High-spatial-resolution distributed optical fiber sound wave sensing system based on FPGA real-time demodulation

Abstract

The invention discloses a high-spatial-resolution distributed optical fiber sound wave sensing system based on FPGA real-time demodulation. The system comprises a narrow linewidth laser source module, a first coupler, a high-bandwidth sweep frequency signal modulation module, a circulator, a coherent receiving and photoelectric conversion module and an FPGA integrated module. A laser source emitted by the narrow linewidth laser source module is divided into about 10% of reference light and about 90% of detection light through the first coupler. The FPGA integrated module is connected with the high-bandwidth sweep frequency signal modulation module, and transmits a sweep frequency radio frequency pulse signal and a frequency shift radio frequency pulse signal to the high-bandwidth sweep frequency signal modulation module; the high-bandwidth sweep frequency signal modulation module is connected with the sensing optical fiber through the circulator so as to output sweep frequency optical pulses; the sensing optical fiber is connected with the coherent receiving and photoelectric conversion module through the circulator to transmit Rayleigh backscattered light; the coherent receiving and photoelectric conversion module is connected with the FPGA integrated module, and the FPGA integrated module carries out signal processing to obtain vibration information on the optical fiber.

Description

Technical Field

[0001] This invention relates to the field of rail transit technology, and more specifically to a high spatial resolution distributed fiber optic acoustic wave sensing system based on FPGA real-time demodulation. Background Technology

[0002] Distributed fiber optic acoustic wave sensing systems possess numerous advantages, including distributed sensing, resistance to electromagnetic interference, high measurement accuracy, and resistance to environmental interference, leading to their widespread application in fields such as structural health monitoring, seismic exploration, cable monitoring, and perimeter security. Traditional single-frequency pulse-based distributed fiber optic acoustic wave sensing systems are limited by the performance of modulation devices, with pulse widths as small as 20 ns. Therefore, they can only achieve spatial resolutions of 2 meters or more. Furthermore, the constraint between pulse width and sensing distance makes long-distance transmission difficult under high spatial distribution conditions.

[0003] A novel linear frequency-modulated optical optical sensor (DAS) technique has been proposed in recent years. By transmitting long linearly swept frequency pulses into the optical fiber and performing matched filtering on the coherent Rayleigh backscattered light at the receiving end, the long probe pulse is compressed into a narrow probe pulse while retaining its high energy advantage. Therefore, it can simultaneously achieve sub-meter-level high spatial resolution and long detection distance. The system's spatial resolution is determined by the swept frequency bandwidth, not the pulse width, as shown in the following formula:

[0004]

[0005] Where c is the speed of light, n is the refractive index of the optical fiber, and B is the sweep bandwidth, that is, a 100MHz modulation bandwidth can achieve a spatial resolution of about 1m.

[0006] For example, CN118464176 A discloses a distributed fiber optic acoustic wave sensing method and system. It employs an unmatched filter to digitally filter and window the analytical sensing signal, achieving the desired windowing effect without causing energy loss in the detection pulse. The method is further optimized using signal compression processing techniques. CN118464176 A also discloses a distributed fiber optic acoustic wave sensing system and device based on a frequency-shifting loop. This system obtains a high-bandwidth swept-frequency optical signal through frequency-shifting loop splicing, achieving high spatial resolution vibration detection. However, this method suffers from signal gaps in the time domain in the high-bandwidth modulated signal obtained through splicing, and requires high consistency in delay between each iteration.

[0007] However, none of the above-mentioned existing technologies have solved the difficult problems in linear frequency modulation DAS technology: (1) Real-time signal processing is difficult. Whether it is matched filtering or unmatched filtering, in the long-distance high bandwidth detection scenario, the filter and the beat frequency signal need to be convolved and phase demodulated. The amount of data processing is huge, and the traditional host computer processing method is difficult to achieve real-time demodulation; (2) Clock synchronization problem between modulation signal generation and data acquisition. The existing technical solution for modulation signal generation is to use a waveform generator, while the data acquisition card is a separate device. The clock sources of the two are different, which makes it difficult to trigger acquisition synchronization and there is noise; (3) High bandwidth modulation signal generation. The bandwidth of traditional acousto-optic modulators is difficult to break through ±20% of the frequency shift, while electro-optic modulators have high modulation bandwidth but poor extinction ratio.

[0008] Therefore, new technologies and equipment are needed to at least partially solve the problems existing in the aforementioned prior art. Summary of the Invention

[0009] To address the above problems, this invention proposes a high spatial resolution distributed fiber optic acoustic wave sensing system based on FPGA real-time demodulation, comprising: a narrow linewidth laser source module, a first coupler, a high bandwidth swept frequency signal modulation module, a circulator, a coherent receiving and photoelectric conversion module, and an FPGA integrated module.

[0010] The laser source emitted by the narrow linewidth laser source module is split into approximately 10% reference light and approximately 90% probe light through the first coupler. The 10% reference light is connected to the coherent receiving and photoelectric conversion module, and the 90% probe light is connected to the high bandwidth sweep frequency signal modulation module to realize laser transmission.

[0011] The FPGA integrated module is connected to the high-bandwidth sweep frequency signal modulation module to transmit sweep frequency RF pulse signals and frequency shift RF pulse signals to the high-bandwidth sweep frequency signal modulation module.

[0012] The high-bandwidth sweep frequency signal modulation module is connected to the sensing optical fiber through a circulator to output a sweep frequency optical pulse train;

[0013] The sensing fiber is connected to the coherent receiver and photoelectric conversion module via a circulator to transmit Rayleigh backscattered light.

[0014] The coherent receiving and photoelectric conversion module is connected to the FPGA integrated module to convert the beat frequency signal into an electrical signal and transmit the electrical signal to the FPGA integrated module. The FPGA integrated module performs signal processing to obtain the vibration information on the optical fiber.

[0015] According to an embodiment of the present invention, the high-bandwidth sweep frequency signal modulation module includes a high extinction ratio electro-optic modulator, a second coupler, an acousto-optic modulator, a bias controller, and a pulse signal erbium-doped fiber amplifier.

[0016] According to an embodiment of the present invention, the high extinction ratio electro-optic modulator splits the light into 1% and 99% by a second coupler, wherein the 1% laser source enters the bias controller to realize the bias control of the high extinction ratio electro-optic modulator; the 99% laser source enters the acousto-optic modulator to modulate into high extinction ratio pulsed light, and then the gain is amplified by the pulse signal erbium-doped fiber amplifier.

[0017] According to an embodiment of the present invention, both the high extinction ratio electro-optic modulator and the acousto-optic modulator are connected to the FPGA integrated module via radio frequency signal lines. The high extinction ratio electro-optic modulator obtains a swept-frequency radio frequency pulse signal, and the acousto-optic modulator obtains a frequency-shifted radio frequency pulse signal.

[0018] According to an embodiment of the present invention, the high extinction ratio electro-optic modulator achieves signal modulation with a bandwidth of over 200MHz under the action of a swept-frequency radio frequency pulse signal, and the acousto-optic modulator achieves modulation based on the center frequency f under the action of a frequency-shifted radio frequency pulse signal. AOM The frequency is shifted and a pulse signal is generated, thereby obtaining a high-bandwidth swept frequency signal pulse signal.

[0019] According to an embodiment of the present invention, the frequency-shifting radio frequency pulse signal and the frequency-sweeping radio frequency pulse signal are set with a fixed delay, the delay duration being the time it takes for the laser source to reach the acousto-optic modulator from the electro-optic modulator.

[0020] According to an embodiment of the present invention, the FPGA integrated module includes a DAC module, an ADC module, a DDR module, a PCIe interface module, and an FPGA chip.

[0021] According to an embodiment of the present invention, the FPGA chip embeds a signal demodulation processing algorithm to realize real-time data processing.

[0022] According to an embodiment of the present invention, the processing algorithm steps are as follows:

[0023] S1: Based on the swept frequency modulation signal, implement a windowed matched filter inside the FPGA;

[0024] S2: The beat frequency signal obtained from coherent reception is converted into an electrical signal by a photoelectric conversion module, and then into a digital signal by an ADC module. After processing by a matched filter, the pulse-compressed analytical signal is obtained; and

[0025] S3: Perform IQ quadrature phase demodulation on the pulse-compressed analytical signal to obtain the phase and amplitude information of the entire line, and then upload it to the host computer via the PCIe bus.

[0026] According to an embodiment of the present invention, in S1, the windowed matched filter is implemented based on an FIR filter structure, and the order N and coefficients Q of the FIR filter are positively correlated with the swept frequency modulation signal M, as shown in the following equation:

[0027]

[0028] Q∝M

[0029] N∝t

[0030] Where: A is the amplitude of the sweep frequency signal, Let f be the initial phase, c be the rate of change of frequency, f0 be the initial frequency, and t be the time.

[0031] According to an embodiment of the present invention, in S3, the IQ quadrature phase demodulation operation includes generating orthogonal intrinsic signals I and Q, multiplying the I and Q signals by the compressed analytical signal, low-pass filtering, arctangent operation, and square root operation to obtain phase and amplitude data.

[0032] According to an embodiment of the present invention, the intrinsic signal center frequency is calculated according to the following formula.

[0033]

[0034] Where: f AOM f is the frequency shifter of the acousto-optic modulator. start f is the start frequency of the frequency sweep. end This is the sweep termination frequency.

[0035] According to an embodiment of the present invention, the low-pass filter used for low-pass filtering is an FIR filter.

[0036] According to an embodiment of the present invention, the FPGA integrated module transmits data and interacts with the host computer through a high-speed PCIe interface.

[0037] According to an embodiment of the present invention, the coherent receiving and photoelectric conversion module includes a third coupler and a photodetector.

[0038] This invention proposes a high spatial resolution distributed fiber optic acoustic wave sensing system based on FPGA real-time demodulation. It solves the clock synchronization problem between the modulation signal and the trigger pulse through integrated AD and DA design; optimizes the processing of complex algorithms through FPGA-based logic design, achieving real-time signal processing of the distributed fiber optic acoustic wave sensing system based on a linear frequency sweep scheme; and achieves high spatial resolution real-time vibration detection below 0.5m by combining electro-optic modulators and acousto-optic modulators and designing the timing of the RF modulation signal. Attached Figure Description

[0039] Figure 1 This is a schematic diagram of the high spatial resolution distributed fiber optic acoustic wave sensing system based on FPGA real-time demodulation according to an embodiment of the present invention.

[0040] Figure 2 This is a flowchart of the signal processing algorithm in a high spatial resolution distributed fiber optic acoustic wave sensing system based on FPGA real-time demodulation according to an embodiment of the present invention.

[0041] Figure 3 This is the original signal time-domain diagram of a high spatial resolution distributed fiber optic acoustic wave sensing system based on FPGA real-time demodulation according to an embodiment of the present invention.

[0042] Figure 4 The pulse compression analysis signal is the result of the high spatial resolution distributed fiber optic acoustic wave sensing system based on FPGA real-time demodulation according to the embodiment of the present invention.

[0043] Figure 5 The phase signal after IQ demodulation of the high spatial resolution distributed fiber optic acoustic wave sensing system based on FPGA real-time demodulation according to an embodiment of the present invention; and

[0044] Figure 6 The image shows the real-time demodulation result of a 100Hz vibration signal from a high spatial resolution distributed fiber optic acoustic wave sensing system based on FPGA real-time demodulation according to an embodiment of the present invention. Detailed Implementation

[0045] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The content shown is intended to fully illustrate the content of the present invention, but is not intended to limit the present invention.

[0046] Figure 1 This is a schematic diagram of the high spatial resolution distributed fiber optic acoustic wave sensing system based on FPGA real-time demodulation according to an embodiment of the present invention. Figure 2 This is a flowchart of the signal processing algorithm in a high spatial resolution distributed fiber optic acoustic wave sensing system based on FPGA real-time demodulation according to an embodiment of the present invention.

[0047] Referring to the accompanying drawings, the high spatial resolution distributed fiber optic acoustic wave sensing system based on FPGA real-time demodulation according to an embodiment of the present invention may include an optical system and an electrical system. More specifically, the high spatial resolution distributed fiber optic acoustic wave sensing system based on FPGA real-time demodulation according to the embodiment may include a narrow linewidth laser source module, a first coupler, a high bandwidth swept frequency signal modulation module, a circulator, a coherent receiving and photoelectric conversion module, and an FPGA integrated module.

[0048] Narrow linewidth laser source modules generate laser light as light sources. For example, they can be narrow linewidth lasers, which are laser devices characterized by extremely narrow spectral linewidths, typically ranging from 1 kHz to 200 kHz, preferably from 1 kHz to 10 kHz.

[0049] The laser source emitted by the narrow linewidth laser source module is split into a 10% reference beam and a 90% probe beam via a first coupler. The 10% reference beam is connected to the coherent receiving and photoelectric conversion module, while the 90% probe beam is connected to the high-bandwidth swept-frequency signal modulation module to achieve laser transmission.

[0050] The high-bandwidth swept-frequency signal modulation module includes a high extinction ratio electro-optic modulator, a second coupler, an acousto-optic modulator, a bias controller, and a pulse signal erbium-doped fiber amplifier. The high extinction ratio electro-optic modulator receives laser light from the first coupler, which is then split into 1% and 99% by the second coupler. The 1% laser source enters the bias controller to achieve bias control of the high extinction ratio electro-optic modulator. The 99% laser source enters the acousto-optic modulator, modulating it into high extinction ratio pulsed light, which is then amplified by the pulse signal erbium-doped fiber amplifier. The high extinction ratio electro-optic modulator is connected to the FPGA integrated module, for example, through an RF signal line, to obtain a swept-frequency RF pulse signal; the acousto-optic modulator is also connected to the FPGA integrated module, for example, through an RF signal line, to obtain a frequency-shifted RF pulse signal. The high extinction ratio electro-optic modulator can achieve signal modulation with a bandwidth of over 200MHz under the action of the swept-frequency RF pulse signal, and the acousto-optic modulator can achieve modulation based on the center frequency f under the action of the frequency-shifted RF pulse signal. AOM The frequency is shifted and a pulse signal is generated to obtain a high-bandwidth swept frequency signal pulse signal; the frequency-shifted radio frequency pulse signal and the swept radio frequency pulse signal are set with a fixed delay, the delay time being the time it takes for the laser source to reach the acousto-optic modulator from the electro-optic modulator.

[0051] The erbium-doped fiber amplifier is connected to the sensing fiber via a circulator, thereby outputting a swept-frequency optical pulse train to the sensing fiber. The sensing fiber is connected to the coherent receiver and photoelectric conversion module via the circulator, transmitting Rayleigh backscattered light.

[0052] The coherent receiving and photoelectric conversion module may include a third coupler and a photodetector to receive laser light from the first coupler and Rayleigh backscattered light from the sensing fiber, and convert the beat frequency signal into an electrical signal for transmission to the FPGA integrated module.

[0053] FPGA integrated modules are used for signal generation, acquisition, and processing. For example, they transmit swept-frequency and frequency-shifted RF pulse signals to high-bandwidth swept-frequency modulation modules, and process electrical signals from coherent receiving and photoelectric conversion modules to obtain vibration information along the optical fiber. More specifically, FPGA integrated modules may include DAC modules, ADC modules, DDR modules, PCIe interface modules, FPGA chips, clock modules, and power supply modules. The DAC (Digital-to-Analog Converter) converts digital signals to analog signals, while the ADC (Analog-to-Digital Converter) converts analog signals to digital signals. The DDR module provides high-speed on-chip memory, supporting data caching and fast access. The PCIe interface module enables high-speed communication between the FPGA and the host computer or peripherals (such as data transmission and expansion card connections). The clock module generates and manages the sampling clock, ensuring timing synchronization. The power supply module provides a stable power supply to the entire FPGA integrated module.

[0054] The FPGA chip is the core component of the entire FPGA integrated module, in which signal demodulation and processing algorithms are embedded to achieve real-time data processing.

[0055] Reference Appendix Figure 2 The processing algorithm of the embodiment of the present invention is as follows:

[0056] S1: Based on the swept-frequency modulation signal, a windowed matched filter is implemented inside the FPGA. This windowed matched filter is generated based on an FIR filter, whose order and coefficients are determined by the swept-frequency modulation signal. More specifically, the order N and coefficients Q of the FIR filter are positively correlated with the swept-frequency modulation signal M, as shown in the following equation:

[0057]

[0058] Q∝M

[0059] N∝t

[0060] Where: A is the amplitude of the sweep frequency signal, Let f be the initial phase, c be the rate of change of frequency, f0 be the initial frequency, and t be the time.

[0061] S2: The beat frequency signal obtained by coherent reception is converted into an electrical signal by the photoelectric conversion module, and then into a digital signal by the ADC module. After being processed by the matched filter, the pulse compressed analytical signal can be obtained.

[0062] S3: Perform IQ quadrature phase demodulation on the pulse-compressed analytical signal to obtain the phase and amplitude information of the entire line, and then upload it to the host computer via the PCIe bus.

[0063] More specifically, the IQ quadrature phase demodulation operation includes generating orthogonal intrinsic signals I and Q, multiplying the I and Q signals by the compressed analytical signal respectively, low-pass filtering, arctangent operation, and square root operation to obtain phase and amplitude data. The low-pass filtering uses an FIR filter, and the center frequency of the intrinsic signal is calculated using the following formula:

[0064]

[0065] Where: f AOM f is the frequency shifter of the acousto-optic modulator. start f is the start frequency of the frequency sweep. end This is the sweep termination frequency.

[0066] Appendix Figure 3-6 The diagram shows the results of FPGA-based real-time demodulation of the high spatial resolution distributed fiber optic acoustic wave sensing system based on real-time demodulation of FPGA according to the present invention. Figure 3 For the original signal time domain diagram, Figure 4 The analyzed signal after pulse compression Figure 5 The phase signal after IQ demodulation Figure 6 The image shows the real-time demodulation result of a 100Hz vibration signal. The results demonstrate that this invention achieves optimized processing of complex algorithms through FPGA-based logic design, realizing real-time signal processing for a distributed fiber optic acoustic wave sensing system based on a linear frequency sweep scheme. By combining electro-optic modulators and acousto-optic modulators, and designing the timing sequence of the RF modulation signal, it can achieve high-bandwidth modulation signal output above 200MHz, enabling real-time vibration detection with high spatial resolution, such as real-time vibration detection with a spatial resolution below 0.5m.

[0067] The above description of the embodiments is intended to enable those skilled in the art to understand and apply the present invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the embodiments described herein, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A high spatial resolution distributed fiber optic acoustic wave sensing system based on FPGA real-time demodulation, characterized in that, include: Narrow linewidth laser source module, first coupler, high bandwidth swept frequency signal modulation module, circulator, coherent receiver and photoelectric conversion module, and FPGA integrated module; The laser source emitted by the narrow linewidth laser source module is split into approximately 10% reference light and approximately 90% probe light through the first coupler. The 10% reference light is connected to the coherent receiving and photoelectric conversion module, and the 90% probe light is connected to the high bandwidth sweep frequency signal modulation module to realize laser transmission. The FPGA integrated module is connected to the high-bandwidth sweep frequency signal modulation module to transmit sweep frequency RF pulse signals and frequency shift RF pulse signals to the high-bandwidth sweep frequency signal modulation module. The high-bandwidth sweep frequency signal modulation module is connected to the sensing optical fiber through a circulator to output a sweep frequency optical pulse train; The sensing fiber is connected to the coherent receiver and photoelectric conversion module via a circulator to transmit Rayleigh backscattered light. The coherent receiving and photoelectric conversion module is connected to the FPGA integrated module to convert the beat frequency signal into an electrical signal and transmit the electrical signal to the FPGA integrated module. The FPGA integrated module performs signal processing to obtain the vibration information on the optical fiber.

2. The high spatial resolution distributed fiber optic acoustic wave sensing system based on FPGA real-time demodulation according to claim 1, characterized in that: The high-bandwidth sweep frequency signal modulation module includes a high extinction ratio electro-optic modulator, a second coupler, an acousto-optic modulator, a bias controller, and a pulse signal erbium-doped fiber amplifier.

3. The high spatial resolution distributed fiber optic acoustic wave sensing system based on FPGA real-time demodulation according to claim 2, characterized in that: The high extinction ratio electro-optic modulator is split into 1% and 99% laser sources by a second coupler, with the 1% laser source entering the bias controller to achieve bias control of the high extinction ratio electro-optic modulator; 99% of the laser source enters the acousto-optic modulator, modulates it into high extinction ratio pulsed light, and then amplifies the gain by the erbium-doped fiber amplifier.

4. The high spatial resolution distributed fiber optic acoustic wave sensing system based on FPGA real-time demodulation according to claim 1, characterized in that: Both the high extinction ratio electro-optic modulator and the acousto-optic modulator are connected to the FPGA integrated module via RF signal lines. The high extinction ratio electro-optic modulator obtains a swept-frequency RF pulse signal, while the acousto-optic modulator obtains a frequency-shifted RF pulse signal.

5. The high spatial resolution distributed fiber optic acoustic wave sensing system based on FPGA real-time demodulation according to claim 1, characterized in that: High extinction ratio electro-optic modulators achieve signal modulation with bandwidths exceeding 200MHz under the action of swept-frequency radio frequency pulse signals, while acousto-optic modulators achieve modulation based on the center frequency f under the action of frequency-shifted radio frequency pulse signals. AOM The frequency is shifted and a pulse signal is generated, thereby obtaining a high-bandwidth swept frequency signal pulse signal.

6. The high spatial resolution distributed fiber optic acoustic wave sensing system based on FPGA real-time demodulation according to claim 1, characterized in that: The FPGA integrated module includes a DAC module, an ADC module, a DDR module, a PCIe interface module, and an FPGA chip.

7. A high spatial resolution distributed fiber optic acoustic wave sensing system based on FPGA real-time demodulation according to claim 6, characterized in that: The FPGA chip embeds a signal demodulation processing algorithm to achieve real-time data processing.

8. A high spatial resolution distributed fiber optic acoustic wave sensing system based on FPGA real-time demodulation according to claim 7, characterized in that, The processing algorithm steps are as follows: S1: Based on the swept frequency modulation signal, implement a windowed matched filter inside the FPGA; S2: The beat frequency signal obtained by coherent reception is converted into an electrical signal by the photoelectric conversion module, and then into a digital signal by the ADC module. After being processed by the matched filter, the pulse compressed analytical signal is obtained. as well as S3: Perform IQ quadrature phase demodulation on the pulse-compressed analytical signal to obtain the phase and amplitude information of the entire line, and then upload it to the host computer via the PCIe bus.

9. A high spatial resolution distributed fiber optic acoustic wave sensing system based on FPGA real-time demodulation according to claim 8, characterized in that, In S1, the windowed matched filter is implemented based on the FIR filter structure. The order N and coefficients Q of the FIR filter are positively correlated with the swept frequency modulation signal M, as shown in the following equation: Q∝M N∝t Where: A is the amplitude of the sweep frequency signal, Let f be the initial phase, c be the rate of change of frequency, f0 be the initial frequency, and t be the time.

10. A high spatial resolution distributed fiber optic acoustic wave sensing system based on FPGA real-time demodulation according to claim 8, characterized in that, In S3, the IQ quadrature phase demodulation operation includes generating orthogonal intrinsic signals I and Q, multiplying the I and Q signals by the compressed analytical signal, low-pass filtering, arctangent operation, and square root operation to obtain phase and amplitude data.

11. A high spatial resolution distributed fiber optic acoustic wave sensing system based on FPGA real-time demodulation according to claim 10, characterized in that, The center frequency of the intrinsic signal is calculated using the following formula. Where: f AOM f is the frequency shifter of the acousto-optic modulator. start f is the start frequency of the frequency sweep. end This is the sweep termination frequency.

12. The high spatial resolution distributed fiber optic acoustic wave sensing system based on FPGA real-time demodulation according to claim 10, characterized in that, The low-pass filter used for low-pass filtering is an FIR filter.

13. A high spatial resolution distributed fiber optic acoustic wave sensing system based on FPGA real-time demodulation according to claim 6, characterized in that, The FPGA integrated module transmits data and interacts with the host computer through a high-speed PCIe interface.

14. The high spatial resolution distributed fiber optic acoustic wave sensing system based on FPGA real-time demodulation according to claim 1, characterized in that: The coherent receiving and photoelectric conversion module includes a third coupler and a photodetector.

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