Rapid demodulation device and method for related coding phi-OTDR (Optical Time Domain Reflectometer)

Abstract

The invention belongs to the technical field of optical fiber sensing, and provides a rapid demodulation device and method for correlation coding phi-OTDR, and the device comprises a laser, a coupler, a phase modulator, an acousto-optic modulator, an erbium-doped optical fiber amplifier, a circulator, a sensing optical fiber, a photoelectric detector, a low-pass filter, a rapid cross-correlation calculation module, a data collection card, and an upper computer. To-be-measured vibration acts on the sensing optical fiber, and after backward Rayleigh scattering signals are subjected to coded modulation and rapid cross-correlation decoding, vibration positioning and restoration are achieved through the system. The device and the method are applied to an optical fiber sensing technology, compared with a traditional phi-OTDR demodulation technology, the problems of high decoding delay and poor real-time performance are essentially solved, hardware acceleration decoding and preposed frequency shift are combined, and the device and the method have the advantages of low sampling rate acquisition, small data processing amount, high processing speed, high signal-to-noise ratio and the like. The method can be widely applied to the field of distributed vibration sensing and real-time monitoring.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic sensing technology and relates to a long-distance fast Φ-OTDR device for distributed fiber optic vibration detection. Specifically, it relates to a distributed fiber optic vibration sensing device with low sampling rate acquisition, fast processing speed, and hardware acceleration decoding technology using fast Walsh transform. Background Technology

[0002] Phase-sensitive optical time-domain reflectometry (OTDR), based on the backscattering Rayleigh principle, is a typical distributed fiber optic vibration sensing technology capable of simultaneously detecting and locating vibration events at multiple points. This technology integrates sensing and transmission, utilizing the optical fiber itself as the sensing medium. By detecting the modulation of characteristic parameters such as intensity, frequency, and phase of the backscattered Rayleigh light in the fiber by external vibrations, it can detect and locate vibration events along the entire fiber optic line. Compared to traditional electrical sensors, the Φ-OTDR system possesses significant advantages such as intrinsic safety, resistance to electromagnetic interference, corrosion resistance, and ease of network deployment for wide-area coverage. Therefore, it has been widely applied in many fields, including rail transit monitoring, pipeline safety monitoring, power transmission line monitoring, and seismic wave detection.

[0003] Maintaining high spatial resolution over long distances requires narrow pulses, which results in extremely weak backscattered Rayleigh light signals and severely degrades the system's signal-to-noise ratio (SNR) and detection sensitivity. To overcome this contradiction, pulse coding technology has been introduced into Φ-OTDR systems. This technology transmits a sequence of coded pulses with good autocorrelation properties (such as Gray code or Legendre code), which are then decoded at the receiver using a cross-correlation algorithm. Without sacrificing spatial resolution, it significantly increases the energy of the detection pulses, effectively overcoming the limitations of single-pulse systems and leading to a significant improvement in the system's SNR and detection sensitivity.

[0004] The signal-to-noise ratio (SNR) gain of pulse coding relies on backend cross-correlation decoding, a complex and computationally intensive operation. Existing technologies implement this in software, resulting in high data processing latency, making real-time performance a key limitation of the system. This problem is even more pronounced in medium- to long-distance applications requiring longer codes, restricting its application in real-time monitoring. There is an urgent need to effectively address the core issue of low computational efficiency in cross-correlation decoding while retaining the SNR advantages of pulse coding. This invention aims to overcome this technical bottleneck and provide an efficient and fast demodulation scheme for correlation-coded Φ-OTDR systems. Summary of the Invention

[0005] To address the technical bottleneck of existing correlation-coded Φ-OTDR systems, which suffer from significantly reduced real-time performance due to the additional computational load of cross-correlation decoding algorithms, this invention proposes a fast demodulation device and method for correlation-coded Φ-OTDR systems. The aim is to overcome the shortcomings of existing correlation-coded Φ-OTDR technology in terms of real-time performance of vibration localization demodulation.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: a fast demodulation device for correlation-encoded Φ-OTDR, comprising: a laser, a first coupler, a phase modulator, an acousto-optic modulator, an optical filter, an erbium-doped fiber amplifier, a circulator, and a sensing fiber connected in sequence; and a second coupler, a photodetector, a low-pass filter, a fast cross-correlation calculation module, a data acquisition card, and a host computer connected in sequence, wherein the second coupler is connected to the first coupler and the circulator respectively; It also includes a first voltage conversion circuit, a sinusoidal oscillation module, a second voltage conversion circuit, an acousto-optic modulator driver, a clock, and a related encoding pulse module; The first clock is connected to the control ports of the sinusoidal oscillation module, the correlated encoding pulse module, and the data acquisition card, respectively; the sinusoidal oscillation module is connected to the control port of the phase modulator through the first voltage conversion circuit; the correlated encoding pulse module is connected to the control port of the acousto-optic modulator through the second voltage conversion circuit and the acousto-optic modulator driver. The fast cross-correlation calculation module includes a first analog-to-digital conversion module, a first data cache module, a preprocessing module, a fast Walsh transform multi-level butterfly budget unit, a second analog-to-digital conversion module, a second data cache module, a clock calibration module, and a second clock.

[0007] Furthermore, the output port of the laser is connected to the input port of the first coupler, the first output port of the first coupler is connected to the input port of the phase modulator, and the second output port is connected to the first input port of the second coupler; the output port of the phase modulator is connected to the input port of the acousto-optic modulator, and the output port of the acousto-optic modulator is connected to the A port of the circulator after passing through an optical filter and an erbium-doped fiber amplifier in sequence; the B port of the circulator is connected to the sensing fiber, and the C port is connected to the second input port of the second coupler; the output port of the second coupler is connected to the input port of the photodetector, and the output port of the photodetector is connected to the host computer after passing through a low-pass filter, a fast cross-correlation calculation module, and a data acquisition card in sequence.

[0008] Furthermore, the first clock is used to provide a unified clock trigger signal for the sinusoidal oscillation module, the related encoding pulse module, and the data acquisition card.

[0009] Furthermore, in the fast cross-correlation calculation module, the output port of the second clock is connected to the sampling clock input port of the first analog-to-digital converter module, the sampling clock input port of the second analog-to-digital converter module, and the reference clock input port of the clock calibration module, respectively. The first output port of the clock calibration module is connected to the timing calibration input port of the first analog-to-digital converter module, and the second output port is connected to the timing calibration input port of the second analog-to-digital converter module. The analog signal input port of the first analog-to-digital converter module receives a first analog signal, and the digital signal output port is connected to the input port of the first data buffer module. The analog signal input port of the second analog-to-digital converter module receives a second analog signal, and the digital signal output port is connected to the input port of the second data buffer module. The output ports of the first and second data buffer modules are both connected to the input port of the preprocessing module. The output port of the preprocessing module is connected to the input port of the fast Walsh transform multi-stage butterfly budget unit. The output port of the fast Walsh transform multi-stage butterfly budget unit is connected to the result output port.

[0010] Furthermore, the phase modulator, driven by the sinusoidal oscillation module, performs a pre-frequency shift on the probe light. f c With respect to the driving frequency shift of the acousto-optic modulator f a In combination, the final frequency of the beat frequency signal after photoelectric detection and low-pass filtering is [value missing]. f a -f c .

[0011] Furthermore, the laser is used to generate a narrow-linewidth continuous light wave, which is split into two outputs by the first coupler, including a probe light and an intrinsic light. The intrinsic light directly enters the input port of the second coupler, and the probe light enters the input port of the phase modulator. The phase modulator is used to pre-shift the frequency of the probe light. The output port of the phase modulator is connected to the input port of the acousto-optic modulator, modulating the continuous probe light into a correlated coded light pulse sequence. After being filtered by an optical filter, the light pulse peak power is amplified by an erbium-doped fiber amplifier. The output port of the erbium-doped fiber amplifier is connected to port A of the circulator and injected into the sensing fiber through port B. The backscattered Rayleigh signal generated by the sensing fiber enters the second coupler through port C of the circulator and beats with the intrinsic light to generate a beat frequency light signal. The photodetector converts the beat frequency optical signal into an electrical signal. After the electrical signal passes through a low-pass filter to remove high-frequency signals, it enters the fast cross-correlation calculation module for correlation decoding. The output port of the fast cross-correlation calculation module is connected to the data acquisition card. The data acquisition card transmits the decoded data to the host computer. After digital quadrature demodulation, the location and reconstruction of the vibration signal along the optical fiber can be realized.

[0012] Furthermore, the encoded sequence generated by the related encoding pulse module is Gray code or Legendre code.

[0013] The present invention also provides a fast demodulation method for correlated encoding Φ-OTDR, based on the aforementioned fast demodulation device, comprising the following steps: 1) After the narrow-linewidth continuous optical wave signal generated by the laser is input into the first coupler, the optical wave signal is separated into intrinsic light and probe light. The intrinsic light enters the second coupler, and the probe light enters the phase modulator, where the frequency of the probe light is pre-shifted. The shifted probe light enters the acousto-optic modulator, where the continuous probe light is modulated into a correlated coded optical pulse sequence, which is then filtered by the optical filter. The resulting filtered signal enters the erbium-doped fiber amplifier to amplify the power of the optical pulse peak, and then enters the sensing fiber through the circulator, where a backscattered Rayleigh signal is generated. This scattered signal returns along the original path through the circulator and beats with the intrinsic light in the second coupler, generating a beat signal which is then input into the photodetector. 2) Generate a specific frequency through a sinusoidal oscillation module f The sinusoidal signal is used to drive the phase modulator via the first voltage conversion circuit, thereby continuously detecting the frequency of the probe light. f c Perform pre-positioning; 3) Triggered by the first clock signal, the related coding pulse module generates a related coding sequence, which is output to the acousto-optic modulator via the second voltage conversion circuit, driving the frequency shift as follows: f a An acousto-optic modulator modulates the frequency-shifted continuous light into a series of correlated coded pulses, which are then injected into the sensing fiber through a circulator. Backscattered Rayleigh light in the sensing fiber is output through the C port of the circulator and, together with the intrinsic light, generates a coded difference-frequency coherent signal at the second coupler, i.e., a signal with a frequency of [missing information]. f a ± f c The signal is converted into an electrical signal by a photodetector, and after passing through a low-pass filter, the frequency of the encoded coherent signal is reduced to [a lower value]. f a - f c ; 4) The low-pass filtered coded coherent signal is input to the first input port of the fast cross-correlation calculation module. The signal at the second input port of the fast cross-correlation calculation module comes from the coded pulse signal generated by the correlation coding pulse module. After efficient calculation in the fast cross-correlation calculation module, the decoded single pulse response data is output. 5) When triggered by the first clock signal, the data acquisition card acquires the output signal of the fast cross-correlation calculation module and transmits it to the host computer for digital quadrature demodulation. The amplitude signal and phase signal are obtained respectively. The positioning curve is obtained by performing moving differential and cumulative averaging on the amplitude signal. The vibration is located by adaptive threshold and fast peak finding algorithm. Finally, the vibration signal is restored by unwinding algorithm on the phase signal.

[0014] Furthermore, in the fast cross-correlation module: under the timing trigger of the stable sampling clock provided by the second clock and the synchronous calibration by the clock calibration module, the first analog-to-digital conversion module and the second analog-to-digital conversion module accurately sample the two input analog signals, convert them into digital signals respectively, and then transmit them to the first data buffer module and the second data buffer module for temporary storage to avoid overflow or loss during high-speed data transmission; subsequently, the digital signals are output from the first data buffer module and the second data buffer module to the preprocessing module; the preprocessed signals enter the fast Walsh transform multi-level butterfly budget unit. This unit is designed based on the fast Walsh transform butterfly operation logic, adopts a multi-level cascaded butterfly array + synchronous control module architecture, and is graded according to the length of the data to be processed. Each level contains an independent butterfly unit and is interconnected through a high-speed bus.

[0015] Furthermore, each butterfly unit is equipped with dual input terminals, a clock synchronization terminal, and an addition / subtraction operation module, enabling efficient completion of basic Fast Walsh Transform operations. The synchronization signal of the first clock precisely controls the timing of each stage of operation, ensuring processing stability. Two signals, one from the correlated encoding pulse module and the other from the low-pass filter, are input to the first-stage butterfly unit after being synchronized by the first clock. They first undergo a forward Fast Walsh Transform to complete the time-domain to Walsh domain conversion, i.e., each stage performs addition and subtraction operations, with the results passed step by step. Then, point-by-point multiplication is used to accelerate the operation by leveraging the fact that Walsh domain multiplication is equivalent to time-domain cross-correlation. Finally, the product result is transformed back to the time-domain signal, i.e., the cross-correlation result, by the inverse Fast Walsh Transform of the same architecture. This result is then output to the data acquisition card via an interface, providing a stable and regular input signal for analysis by the host computer.

[0016] Compared with the prior art, the present invention has the following advantages: This invention provides a correlation-encoded Φ-OTDR system device and method for fast demodulation. Utilizing a pre-shifting device, the frequency band of the encoded pulse coherent signal is first reduced. While ensuring medium-to-long sensing distances and high spatial resolution, the encoding system can acquire data at a low sampling rate, significantly reducing the amount of data processed for decoding and demodulation. Simultaneously, it combines a fast cross-correlation calculation module using Fast Walsh Transform for efficient decoding, rapidly recovering the single-pulse response, and implementing real-time demodulation of the vibration signal on a host computer. Compared to traditional technologies, due to the exponential increase in computational load for cross-correlation decoding, the encoding system using this technology exhibits better real-time performance over medium-to-long distances.

[0017] The fast demodulation device and method provided by this invention are applicable to correlation-coded pulse Φ-OTDR systems such as Gray code and Legendre code. Attached Figure Description

[0018] Figure 1 A schematic diagram of a fast demodulation device for correlated encoding Φ-OTDR provided in an embodiment of the present invention; In the diagram, 1 is the laser, 2 is the first coupler, 3 is the phase modulator, 4 is the acousto-optic modulator, 5 is the optical filter, 6 is the erbium-doped fiber amplifier, 7 is the first voltage conversion circuit, 8 is the sinusoidal oscillation module, 9 is the second voltage conversion circuit, 10 is the acousto-optic modulator driver, 11 is the first clock, 12 is the correlation coding pulse module, 13 is the circulator, 14 is the sensing fiber, 15 is the second coupler, 16 is the photodetector, 17 is the low-pass filter, 18 is the fast cross-correlation calculation module, 19 is the data acquisition card, and 20 is the host computer.

[0019] Figure 2 This is a schematic diagram of the fast cross-correlation calculation module in a fast demodulation device; In the figure, 21 is the first analog-to-digital conversion module, 22 is the first data buffer module, 23 is the preprocessing module, 24 is the fast Walsh transform multi-stage butterfly budget unit, 25 is the second analog-to-digital conversion module, 26 is the second data buffer module, 27 is the clock calibration module, and 28 is the second clock. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] The first objective of this invention is to provide a fast demodulation apparatus for correlation-coded Φ-OTDR. A fast demodulation apparatus for correlation-coded Φ-OTDR, such as... Figure 1 As shown, the system includes a laser 1, a first coupler 2, a phase modulator 3, an acousto-optic modulator 4, an optical filter 5, an erbium-doped fiber amplifier 6, a first voltage converter 7, a sinusoidal oscillation module 8, a second voltage converter 9, an acousto-optic modulator driver 10, a first clock 11, a correlation-encoded pulse module 12, a circulator 13, a sensing fiber 14, a second coupler 15, a photodetector 16, a low-pass filter 17, a fast cross-correlation calculation module 18, a data acquisition card 19, and a host computer 20. Figure 2 As shown, the fast cross-correlation calculation module 18 includes 21 as a first analog-to-digital conversion module, 22 as a first data cache module, 23 as a preprocessing module, 24 as a fast Walsh transform multi-level butterfly budget unit, 25 as a second analog-to-digital conversion module, 26 as a second data cache module, 27 as a clock calibration module, and 28 as a second clock.

[0022] The output port of laser 1 is connected to the input port of the first coupler 2. The first coupler 2 has two output ports. One output port is connected to the input port of phase modulator 3, and the other output port is connected to one input port of the second coupler 15. The output port of phase modulator 3 is connected to the input port of acousto-optic modulator 4. The output port of acousto-optic modulator 4 is connected to the input port of optical filter 5. The output port of optical filter 5 is connected to the input port of erbium-doped fiber amplifier 6. The output port of erbium-doped fiber amplifier 6 is connected to the input port of circulator 13, and one port of circulator 13 is connected to sensing fiber 14. The output port of circulator 13 is connected to the input port of the second coupler 15, and the output port of the second coupler 15 is connected to photodetector 16. The output port of photodetector is connected to the input port of low-pass filter 17. The output port of low-pass filter 17 is connected to the input of fast cross-correlation module 18. The output port of fast cross-correlation module 18 is connected to the input port of data acquisition card 19, and data acquisition card 19 is connected to host computer 20.

[0023] The first clock 11 is connected to the sinusoidal oscillation module 8, the correlation encoding pulse module 12, and the data acquisition card 19. At the same time, the first clock 11 is connected to the phase modulator 3 through the sinusoidal oscillation module 8 and the first voltage converter 7. The first clock 11 is connected to the acousto-optic modulator 4 through the correlation encoding pulse module 12, the second voltage converter 9, and the acousto-optic modulation driver 10.

[0024] In the fast cross-correlation calculation module, signals are input from the first analog-to-digital converter module 21 and the second analog-to-digital converter module 25, respectively. The digital signal output port of the first analog-to-digital converter module 21 is connected to the input port of the first data buffer module 22, and the digital signal output port of the second analog-to-digital converter module 25 is connected to the input port of the second data buffer module 26. The output ports of the first data buffer module 22 and the second data buffer module 26 are connected to the input port of the preprocessing module 23. The output port of the preprocessing module 23 is connected to the input port of the fast Walsh transform multi-stage butterfly budget unit 24, and the output port of the fast Walsh transform multi-stage butterfly budget unit 24 is connected to the result output port. The output port of the clock calibration module 27 is connected to the sampling clock input port of the first analog-to-digital converter module 21 and the reference clock input port of the clock calibration module 28, respectively, and the output port of the clock calibration module 28 is connected to the input ports of the first analog-to-digital converter module 21 and the second analog-to-digital converter module 25.

[0025] The working process of a fast demodulation device for correlated-coded Φ-OTDR is as follows: The narrow-linewidth continuous light wave generated by the laser is split into two outputs by a first coupler, including a probe light and an intrinsic light. The intrinsic light directly enters the input port of a second coupler, while the probe light enters the input port of a phase modulator. The phase modulator pre-shifts the frequency of the probe light, and its output port is connected to the input port of an acousto-optic modulator, modulating the continuous probe light into a correlated-coded light pulse sequence. After filtering by an optical filter, the pulse peak power is amplified by an erbium-doped fiber amplifier. The output port of the erbium-doped fiber amplifier is connected to port A of a circulator and injected into the sensing fiber through port B. The backscattered Rayleigh signal generated by the sensing fiber enters the second coupler through port C of the circulator and beats with the intrinsic light, generating a beat-frequency light signal. Figure 1 In the above, port A is the input port of circulator 13, used to receive the optical signal output from erbium-doped fiber amplifier 6; port B is connected to sensing fiber 14, responsible for transmitting the optical signal input from port A to sensing fiber 14; port C is the output port of circulator 13, used to receive the optical signal returned from sensing fiber 14 and output the signal to the input port of second coupler 15.

[0026] The photodetector converts the beat frequency optical signal into an electrical signal. After the electrical signal passes through a low-pass filter to remove high-frequency signals, it enters the fast cross-correlation calculation module for correlation decoding. The output port of the fast cross-correlation calculation module is connected to the data acquisition card. The data acquisition card transmits the decoded data to the host computer. After digital quadrature demodulation, the location and reconstruction of the vibration signal along the optical fiber can be realized.

[0027] The fast cross-correlation calculation module has two signal input ports. Its core lies in using the Fast Walsh Transform as the signal processing engine. The two input analog signals are accurately sampled by the first analog-to-digital conversion module and the second analog-to-digital conversion module within the module. After being converted into digital signals, they are first temporarily stored by the first data buffer module and the second data buffer module to avoid data overflow or loss during high-speed transmission. After the signal quality is optimized by the preprocessing module, the signals are sent to the Fast Walsh Transform multi-stage butterfly budget unit. This budget unit adopts a multi-stage butterfly structure and realizes the Fast Walsh Transform through parallel operation. It does not require complex hardware multipliers and can efficiently complete the conversion of signals from the time domain to the Walsh domain, quickly extract signal features, and thus reduce the time cost of signal transformation processing.

[0028] The output port of the sinusoidal oscillation module is connected to a first voltage conversion circuit, and the output port of the first voltage conversion circuit is connected to a phase modulator to drive the phase modulator to pre-shift the frequency of the probe light. The correlation coding pulse module is used to emit a correlation coding pulse sequence, and its output port is connected to a second voltage conversion circuit. The output port of the second voltage conversion circuit is connected to an acousto-optic modulator driver to drive the acousto-optic modulator and modulate the correlation coding light pulse sequence.

[0029] The encoded sequence generated by the related encoding pulse module is either Gray code or Legendre code.

[0030] The output port of the first clock is connected to the sine oscillation module, the correlated encoding pulse module, and the data acquisition card, respectively, to provide a unified clock trigger signal for the sine oscillation module, the correlated encoding pulse module, and the data acquisition card, so as to ensure the timing synchronization of the entire system.

[0031] The second objective of this invention is to provide a fast demodulation method for correlation-coded Φ-OTDR, comprising the following steps: The narrow-linewidth continuous light wave output by the laser is split into probe light and intrinsic light by the first coupler. The probe light enters the phase modulator, while the intrinsic light is directly transmitted to the second coupler. The first clock synchronously triggers the sinusoidal oscillation module and the correlation encoded pulse module. The former drives the phase modulator to frequency modulate the probe light through the first voltage converter, while the latter drives the acousto-optic modulator through the second voltage converter and the acousto-optic modulator to modulate the continuous probe light into a correlation encoded light pulse sequence. After the stray light of this sequence is filtered out by the optical filter, the peak power is amplified by the erbium-doped fiber amplifier and then injected into the sensing fiber through the circulator. The backscattered Rayleigh echo signal generated along the fiber returns along the original path and is guided to the second coupler through the circulator. It beats with the intrinsic light to generate a beat frequency optical signal. The optical signal corresponding to this signal and the intrinsic light is converted into a corresponding electrical signal by the photodetector. After the high-frequency noise is filtered out by the low-pass filter, the electrical signal is input into the fast cross-correlation calculation module to complete the cross-correlation decoding with the preset encoded sequence. The decoded signal is acquired by the data acquisition card and transmitted to the host computer, ultimately realizing the positioning and waveform reconstruction of the vibration signal along the sensing fiber.

[0032] In the fast cross-correlation calculation module, the second clock provides a sampling clock to both the first and second analog-to-digital (ADC) modules, and simultaneously outputs a reference clock to the clock calibration module. The clock calibration module synchronously calibrates the timing of the first and second ADC modules based on this reference clock. Meanwhile, signals output from the correlation encoding pulse module and the low-pass filter are input to the first and second ADC modules, respectively. The first ADC module outputs the converted digital signal to the first data buffer module for temporary storage, and the second ADC module outputs its digital signal to the second data buffer module for temporary storage, preventing data loss or overflow. Subsequently, the first and second data buffer modules transmit the signal to the preprocessing module for optimization. The preprocessed signal is input to the fast Walsh transform multi-stage butterfly budget unit, where multi-stage butterfly operations complete the signal transformation, and the final result is output from the output terminal.

[0033] In the host computer, the signal transmitted by the data acquisition card is digitally orthogonally demodulated to extract the amplitude and phase signals separately. Subsequently, the amplitude signal undergoes moving differential and cumulative averaging processing to generate a curve for vibration localization. Through adaptive threshold setting and a fast peak-finding algorithm, the location of the vibration time is accurately identified. Simultaneously, a dewinding algorithm is applied to the phase signal to reconstruct the true changes in the vibration waveform. Finally, by combining the localization results and the reconstructed waveform, high-precision detection and waveform reconstruction of the vibration signal along the sensing fiber are achieved.

[0034] Furthermore, the key modules and parameters involved in the method include: a first clock to ensure the timing consistency of the sinusoidal oscillation module and the associated encoded pulse module; and frequency modulation achieved through a phase modulator, which forwards the probe light frequency.f c Acousto-optic modulator drives frequency shift f a A related coded pulse sequence is generated; after photoelectric detection and low-pass filtering, the frequency of the beat frequency signal is reduced to [a value missing]. f a -f c To simplify processing, a fast cross-correlation calculation module utilizes a preset encoding sequence for efficient decoding, improving system response speed. The entire process, through hardware collaboration and algorithm optimization, achieves a fast demodulation method for correlation-encoded Φ-OTDR and vibration sensing.

[0035] The core of the fast cross-correlation calculation module is a multi-level butterfly operation unit based on the fast Walsh transform. This operation unit completes all calculations through parallel addition and subtraction operations, achieving extremely low hardware complexity.

[0036] The specific method for fast decoding and demodulation of the related-encoded Φ-OTDR is as follows: 1) After the narrow-linewidth continuous optical wave signal generated by the laser is input into the first coupler, the optical wave signal is separated into intrinsic light and probe light. The intrinsic light enters the second coupler, and the probe light enters the phase modulator, where the frequency of the probe light is pre-shifted. The shifted probe light enters the acousto-optic modulator, where the continuous probe light is modulated into a correlated coded optical pulse sequence, which is then filtered by the optical filter. The resulting filtered signal enters the erbium-doped fiber amplifier to amplify the power of the optical pulse peak, and then enters the sensing fiber through the circulator, where a backscattered Rayleigh signal is generated. This scattered signal returns along the original path through the circulator and beats with the intrinsic light in the second coupler, generating a beat signal which is then input into the photodetector. 2) Generate a specific frequency through a sinusoidal oscillation module f The sinusoidal signal is used to drive the phase modulator via the first voltage conversion circuit, thereby continuously detecting the frequency of the probe light. f c Perform pre-positioning; 3) Triggered by the first clock signal, the related coding pulse module generates a related coding sequence, which is output to the acousto-optic modulator via the second voltage conversion circuit, driving the frequency shift as follows: f a An acousto-optic modulator modulates the frequency-shifted continuous light into a series of correlated coded pulses, which are then injected into the sensing fiber through a circulator. Backscattered Rayleigh light in the sensing fiber is output through the circulator's C port and, together with the intrinsic light, generates a coded difference-frequency coherent signal at the second coupler, i.e., a signal with a frequency of [missing information]. f a ± f c The signal is converted into an electrical signal by a photodetector, and after passing through a low-pass filter, the frequency of the encoded coherent signal is reduced to [a lower value].f a - f c ; 4) The low-pass filtered coded coherent signal is input to the first input port of the fast cross-correlation calculation module. The signal at the second input port of the fast cross-correlation calculation module comes from the coded pulse signal generated by the correlation coding pulse module. After efficient calculation in the fast cross-correlation calculation module, the decoded single pulse response data is output. 5) When triggered by the first clock signal, the data acquisition card acquires the output signal of the fast cross-correlation calculation module and transmits it to the host computer for digital quadrature demodulation. The amplitude signal and phase signal are obtained respectively. The positioning curve is obtained by performing moving differential and cumulative averaging on the amplitude signal. The vibration is located by adaptive threshold and fast peak finding algorithm. Finally, the vibration signal is restored by unwinding algorithm on the phase signal.

[0037] In the fast cross-correlation module, under the timing trigger of the stable sampling clock provided by the second clock and the synchronous calibration by the clock calibration module, the first analog-to-digital conversion module and the second analog-to-digital conversion module accurately sample the two input analog signals, convert them into digital signals, and then transmit them to the first data buffer module and the second data buffer module for temporary storage to avoid overflow or loss during high-speed data transmission. Subsequently, the digital signals are output from the first data buffer module and the second data buffer module to the preprocessing module. The preprocessed signals enter the fast Walsh transform multi-level butterfly budget unit. This unit is designed based on the fast Walsh transform butterfly operation logic and adopts a "multi-level cascaded butterfly array + synchronous control module" architecture. It is divided into levels according to the length of the data to be processed, with each level containing an independent butterfly unit and interconnected through a high-speed bus. Each butterfly unit is equipped with dual input terminals, a clock synchronization terminal (connected to the first clock 11), and an addition and subtraction operation module, which can efficiently complete the basic operations of the fast Walsh transform. The synchronous control module receives the first clock 11 signal and accurately controls the operation sequence of each level to ensure processing stability. The core of this unit is to accelerate the cross-correlation calculation between the reference coded signal and the sensor echo signal. The two signals (from the correlation coded pulse module 12 and the low-pass filter 17, respectively) are input into the first-stage butterfly unit after being synchronized by the first clock 11. They are first transformed by the forward fast Walsh transform (each stage performs addition and subtraction operations, and the result is passed step by step) to complete the time-domain to Walsh domain conversion. Then, the calculation is accelerated by point-by-point multiplication using the characteristic that "Walsh domain multiplication is equivalent to time-domain cross-correlation". Finally, the product result is transformed by the reverse fast Walsh transform of the same architecture to restore the time-domain signal, that is, the cross-correlation result, and output to the data acquisition card 19 through the interface to provide a stable and regular input signal for the host computer 20 to analyze.

[0038] Compared with the prior art, the present invention has the following advantages: 1. This invention processes the backscattered Rayleigh signal of sensing optical fiber by combining correlation-coded pulse modulation and fast cross-correlation decoding, overcoming the problems of strong noise interference and slow decoding rate in traditional Φ-OTDR demodulation. It proposes a fast demodulation device and method for correlation-coded Φ-OTDR, providing a new solution for efficient demodulation of distributed optical fiber vibration sensing. 2. Compared with the conventional Φ-OTDR single-pulse demodulation technology, this invention utilizes the anti-interference characteristics of correlation coding and the high-speed computing capability of fast cross-correlation to simplify the demodulation circuit structure while further improving the signal-to-noise ratio and demodulation efficiency, thus achieving higher spatial resolution.

[0039] This invention is applied to correlation-coded Φ-OTDR demodulation technology, which has the following advantages compared with traditional Φ-OTDR demodulation technology: it combines correlation-coded pulse modulation with fast cross-correlation decoding to overcome the problems of strong noise interference and slow decoding rate; it simplifies the demodulation circuit structure and improves the signal-to-noise ratio and demodulation efficiency; it achieves higher spatial resolution and provides an efficient demodulation solution for distributed optical fiber vibration sensing, which can be widely used in the field of vibration monitoring along optical fibers.

[0040] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A fast demodulation device for coherent coded Φ-OTDR, characterized by, The application relates to a laser sensor system, which comprises a laser (1), a first coupler (2), a phase modulator (3), an acousto-optic modulator (4), an optical filter (5), an erbium-doped fiber amplifier (6), a circulator (13) and a sensing optical fiber (14) connected in sequence, and a second coupler (15), a photodetector (16), a low-pass filter (17), a fast cross-correlation calculation module (18), a data acquisition card (19) and an upper computer (20) connected in sequence, wherein the second coupler (15) is connected with the first coupler (2) and the circulator (13) respectively. The application further comprises a first voltage conversion circuit (7), a sinusoidal oscillation module (8), a second voltage conversion circuit (9), an acousto-optic modulator drive (10), a clock (11) and a correlation coding pulse module (12). The first clock (11) is connected with the control ports of the sinusoidal oscillation module (8), the correlation coding pulse module (12) and the data acquisition card (19) respectively; the sinusoidal oscillation module (8) is connected with the control port of the phase modulator (3) through the first voltage conversion circuit (7); the correlation coding pulse module (12) is connected with the control port of the acousto-optic modulator (4) through the second voltage conversion circuit (9) and the acousto-optic modulator drive (10). The fast cross-correlation calculation module (18) comprises a first analog-digital conversion module (21), a first data buffer module (22), a preprocessing module (23), a fast Walsh transform multi-stage butterfly precalculation unit (24), a second analog-digital conversion module (25), a second data buffer module (26), a clock calibration module (27) and a second clock (28). The output port of the laser (1) is connected with the input port of the first coupler (2); the first output port of the first coupler (2) is connected with the input port of the phase modulator (3), and the second output port is connected with the first input port of the second coupler (15); the output port of the phase modulator (3) is connected with the input port of the acousto-optic modulator (4); the output port of the acousto-optic modulator (4) is connected with the A port of the circulator (13) through the optical filter (5) and the erbium-doped fiber amplifier (6) in sequence; the B port of the circulator (13) is connected with the sensing optical fiber (14), and the C port is connected with the second input port of the second coupler (15); the output port of the second coupler (15) is connected with the input port of the photodetector (16); the output port of the photodetector (16) is connected with the upper computer (20) through the low-pass filter (17), the fast cross-correlation calculation module (18) and the data acquisition card (19) in sequence.

2. A fast demodulation device for coherent coded Φ-OTDR according to claim 1, characterized in that: The first clock (11) is used for providing a unified clock trigger signal for the sinusoidal oscillation module (8), the correlation coding pulse module (12) and the data acquisition card (19).

3. A fast demodulation device for coherent coded Φ-OTDR according to claim 1, characterized in that: ​ 4. A fast demodulation device for coherent coded Φ-OTDR according to claim 1, characterized in that: In the fast cross-correlation calculation module (18), the output port of the second clock (28) is connected with the sampling clock input port of the first analog-digital conversion module (21), the sampling clock input port of the second analog-digital conversion module (25) and the reference clock input port of the clock calibration module (27) respectively, the first output port of the clock calibration module (27) is connected with the timing calibration input port of the first analog-digital conversion module (21), and the second output port is connected with the timing calibration input port of the second analog-digital conversion module (25); the analog signal input port of the first analog-digital conversion module (21) receives the first analog signal, and the digital signal output port is connected with the input port of the first data buffer module (22); the analog signal input port of the second analog-digital conversion module (25) receives the second analog signal, and the digital signal output port is connected with the input port of the second data buffer module (26); the output port of the first data buffer module (22) and the output port of the second data buffer module (26) are connected with the input port of the pre-processing module (23); the output port of the pre-processing module (23) is connected with the input port of the fast Walsh transform multi-stage butterfly pre-calculation unit (24); and the output port of the fast Walsh transform multi-stage butterfly pre-calculation unit (24) is connected with the result output port.

5. A fast demodulation device for coherent coded Φ-OTDR according to claim 1, characterized in that: The phase modulator (3) drives the probe light to the front frequency shift amount f c , and the drive frequency shift amount of the acousto-optic modulator (4) f a , and the final frequency of the beat signal after photoelectric detection and low-pass filtering is f a - f c .

6. A fast demodulation device for coherent coded Φ-OTDR according to claim 1, characterized in that: The laser (1) is used for generating narrow linewidth continuous light wave, which is divided into two paths through the first coupler (2) and includes one path of detection light and one path of eigenlight; the eigenlight directly enters the input port of the second coupler (15), and the detection light enters the input port of the phase modulator (3); the phase modulator (3) is used for pre-shifting the frequency of the detection light, the output port of the phase modulator (3) is connected with the input port of the acousto-optic modulator (4), the continuous detection light is modulated into a related coding light pulse sequence, after filtering through the optical filter (5), the light pulse peak power is amplified through the erbium-doped fiber amplifier (6), the output port of the erbium-doped fiber amplifier (6) is connected with the A port of the circulator (13), and the backward Rayleigh scattering signal generated by the sensing optical fiber (14) enters the second coupler (15) through the C port of the circulator (13) and beats with the eigenlight to generate beat frequency light signal; The photoelectric detector (16) converts the beat frequency light signal into an electric signal, the electric signal enters the fast cross-correlation calculation module (18) after filtering out high frequency signals through the low pass filter (17) to perform related decoding operation, the output port of the fast cross-correlation calculation module (18) is connected with the data acquisition card (19), the data acquisition card (19) transmits the decoded data to the host computer (20), and after digital quadrature demodulation, the positioning and restoration of the fiber vibration signal along the line can be realized.

7. A fast demodulation device for coherent coded Φ-OTDR according to claim 3, characterized in that: The coding sequence generated by the related coding pulse module (12) is Gray code or Legendre code.

8. A fast demodulation method for coherent Φ-OTDR, based on the fast demodulation device according to any one of claims 1-7, characterized in that, The method comprises the following steps: The laser (1) is used for generating narrow linewidth continuous light wave, which is divided into two paths through the first coupler (2) and includes one path of detection light and one path of eigenlight; the eigenlight directly enters the input port of the second coupler (15), and the detection light enters the input port of the phase modulator (3); the phase modulator (3) is used for pre-shifting the frequency of the detection light, the output port of the phase modulator (3) is connected with the input port of the acousto-optic modulator (4), the continuous detection light is modulated into a related coding light pulse sequence, after filtering through the optical filter (5), the light pulse peak power is amplified through the erbium-doped fiber amplifier (6), the output port of the erbium-doped fiber amplifier (6) is connected with the A port of the circulator (13), and the backward Rayleigh scattering signal generated by the sensing optical fiber (14) enters the second coupler (15) through the C port of the circulator (13) and beats with the eigenlight to generate beat frequency light signal; The photoelectric detector (16) converts the beat frequency light signal into an electric signal, the electric signal enters the fast cross-correlation calculation module (18) after filtering out high frequency signals through the low pass filter (17) to perform related decoding operation, the output port of the fast cross-correlation calculation module (18) is connected with the data acquisition card (19), the data acquisition card (19) transmits the decoded data to the host computer (20), and after digital quadrature demodulation, the positioning and restoration of the fiber vibration signal along the line can be realized. The coding sequence generated by the related coding pulse module (12) is Gray code or Legendre code. The method comprises the following steps: 1) The narrow linewidth continuous wave signal generated by the laser (1) is input into the first coupler (2), and the wave signal is divided into characteristic light and probe light; the characteristic light enters the second coupler (15), and the probe light enters the phase modulator (3) to pre-shift the frequency of the probe light, and the shifted probe light enters the acousto-optic modulator (4) to modulate the continuous probe light into a related coded light pulse sequence and then enters the optical filter (5) for filtering; The obtained filtered signal enters the erbium-doped fiber amplifier (6) to amplify the power of the optical pulse, and then enters the sensing optical fiber (14) through the circulator (13), and generates a backward Rayleigh scattering signal in the sensing optical fiber (14); the scattering signal returns to the original route through the circulator (13), and beats with the characteristic light in the second coupler (15) to generate a beat signal and then input into the photodetector (16); 2) generating a sinusoidal signal of a specific frequency by a sinusoidal oscillation module (8) and driving the phase modulator (3) by a first voltage conversion circuit (7) to shift the frequency of the continuous probe light f f c performing a pre-shift​ 3) The correlation coding pulse module (12) generates a correlation coding sequence under the trigger of the first clock (11) signal, which is output to the acousto-optic modulator (10) through the second voltage conversion circuit (9) for driving. The driving frequency shift is f a The acousto-optic modulator (10) modulates the continuous light with frequency shift into a series of correlation coding pulses, which are injected into the sensing optical fiber (14) through the circulator (13). The backscattered Rayleigh light in the sensing optical fiber is output through the C port of the circulator (13), and generates a coded difference frequency coherent signal with the eigenlight at the second coupler (15), i.e. the frequency is f a ± f c The coded coherent signal is converted into an electrical signal by the photodetector (16), and after low-pass filtering, the frequency of the coded coherent signal is reduced to f a - f c ; 4) The low-pass filtered coded coherent signal is connected to the first input port of the fast cross-correlation calculation module (18), and the signal of the second input port of the fast cross-correlation calculation module (18) comes from the coded pulse signal generated by the related coded pulse module (12); after efficient operation in the fast cross-correlation calculation module (18), the output signal of the fast cross-correlation calculation module (18) is decoded into a single pulse response data; 5) Under the trigger of the first clock (11) signal, the data acquisition card collects the output signal of the fast cross-correlation calculation module (18) and transmits it to the upper computer (20) for digital quadrature demodulation, respectively obtaining the amplitude signal and the phase signal, obtaining the positioning curve by moving difference and cumulative average of the amplitude signal, positioning the vibration by adaptive threshold and fast peak seeking algorithm, and finally restoring the vibration signal by unwrapping algorithm on the phase signal.

9. A fast demodulation method for coherent coded Φ-OTDR according to claim 8, characterized in that: In the fast cross-correlation module (18): under the stable sampling clock provided by the second clock (28) and the timing trigger synchronized and calibrated by the clock calibration module (27), the first analog-to-digital conversion module (21) and the second analog-to-digital conversion module (25) accurately sample the input two-way analog signal, respectively convert it into a digital signal, and then transmit it to the first data buffer module (22) and the second data buffer module (26) for temporary storage to avoid overflow or loss during high-speed data transmission; then the digital signal is output from the first data buffer module (22) and the second data buffer module (26) to the preprocessing module (23); the preprocessed signal enters the fast Walsh transform multi-stage butterfly precalculation unit (24), which is designed based on the fast Walsh transform butterfly operation logic, adopts a multi-stage cascaded butterfly array + synchronous control module architecture, and is divided into stages according to the length of the data to be processed, each stage contains an independent butterfly unit and is interconnected through a high-speed bus.

10. A fast demodulation method for coherent coded Φ-OTDR according to claim 9, characterized in that: The single butterfly unit is equipped with double input ends, clock synchronization ends and addition and subtraction operation modules, and can efficiently complete basic operation of fast Walsh transform; the synchronization signal of the first clock accurately controls the operation time sequence of each stage, and guarantees the stability of processing; two signals respectively from a correlation coding pulse module (12) and a low-pass filter (17) are input into the first stage butterfly unit after being synchronized by the first clock (11), and are first converted into Walsh domain through forward fast Walsh transform, that is, each stage performs addition and subtraction operation, and the result is transmitted stage by stage; then, the Walsh domain product is obtained through point-by-point multiplication, and the operation is accelerated by using the characteristic that the Walsh domain product is equivalent to the time domain cross-correlation; finally, the final product result is converted into a time domain signal through inverse fast Walsh transform of the same structure, that is, a cross-correlation result, and is output to a data acquisition card through an interface, so as to provide a stable and regular input signal for analysis of an upper computer.

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