A real-time demodulation system for linear frequency modulation distributed optical fiber acoustic wave sensing based on polyphase decomposition

Abstract

Disclosed is a linear frequency modulation distributed optical fiber acoustic wave sensing real-time demodulation system based on polyphase decomposition, which comprises a coherent receiving and photoelectric conversion module and an FPGA signal processing integrated module; the FPGA signal processing integrated module comprises an ADC module and an FPGA chip, and the FPGA chip is embedded with a matching filter and demodulation processing algorithm based on a polyphase decomposition technology; the processing algorithm comprises the following steps: S1: generating a windowed matching filter and matching filter coefficients h, and generating polyphase sub-matching filters; the matching filter coefficients h are split into G groups of sub-matching filter coefficients according to a rule; S2: converting original high-speed serial data into G parallel data streams, and reducing the speed of each stream to 1 / G of the original speed; S3: sending the above G parallel data into corresponding polyphase sub-matching filters generated in S1 respectively for convolution operation, so as to realize distributed parallel matching filtering; and S4: performing phase demodulation processing.

Description

Technical Field

[0001] This invention relates to the field of distributed fiber optic acoustic sensing, and more specifically to a real-time demodulation system for linear frequency modulated distributed fiber optic acoustic wave sensing based on polyphase decomposition. Background Technology

[0002] Distributed fiber optic acoustic sensing (DAS) systems, with their advantages of distributed sensing, resistance to electromagnetic interference, high measurement accuracy, and strong environmental adaptability, have been widely used in various fields, including the oil and gas energy industry, power cable and energy facility monitoring, transportation, urban public safety and perimeter protection, geological and seismic science, and communications. With rapid technological advancements, typical DAS systems now generally achieve monitoring distances of tens of kilometers, spatial resolution at the meter level, and frequency response ranges covering a wide frequency band from a few hertz to several kilohertz.

[0003] In recent years, a novel linear frequency modulated (LFM) DAS technology has gradually attracted attention. This technology effectively compresses long pulses into narrow pulses by transmitting long-duration linearly swept pulses into the sensing fiber and performing matched filtering on the coherent Rayleigh backscattered signal at the receiving end. This significantly improves system performance while retaining high pulse energy. Combined with advanced phase demodulation schemes, this technology not only effectively suppresses signal fading but also enables quantitative strain measurement. Therefore, LFM DAS systems can simultaneously achieve sub-meter spatial resolution, long detection distances exceeding tens of kilometers, and a wider frequency response capability.

[0004] With the continuous improvement of detection distance and spatial resolution, the amount of data generated by the system is also increasing dramatically. In linear sweep frequency DAS systems, the amount of data generated per second can reach the GB level, which places extremely high demands on the performance of demodulation equipment. Currently, the mainstream processing methods can be divided into two categories: one is offline processing, which requires filters and beat frequency signals to perform convolution and phase demodulation operations in long-distance, high-bandwidth detection scenarios, resulting in a huge data processing burden, and traditional host computers are difficult to achieve real-time processing; the other is to achieve real-time demodulation using the parallel architecture of FPGA, but when performing matched filtering operations, a high-order filter is directly created according to the filter coefficients, which consumes a large number of DSP computing units and usually requires operation at extremely high clock frequencies, posing severe challenges to the hardware resources and timing convergence of FPGA. Summary of the Invention

[0005] In view of this, this paper proposes an FPGA real-time demodulation scheme based on multiphase decomposition technology, which is applied to a linear frequency modulated distributed fiber acoustic wave sensing (DAS) system to improve performance while effectively reducing hardware complexity and improving timing margin.

[0006] More specifically, the present invention proposes a real-time demodulation system for chirped distributed optical fiber acoustic sensing based on polyphase decomposition, comprising:

[0007] A real-time demodulation system for chirped distributed optical fiber acoustic sensing based on polyphase decomposition, comprising a coherent receiving and photoelectric conversion module and an FPGA signal processing integrated module, wherein the FPGA signal processing integrated module includes an ADC module and an FPGA chip, characterized in that a matched filtering and demodulation processing algorithm based on polyphase decomposition technology is embedded in the FPGA chip to realize real-time data processing;

[0008] The processing algorithm comprises the following steps:

[0009] S1: According to the waveform characteristics of the linear frequency-sweep modulated signal, generate a windowed matched filter and matched filter coefficients h, adopt a polyphase decomposition structure to generate polyphase sub-matched filters, and divide the matched filter coefficients h into G groups of sub-matched filter coefficients according to the following rule:

[0010] e k [g]= h[k + g * G]

[0011] wherein: k is the polyphase branch index (0≤k<G), g is the internal coefficient index of each branch, e k represents the filter coefficient of the k-th polyphase branch;

[0012] S2: The beat frequency digital signal output by the coherent receiving and photoelectric conversion module and sampled by the ADC is subjected to delay alignment processing according to the number of polyphase branches G, with sequential delay of G clock cycles, so as to convert the original high-speed serial data into G parallel data streams, and the rate of each stream is reduced to 1 / G of the original rate;

[0013] S3: The above G paths of parallel data are respectively sent into the corresponding polyphase sub-matched filters generated in S1 for convolution operation, so as to realize distributed parallel matched filtering;

[0014] S4: The filtered results of each polyphase branch are merged and sequentially reorganized according to the original time sequence relationship, restored to a path of time-domain signal, then filtered by a band-pass filter, and subjected to phase demodulation processing.

[0015] According to an embodiment of the present invention, the real-time demodulation system for chirped distributed optical fiber acoustic sensing based on polyphase decomposition further comprises a narrow linewidth laser source module, a first coupler, a frequency-sweep signal modulation module and a circulator;

[0016] The laser source emitted by the narrow linewidth laser source module is split into approximately 10% reference light and approximately 90% probe light via a 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 sweep frequency signal modulation module to achieve laser transmission. The FPGA signal processing integration module is connected to the sweep frequency signal modulation module to transmit sweep frequency RF pulse signals and frequency shift RF pulse signals to the sweep frequency signal modulation module. The sweep frequency signal modulation module is connected to the sensing fiber via a circulator to output a sweep frequency optical pulse train. The sensing fiber is connected to the coherent receiving and photoelectric conversion module via a circulator to transmit Rayleigh backscattered light. The coherent receiving and photoelectric conversion module is connected to the FPGA signal processing integration module to convert the beat frequency signal into an electrical signal and transmit the electrical signal to the FPGA signal processing integration module. The FPGA signal processing integration module performs signal processing to obtain vibration information on the optical fiber.

[0017] According to an embodiment of the present invention, the FPGA module is connected to the sweep frequency modulation module via a high-speed radio frequency signal line to perform sweep frequency pulse signal modulation.

[0018] According to an embodiment of the present invention, the sub-matched filter is implemented based on an FIR filter structure, and the order N and coefficients P of the FIR filter are positively correlated with the swept frequency modulation signal M, as shown in the following equation.

[0019]

[0020] P ∝ M / G

[0021] N ∝ t / G

[0022] Where: A is the amplitude of the sweep frequency signal, Let c be the initial phase and c be the rate of change of frequency. t represents the initial frequency and t represents time.

[0023] According to an embodiment of the present invention, the phase demodulation process is an IQ quadrature phase demodulation operation, which 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.

[0024] According to an embodiment of the present invention, the intrinsic signal center frequency is calculated according to the following formula.

[0025]

[0026] 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.

[0027] According to an embodiment of the present invention, the coherent receiving and photoelectric conversion module includes a third coupler and a photodetector. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the structure of a real-time demodulation system for linear frequency modulation distributed optical fiber acoustic wave sensing based on a multiphase decomposition according to an embodiment of the present invention.

[0029] Figure 2 The flowchart shows the signal processing algorithm in the real-time demodulation system of linear frequency modulated distributed optical fiber acoustic wave sensing based on polyphase decomposition according to an embodiment of the present invention.

[0030] Figure 3 The signal obtained by performing matched filtering on the collected Rayleigh backscattered light according to the embodiment of the present invention;

[0031] Figure 4 The signal obtained by matched filtering based on polyphase decomposition according to an embodiment of the present invention;

[0032] Figure 5 To apply a 100Hz sinusoidal vibration to an optical fiber according to an embodiment of the present invention, a single-point vibration phase diagram is obtained by using matched filtering followed by IQ demodulation.

[0033] Figure 6 To apply a 100Hz sinusoidal vibration to an optical fiber according to an embodiment of the present invention, a single-point vibration phase diagram is obtained by matched filtering based on polyphase decomposition and then demodulation using IQ. Detailed Implementation

[0034] 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.

[0035] Figure 1 This is a schematic diagram of the structure of a real-time demodulation system for linear frequency modulation distributed optical fiber acoustic wave sensing based on a multiphase decomposition according to an embodiment of the present invention. Figure 2 This is a flowchart of the signal processing algorithm in a real-time demodulation system for linear frequency modulated distributed optical fiber acoustic wave sensing based on multiphase decomposition according to an embodiment of the present invention.

[0036] Referring to the accompanying drawings, the real-time demodulation system for linear frequency modulated distributed fiber optic acoustic wave sensing based on polyphase decomposition according to an embodiment of the present invention may include an optical system and an electrical system. More specifically, the real-time demodulation system for linear frequency modulated distributed fiber optic acoustic wave sensing based on polyphase decomposition according to the embodiment may include a narrow linewidth laser source module, a first coupler, a swept frequency signal modulation module, a circulator, a coherent receiving and photoelectric conversion module, and an FPGA signal processing integrated module.

[0037] 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.

[0038] 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 swept frequency signal modulation module to achieve laser transmission.

[0039] The 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. 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 signal processing integration module, for example, through an RF signal line, to obtain a sweep frequency RF pulse signal. The acousto-optic modulator is also connected to the FPGA signal processing integration module, for example, through an RF signal line, to obtain a frequency-shifted RF pulse signal. The frequency-shifted RF pulse signal and the sweep frequency RF pulse signal are set with a fixed delay, the delay duration being the time it takes for the laser source to travel from the electro-optic modulator to the acousto-optic modulator.

[0040] 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.

[0041] 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 signal processing integrated module.

[0042] The FPGA signal processing integrated module is used for signal generation, acquisition and processing, for example, transmitting frequency-swept radio frequency pulse signals and frequency-shifted radio frequency pulse signals to the frequency-swept signal modulation module, processing electrical signals from the coherent receiving and photoelectric conversion module to obtain vibration information along the optical fiber, and so on. More specifically, the FPGA signal processing integrated module may include a DAC module, an ADC module, a DDR module, a PCIE interface module, an FPGA chip, a clock module, a DDR module, and a power supply module, etc. DAC (Digital-to-Analog Converter) is used to output digital signals as analog signals, and ADC (Analog-to-Digital Converter) completes the conversion of analog signals to digital signals. The DDR module provides high-speed on-chip storage, supporting data caching and fast access. The PCIE interface module is used to realize high-speed communication between the FPGA and the host (upper computer) or peripheral devices (such as data transmission and expansion card connection). The clock module is used to generate and manage sampling clocks to ensure timing synchronization. The power supply module provides stable power supply for the entire FPGA signal processing integrated module.

[0043] The FPGA chip is the core part of the entire FPGA signal processing integrated module, in which a signal demodulation processing algorithm is embedded to realize real-time data processing.

[0044] Refer to the attached Figure 2 , the processing algorithm of the embodiment of the present invention is as follows:

[0045] S1: According to the waveform characteristics of the linear frequency-swept modulation signal, generate a windowed matched filter and a matched filter coefficient h, adopt a polyphase decomposition structure to generate polyphase sub-matched filters, and divide the matched filter coefficient h into G groups of sub-matched filter coefficients according to the following rules:

[0046] e k [g]= h[k + g * G]

[0047] Where: k is the polyphase branch index (0≤k<G), g is the coefficient index inside each branch, e k represents the filtering coefficient of the k-th polyphase branch;

[0048] Wherein, the sub-matched filter is generated based on an FIR filter, and the order and coefficients of the FIR filter are determined by the frequency-swept modulation signal. More specifically, the order N of the FIR filter and the coefficient P are positively correlated with the frequency-swept modulation signal M, as shown in the following formula:

[0049]

[0050] P ∝ M / G

[0051] N ∝ t / G

[0052] Where: A is the amplitude of the frequency-swept 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.

[0053] S2: The beat frequency digital signal output by the coherent receiver and photoelectric conversion module and sampled by the ADC is delayed and aligned according to the number of multi-phase branches G. The original high-speed serial data is delayed by G clock cycles in sequence, and the original high-speed serial data is converted into G parallel data streams, with the rate of each stream reduced to 1 / G of the original.

[0054] S3: The above G parallel data are fed into the corresponding multiphase sub-matched filters generated by S1 for convolution operation to realize distributed parallel matched filtering;

[0055] S4: The results of filtering each multi-phase branch are merged and reassembled according to the original timing relationship to restore a single time-domain signal. This signal is then filtered by a bandpass filter and subjected to phase demodulation processing, such as IQ quadrature phase demodulation. The obtained phase and amplitude information of the entire line can be uploaded to the host computer via the PCIe bus.

[0056] 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:

[0057]

[0058] 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.

[0059] Figure 3 The signal obtained by performing matched filtering on the collected Rayleigh backscattered light according to the embodiment of the present invention; Figure 4 The signal obtained by matched filtering based on polyphase decomposition according to an embodiment of the present invention; Figure 5 To apply a 100Hz sinusoidal vibration to an optical fiber according to an embodiment of the present invention, a single-point vibration phase diagram is obtained by using matched filtering followed by IQ demodulation. Figure 6 To apply a 100Hz sinusoidal vibration to an optical fiber according to an embodiment of the present invention, a single-point vibration phase diagram is obtained by matched filtering based on polyphase decomposition followed by IQ demodulation. Results show that matched filtering is applied to the collected Rayleigh backscattered light respectively (… Figure 3 ) and matched filtering based on polyphase decomposition ( Figure 4The obtained signals show that the signal envelopes are basically consistent. When a 100Hz sinusoidal wave vibration is applied to the optical fiber, compared to obtaining a single-point vibration phase diagram using matched filtering followed by IQ demodulation, the phase diagram obtained by matched filtering based on multiphase decomposition followed by IQ demodulation in this invention can still be obtained with a relatively smooth phase diagram. Figure 6 ).

[0060] In other words, the linear sweep frequency distributed optical fiber acoustic wave sensing (DAS) system based on polyphase decomposition technology proposed in this invention uses an FPGA real-time demodulation scheme to decompose the matched filter into multiple parallel low-rate sub-filter operations by reconstructing the originally centralized and high-load filtering operation into multiple small-scale modules that can work together. This design breaks down the single large-scale resource requirement into multiple small-scale modules that can work together, which not only significantly reduces the consumption of key hardware resources such as DSP in the FPGA, but also reduces the system's timing requirements through parallel processing, alleviates the pressure of placement and routing, and ultimately achieves a simultaneous improvement in system resource utilization efficiency and timing margin while ensuring that the demodulation performance remains basically unchanged.

[0061] 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 real-time demodulation system for linear frequency modulated distributed fiber optic acoustic wave sensing based on polyphase decomposition, comprising a coherent receiving and photoelectric conversion module and an FPGA signal processing integration module, wherein the FPGA signal processing integration module comprises an ADC module and an FPGA chip, characterized in that, The FPGA chip embeds matched filtering and demodulation processing algorithms based on multiphase decomposition technology to achieve real-time data processing; The processing algorithm includes the following steps: S1: Based on the waveform characteristics of the linear sweep frequency modulation signal, a windowed matched filter and its coefficients h are generated. A polyphase decomposition structure is then used to generate polyphase sub-matched filters, and the matched filter coefficients h are divided into G groups of sub-matched filter coefficients according to the following rules: e k [g]= h[k + g * G] wherein: k is the polyphase branch index (0≤k<G), g is the coefficient index inside each branch, e k represents the filter coefficient of the k-th polyphase branch; S2: The beat frequency digital signal output by the coherent receiver and photoelectric conversion module and sampled by the ADC is delayed and aligned according to the number of multi-phase branches G. The original high-speed serial data is delayed by G clock cycles in sequence, and the original high-speed serial data is converted into G parallel data streams, with the rate of each stream reduced to 1 / G of the original. S3: The above G parallel data are fed into the corresponding multiphase sub-matched filters generated by S1 for convolution operation to realize distributed parallel matched filtering; S4: The results of filtering each multi-phase branch are merged and reassembled according to the original timing relationship to restore a single time-domain signal. This signal is then filtered by a bandpass filter and subjected to phase demodulation.

2. The real-time demodulation system for linear frequency modulation distributed optical fiber acoustic wave sensing based on multiphase decomposition according to claim 1, characterized in that: It also includes a narrow linewidth laser source module, a first coupler, a swept frequency signal modulation module, and a circulator; The laser source emitted by the narrow linewidth laser source module is split into approximately 10% reference light and approximately 90% probe light via a 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 sweep frequency signal modulation module to achieve laser transmission. The FPGA signal processing integration module is connected to the sweep frequency signal modulation module to transmit sweep frequency RF pulse signals and frequency shift RF pulse signals to the sweep frequency signal modulation module. The sweep frequency signal modulation module is connected to the sensing fiber via a circulator to output a sweep frequency optical pulse train. The sensing fiber is connected to the coherent receiving and photoelectric conversion module via a circulator to transmit Rayleigh backscattered light. The coherent receiving and photoelectric conversion module is connected to the FPGA signal processing integration module to convert the beat frequency signal into an electrical signal and transmit the electrical signal to the FPGA signal processing integration module. The FPGA signal processing integration module performs signal processing to obtain vibration information on the optical fiber.

3. The real-time demodulation system for linear frequency modulation distributed optical fiber acoustic wave sensing based on multiphase decomposition according to claim 2, characterized in that, The FPGA module is connected to the sweep frequency modulation module via a high-speed radio frequency signal line to perform sweep frequency pulse signal modulation.

4. The real-time demodulation system for linear frequency modulation distributed optical fiber acoustic wave sensing based on polyphase decomposition according to claim 1, characterized in that: The sub-matched filter is implemented based on the FIR filter structure. The order N and coefficients P of the FIR filter are positively correlated with the swept frequency modulation signal M, as shown in the following equation. P ∝ M / G N ∝ t / G Where: A is the amplitude of the sweep frequency signal, Let c be the initial phase and c be the rate of change of frequency. t represents the initial frequency and t represents time.

5. The real-time demodulation system for linear frequency modulation distributed optical fiber acoustic wave sensing based on multiphase decomposition according to claim 1, characterized in that, The phase demodulation process is an IQ quadrature phase demodulation operation, which 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.

6. The real-time demodulation system for linear frequency modulated distributed fiber optic acoustic wave sensing based on multiphase decomposition according to claim 5, 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.

7. The real-time demodulation system for linear frequency modulation distributed optical fiber acoustic wave sensing based on multiphase decomposition 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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