0043] (3) Bandpass filtering is performed on the segmented data according to the chirped pulse sweep width corresponding to the window to obtain the Rayleigh scattering digital signal of a single chirped pulse;

[0044] (4) Perform matched filtering on the bandpass filtered data based on the center frequency and chirp bandwidth of the chirped pulse corresponding to the window;

[0045] (5) The data after matched filtering is processed by the rotating vector method to suppress coherent fading;

[0046] (6) Based on the Hilbert principle, calculate the vibration phase information of each point on the optical fiber reflected by the Rayleigh scattering signal of a single chirped pulse after suppressing coherent fading of the data.

[0047] (7) Call the denoising algorithm in the denoising algorithm library to filter the demodulated vibration phase data. The denoising algorithm called includes any one of spectral subtraction, wavelet denoising algorithm and denoising graph neural network.

[0048] (8) Based on the timing relationship of the four-frequency chirped pulses, the noise-reduced vibration phase information is combined to construct vibration phase change data of each point on the optical fiber within a certain time period, and then sent to the storage hard disk for data storage.

[0049] The above embodiments are merely specific examples to further illustrate the purpose, technical solution, and beneficial effects of the present invention, and the present invention is not limited thereto. Any modifications, equivalent substitutions, improvements, etc., made within the scope of the disclosure of the present invention are included within the protection scope of the present invention.

Claims

1. A distributed optical fiber sensing system based on four-frequency chirped modulation, characterized in that: It includes a narrow linewidth laser module, a signal modulation module, a filler light modulation module, a coherent detection module, and a signal demodulation and noise reduction module; The narrow-linewidth laser module splits the laser generated by the narrow-linewidth laser into a probe beam and a local oscillator beam via a 10 / 90 polarization-maintaining coupler. The probe beam is then passed through the signal modulation module to obtain a modulated four-frequency multiplexed chirped pulse signal with different center frequencies. The four-frequency multiplexed chirped pulse signal enters the filler light modulation module, where it undergoes multiplexing, amplification, and demultiplexing to obtain an amplified four-frequency chirped pulse signal. This amplified four-frequency chirped pulse signal is then sent into the sensing fiber to excite backscattered Rayleigh light. The backscattered Rayleigh light is then sent to the coherent detection module and interfered with the local oscillator beam. After being converted into a Rayleigh scattering analog electrical signal by a balanced detector, the Rayleigh scattering analog electrical signal is input to the signal demodulation and noise reduction module. The signal is then acquired by the acquisition card, and the acquired signal is sent to the GPU for four-frequency chirped signal phase demodulation and noise reduction processing. Finally, the signal is sent to the storage hard disk for storage.

2. The distributed optical fiber sensing system based on four-frequency chirped modulation according to claim 1, characterized in that: The signal modulation module includes a chirped modulation board, a synchronization control board, AOM-1, 1×2AOM, two 1×2 fiber optic splitters, AOM-2, AOM-3, AOM-4, AOM-5 and a 1×4 coupler; The continuous probe light passes through AOM-1 and, under the modulation of the chirped modulation board and the synchronization control board, modulates the injected probe light into a chirped pulse signal S. The chirped pulse signal S is then frequency-shifted by 1×2 AOMs to output chirped pulse signals S0 and S1, respectively. S0 and S1 are then split into two optical signals by two 1×2 fiber splitters. After splitting, one of the chirped pulse signals S0 is injected into AOM-2, where it is modulated into a chirped pulse signal S2 under the control of the synchronization control board. Another path is injected into AOM-3, which is distributed and modulated into a chirped pulse signal S3 under the control of the synchronization control board; after the chirped pulse signal S1 is split, one path is injected into AOM-4, which is modulated into a chirped pulse signal S4 under the control of the synchronization control board; the other path is injected into AOM-5, which is modulated into a chirped pulse signal S5 under the control of the synchronization control board; the four modulated chirped pulse signals S2, S3, S4 and S5 are injected into a 1×4 coupler and combined into a four-frequency multiplexed chirped pulse signal.

3. The distributed optical fiber sensing system based on four-frequency chirped modulation according to claim 2, characterized in that: All four chirped pulse signals are pulse signals with equal time delay difference. The time delay difference between adjacent pulses in the combined four-frequency multiplexed chirped pulse signal is T0.

4. The distributed optical fiber sensing system based on four-frequency chirped modulation according to claim 1, characterized in that: The filling optical modulation module includes a filling laser, AOM-6, WDM-1, EDFA-1 and WDM-2 connected in sequence; The fill laser generates a fill light signal with a wavelength different from the probe light. It is injected into AOM-6 and modulated under the control of the synchronization control board to generate a fill light signal with a peak-to-valley value that is exactly opposite to that of the probe light. The modulated fill light signal and the four-frequency multiplexed chirped pulse signal are both injected into WDM-1 for multiplexing. The combined light signal is injected into EDFA-1 for optical amplification and then injected into WDM-2 for demultiplexing to obtain the amplified four-frequency chirped pulse signal.

5. The distributed optical fiber sensing system based on four-frequency chirped modulation according to claim 2, characterized in that: The coherent detection module includes EDFA-2, AOM-7, a coupler, and a BPD; The amplified four-frequency chirped pulse signal is injected into the sensing fiber through a circulator, and the returned backscattered Rayleigh light is injected into EDFA-2 for optical amplification; the split local oscillator light is injected into AOM-7 and then frequency-shifted; the amplified Rayleigh scattered light and the frequency-shifted local oscillator light are simultaneously injected into the coupler to generate an optical interference signal; the optical interference signal is injected into BPD to be converted into an electrical signal and sent to the signal demodulation and noise reduction module.

6. The distributed optical fiber sensing system based on four-frequency chirped modulation according to claim 5, characterized in that: The optical interference signal generated by the backscattered Rayleigh light and the frequency-shifted local oscillator light after beam combining has four frequency bands. The center frequency intervals of the four frequency bands are equal and their frequency components are separated in the optical domain. Corresponding to the chirped pulse signals S2, S3, S4 and S5, the center frequencies of the four-frequency multiplexed chirped optical interference signal are F1-F2, 3F2-F1, F1 and 2F2-F1, respectively. The frequency component bandwidth is ΔF, and the related frequencies satisfy the relationship F1=1.75F2 and ΔF<0.5F2.

7. The distributed optical fiber sensing system based on four-frequency chirped modulation according to claim 1, characterized in that: The signal demodulation and noise reduction module includes a data acquisition card, a GPU phase demodulation and noise reduction module, and a storage hard drive; The signal is converted into a Rayleigh scattering analog electrical signal by BPD and sent to the acquisition card to be converted into a digital signal that can be processed by the GPU. Then it is sent to the GPU phase demodulation and noise reduction module to realize frequency division phase demodulation, noise reduction processing and data synthesis. The processed digital sensing signal is sent to the storage hard disk.

8. The distributed optical fiber sensing system based on four-frequency chirped modulation according to claim 1, characterized in that, The signal demodulation and noise reduction module's processing flow includes the following steps: (1) Using a data acquisition card, the electrical signal converted by BPD is sampled at a fixed sampling frequency, converted into a digital signal that can be processed by the GPU, and sent to the GPU phase demodulation and noise reduction module. (2) Based on the four-frequency chirped pulse signal, the original data is segmented along the time axis using a sliding time window with a time delay difference of T0; (3) Bandpass filtering is performed on the segmented data according to the frequency sweep range of the chirped pulse corresponding to the window to obtain the Rayleigh scattering digital signal of a single chirped pulse; (4) Perform matched filtering on the bandpass filtered data based on the center frequency and chirp bandwidth of the chirped pulse corresponding to the window; (5) The data after matched filtering is processed by the rotating vector method to suppress coherent fading; (6) Based on the Hilbert principle, calculate the vibration phase information of each point on the optical fiber reflected by the Rayleigh scattering signal of a single chirped pulse after suppressing coherent fading of the data. (7) Call the denoising algorithm in the denoising algorithm library to filter the demodulated vibration phase data. The denoising algorithm called includes any one of spectral subtraction, wavelet denoising algorithm and denoising graph neural network. (8) Based on the timing relationship of the four-frequency chirped pulses, the noise-reduced vibration phase information is combined to construct vibration phase change data of each point on the optical fiber within a certain time period, and then sent to the storage hard disk for data storage.

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