Frequency-shift time-gated heterodyne distributed acoustic sensing system and demodulation algorithm

Abstract

The application provides a heterodyne distributed acoustic wave sensing system based on frequency shift time gating and a demodulation algorithm, and belongs to the technical field of distributed acoustic wave sensing. A light coupling module divides continuous laser into two paths of signal light and local oscillator light. A light modulation module sequentially performs pulse modulation and frequency shift modulation on the signal light, and outputs the modulated light pulse. A sensing optical fiber receives the light pulse output by the light modulation module, and generates a backscattering signal based on the back Rayleigh scattering effect. A light mixing module coherently mixes the backscattering signal and the local oscillator light. A photoelectric detection module converts the coherently mixed light signal into an electric signal. A signal processing unit collects the electric signal and extracts vibration information at different positions of the optical fiber through a phase demodulation algorithm. The application realizes synchronous integration of pulse and frequency shift modulation, optimizes the demodulation algorithm, improves the system stability, sensing accuracy and response efficiency, reduces the hardware cost and energy consumption, and is suitable for multi-scene application through innovative light path design and signal processing strategy.

Description

Technical Field

[0001] This invention belongs to the field of distributed acoustic wave sensing technology, specifically a heterodyne distributed acoustic wave sensing system and demodulation algorithm based on a frequency-shift time gate. Background Technology

[0002] Distributed acoustic sensing (DAS) technology, a novel sensing technology based on fiber Rayleigh scattering, has been widely applied in several key fields, including oil and gas pipeline monitoring, power line inspection, geological disaster early warning, and security perimeter protection, thanks to its advantages such as fully distributed sensing, resistance to electromagnetic interference, flexible deployment, and controllable cost. Its core principle is to inject a probe light signal into the sensing fiber and utilize the phase and amplitude information carried by the backscattered Rayleigh light to invert the vibration disturbance along the fiber, thereby achieving accurate perception of spatial location and event information.

[0003] In existing technologies, such as Figure 10 As shown, in a coherent distributed acoustic sensing (DAS) system based on phase-generated carrier (PGC) technology, different modulation depths result in different gains, leading to amplitude measurement distortion and introducing nonlinear errors. High sampling rates result in high hardware costs and computational limitations, long signal processing delays, and poor real-time performance. High computational resource consumption becomes a bottleneck for real-time performance. A typical PGC-based signal demodulation scheme is as follows: Figure 11 .like Figure 12 As shown, the technology of vibration measurement and localization by transmitting linearly frequency-modulated pulses (LFM) and combining it with digital signal processing requires a high-quality, wide-range linearly swept laser source (often achieved through external modulation) and a high-speed data acquisition and processing system. The hardware cost is typically much higher than that of a direct-detection φ-OTDR system. Algorithms such as matched filtering, frequency demultiplexing, and fading suppression require complex digital signal processing, placing high demands on the processor's computing power. Similar to other high-sensitivity DAS technologies, when monitoring events like large earthquakes that generate strain signals with extremely large dynamic ranges, there is a risk of signal saturation. Summary of the Invention

[0004] This invention provides a heterodyne distributed acoustic wave sensing system and demodulation algorithm based on a frequency-shift time gate to address the shortcomings of existing technologies.

[0005] On one hand, the present invention provides a heterodyne distributed acoustic wave sensing system based on a frequency-shift time gate, comprising: Narrow linewidth lasers are used to generate continuous laser beams.

[0006] The optical coupling module, connected to a narrow-linewidth laser, is used to split the continuous laser into two paths: a signal beam and a local oscillator beam.

[0007] The optical modulation module, connected to the optical coupling module, is used to sequentially perform pulse modulation and frequency shift modulation on the signal light and output the modulated optical pulse.

[0008] The sensing fiber is used to receive the optical pulses output by the optical modulation module and generate a back echo signal based on the backscattering Rayleigh effect.

[0009] The optical mixing module, connected to the optical coupling module, is used to coherently mix the back echo signal with the local oscillator light.

[0010] The photoelectric detection module, connected to the optical mixing module, is used to convert the coherently mixed optical signal into an electrical signal.

[0011] The signal processing unit, connected to the photoelectric detection module, is used to collect electrical signals and extract vibration information at different locations of the optical fiber through a phase demodulation algorithm.

[0012] The heterodyne distributed acoustic wave sensing system based on frequency shift time gate provided by the present invention further includes a signal generator for providing a driving signal and a synchronization trigger signal to the optical modulation module. The synchronization trigger signal is used to control the signal processing unit to start acquisition at the start of optical pulse emission.

[0013] The heterodyne distributed acoustic wave sensing system based on frequency shift time gate provided by the present invention further includes a circulator for realizing unidirectional transmission of optical signals. The modulated optical pulse is injected into the sensing optical fiber through the circulator, and the back echo signal is output to the optical mixing module through the circulator.

[0014] The heterodyne distributed acoustic wave sensing system based on frequency shift time gate provided by the present invention further includes a clock synchronization module for ensuring clock synchronization between the signal generator and the signal processing unit, thereby eliminating clock jitter error.

[0015] According to the heterodyne distributed acoustic wave sensing system based on a frequency-shift time gate provided by the present invention, the optical modulation module includes a phase modulator and a semiconductor optical amplifier. The phase modulator is used to implement frequency-shift modulation by introducing a constant frequency offset through applying a linear phase modulation signal synchronized with the pulse time gate. The semiconductor optical amplifier is used to implement pulse modulation, with its pulse width set in the range of 50ns to 500ns and synchronized with the phase modulation signal.

[0016] According to the heterodyne distributed acoustic wave sensing system based on frequency shift time gate provided by the present invention, the modulation phase of the phase modulator satisfies , where k is a coefficient related to phase modulation, and t is a time variable. The modulated signal light field satisfies The local oscillator light field satisfies .in Let be the amplitude constant of the light field. This refers to the angular frequency of the output light from a narrow-linewidth laser. The coefficients are related to frequency shift. is the initial phase constant, and j is the imaginary unit.

[0017] According to the heterodyne distributed acoustic wave sensing system based on frequency shift time gate provided by the present invention, the phase modulator has a pulse width Phase change introduced within the corresponding time period satisfy The frequency of the intermediate frequency signal and The sampling frequency of the signal processing unit satisfy .

[0018] According to the heterodyne distributed acoustic wave sensing system based on frequency shift time gate provided by the present invention, the optical modulation module can be replaced by an acousto-optic modulator. The acousto-optic modulator simultaneously realizes pulse modulation and frequency shift modulation functions, and the signal generator only needs to provide one gate time control drive signal and one synchronization trigger signal.

[0019] According to the heterodyne distributed acoustic wave sensing system based on frequency shift time gate provided by the present invention, the optical mixing module includes a 90° optical mixer, and the photoelectric detection module includes two balanced detectors. After the local oscillator light and the back echo signal are mixed by the 90° optical mixer, they are received by the two balanced detectors and output as I(t) signals and Q(t) signals, respectively.

[0020] On the other hand, the present invention also provides a heterodyne distributed acoustic wave sensing demodulation algorithm based on frequency shift time gate, including: Perform a Hilbert transform on the electrical signal to obtain the transformed signal. ,in For Hilbert transformation operators.

[0021] The phase is calculated based on the transformed signal, and phase unwrapping is performed. The unwrapping formula is as follows: ,in This indicates the phase after unwinding. This is a phase unwinding operator used to eliminate phase jumps. It is the arctangent function. for The imaginary part, for The real part.

[0022] Perform time-domain difference analysis on the unwound phase to obtain .

[0023] Phase difference sequences corresponding to different distance positions are extracted according to pulse sequence number, and Fourier transform is performed on each sequence to obtain the vibration frequency information at each position.

[0024] Based on the vibration frequency information at each location, a two-dimensional distance-frequency distribution map is constructed.

[0025] The heterodyne distributed acoustic wave sensing system and demodulation algorithm based on frequency-shift time gate provided by this invention simplifies the complex optical path structure of traditional systems by optimizing the combination of optical path modules, effectively reducing system size, hardware complexity, and deployment difficulty. By introducing a clock synchronization module, the clock of signal generation and data processing is aligned, eliminating error interference caused by clock jitter at its source and improving the stability and reliability of the system in long-term operation. By applying a circulator to achieve efficient unidirectional transmission of optical signals, signal loss and interference caused by optical path reflection are reduced, further optimizing signal transmission quality, enabling the system to maintain stable sensing performance in complex and harsh field environments or long-term monitoring scenarios.

[0026] By optimizing the modulation and demodulation mechanisms, the accuracy of vibration signal recognition and the ability to detect weak signals are improved. The strict synchronization design of phase modulation and pulse modulation ensures the stability and consistency of frequency offset, making the intermediate frequency signal characteristics more distinct and effectively improving the identification of vibration signals. Compared with traditional solutions, this invention reduces the sampling frequency requirement, decreasing data acquisition and real-time processing pressure while ensuring signal integrity, thus lowering hardware costs and energy consumption, improving system response speed, and enabling rapid acquisition and analysis of vibration signals. The organic combination of Hilbert transform and phase unwinding technology effectively avoids interference from phase jumps and light intensity fluctuations, improving the accuracy of phase extraction. The combination of an optical mixer and a balanced detector further simplifies the phase demodulation process, enabling direct signal extraction and improving demodulation efficiency and accuracy.

[0027] Adopting a modular and replaceable design approach, it offers diverse technical combination solutions. The optical modulation module can be flexibly selected from combinations of phase modulators and semiconductor optical amplifiers, or acousto-optic modulators, depending on actual needs. The demodulation scheme can also be switched between real sampling and complex sampling architectures as needed, meeting the application requirements of high precision and high flexibility, as well as adapting to scenarios requiring low cost and simplified deployment. Attached Figure Description

[0028] The invention will now be further described with reference to the accompanying drawings.

[0029] Figure 1 This is a schematic diagram of a heterodyne distributed acoustic sensing system based on a frequency-shift time gate in Embodiment 1 of the present invention; Figure 2 This is an explanatory diagram of the signals generated by the signal generator in Embodiment 1 of the present invention; Figure 3 This is the normalized vibration time-domain diagram after demodulation in Embodiment 1 of the present invention; Figure 4This is the vibration signal spectrum diagram in Embodiment 1 of the present invention; Figure 5 This is a distance-frequency two-dimensional distribution map in Embodiment 1 of the present invention; Figure 6 This is a diagram of the frequency shift gating architecture implemented using AOM in Embodiment 2 of the present invention; Figure 7 This is the signal control and timing diagram in Embodiment 2 of the present invention; Figure 8 This is a system architecture diagram of Embodiment 3 of the present invention; Figure 9 This is a system architecture diagram of Embodiment 4 of the present invention; Figure 10 This is a schematic diagram of a PGC-based DAS system solution in the background technology. Figure 11 This is a flowchart of the signal processing of a PGC-based DAS system in the background technology. Figure 12 This is a structural diagram of a DAS system based on TGD-OFDR in the background technology. Detailed Implementation

[0030] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below in conjunction with specific embodiments.

[0031] Example 1: As Figures 1 to 5 As shown, this embodiment sequentially modulates and frequency-shifts the continuous light output from a narrow-linewidth laser, thereby achieving frequency shift within a single time gate. The modulated light pulses are injected into the sensing fiber, and based on the backscattering Rayleigh effect, the resulting back echo signal is coherently mixed with the local reference light. A simple phase demodulation algorithm can then be used to extract vibration information corresponding to different positions along the fiber. The narrow-linewidth laser, as the core light source of the system, directly affects the coherence and detection accuracy of the sensing system due to the linewidth of its output laser. This embodiment selects a narrow-linewidth laser with a linewidth less than 1 kHz to ensure that the optical signal maintains good coherence characteristics during long-distance transmission and scattering, providing a stable optical field foundation for subsequent coherent mixing and phase demodulation.

[0032] like Figure 1As shown, the phase modulator in the optical modulation module is used to implement the "frequency shift" function. By applying a linear phase modulation signal synchronized with the pulse time gate, a constant frequency shift can be introduced within each pulse. The semiconductor optical amplifier (SOA) in the optical modulation module is used to implement "pulse" modulation. Its pulse width is typically set in the range of 50ns to 500ns and must be strictly synchronized with the phase modulation signal. The semiconductor optical amplifier is chosen as the core device for pulse modulation because it has advantages such as high gain, fast switching response, and low insertion loss. It can be flexibly adjusted within a wide pulse width range of 50ns-500ns to adapt to the sensing requirements of different distance ranges. Short pulse widths (50ns-200ns) are suitable for short-range, high spatial resolution sensing scenarios, while long pulse widths (200ns-500ns) can improve the light signal intensity during long-distance transmission and ensure the effective detection of weak scattered signals.

[0033] Channels 1 and 2 of the signal generator produce the drive signals required for frequency shift modulation and pulse modulation, respectively. Channel 3 serves as a synchronization trigger source, controlling the data acquisition unit to start the acquisition process at the beginning of optical pulse transmission. Clock sources 1 and 2 of the clock synchronization module ensure clock synchronization of the transceiver system, thereby eliminating clock jitter errors between the data acquisition card and the signal processing unit (FPGA). The signal generator must have high-precision signal output capability. The peak-to-peak value of the linear voltage waveform output by channel 1 must accurately match the modulation depth requirements of the phase modulator and be continuously adjustable within the range of 0-5V. The pulse modulation signal of channel 2 must meet the fast switching characteristic of rising and falling edges of less than 10ns to avoid modulation distortion caused by pulse edge blurring. The synchronization trigger signal of channel 3 uses TTL level, and the rising edge trigger time error is controlled within 1ns to ensure strict timing alignment between data acquisition and optical pulse transmission.

[0034] like Figure 2 and Figure 1 As shown, the light emitted from the narrow linewidth laser is split into two paths by coupler 1 of the optical coupling module: one path is modulated and used as the signal light. The other path is unmodulated and serves as the local oscillator. The optical coupling module uses a 1×2 fiber coupler, whose splitting ratio can be flexibly configured according to the actual application scenario. In this embodiment, the splitting ratio of the signal light to the local oscillator light is set to 1:1, which ensures that the signal light has sufficient power to be injected into the sensing fiber to generate effective scattering, and also ensures that the local oscillator light has sufficient light intensity to participate in coherent mixing, thereby improving the signal-to-noise ratio of the mixed signal. In the signal optical path, the light wave first passes through the phase modulator, and its modulation phase can be expressed as:

[0035] In the formula, The modulation phase of the phase modulator is a function of time t, k represents the coefficient related to the phase modulation, and t is a time variable.

[0036] The modulated light field can be represented as:

[0037] In the formula, Let be the modulated signal light field, which is a function of time t. Let be the amplitude constant of the light field. This refers to the angular frequency of the output light from a narrow-linewidth laser. This represents the coefficient related to frequency shift, where t is the time variable. This represents the initial phase constant, where j is the imaginary unit.

[0038] The other unmodulated local oscillator signal is:

[0039] In the formula, The local oscillator light field is unmodulated. Let be the amplitude constant of the light field. This refers to the angular frequency of the output light from a narrow-linewidth laser. This represents the initial phase constant, where j is the imaginary unit.

[0040] like Figure 2 As shown, channel 1 represents the drive voltage signal applied to the phase modulator. This drive signal is a linear voltage waveform, and its peak-to-peak value corresponds to the modulation depth of the phase modulation. Channel 2 represents the pulse modulation signal used for the semiconductor optical amplifier (SOA), with the pulse width denoted as... The pulse width must be consistent with the duration of the phase modulation signal in channel 1, and the two must be strictly synchronized. To achieve precise synchronization between pulse modulation and frequency shift modulation, this embodiment uses an FPGA to perform timing control on the output signal of the signal generator. The timing error between the two signals is controlled within 5ns through an internal phase-locked loop (PLL) module, avoiding instability in the intermediate frequency signal due to synchronization deviation and ensuring the accuracy of vibration signal demodulation.

[0041] To achieve heterodyne demodulation, in Within the time frame, the phase modulator needs to introduce a phase change greater than one complete cycle, i.e., it must satisfy the following condition:

[0042] In the formula, Indicates the phase adjuster in The amount of phase change introduced within a time period. This represents the phase value corresponding to a complete cycle.

[0043] According to Rayleigh backscattering principle, the optical signal returning from the sensing fiber and the local oscillator light are coherently mixed by an optical mixing module. The frequency of the resulting intermediate frequency signal is directly related to the modulation phase depth, and its expression is as follows:

[0044] In the formula, Indicates the frequency of the intermediate frequency signal. Indicates the phase adjuster in The amount of phase change introduced within a time period. This represents the phase value corresponding to a complete cycle. This represents the pulse width, which is the duration of the phase-modulated signal.

[0045] Therefore, the minimum frequency of the intermediate frequency signal satisfies:

[0046] Since the phase modulation depth and the corresponding intermediate frequency signal amplitude have little impact on system performance, the system sampling rate setting only needs to follow the Nyquist sampling theorem, that is:

[0047] In the formula, Indicates the sampling frequency.

[0048] Under these conditions, the bandwidth and data volume required by the system can be effectively compressed, thereby significantly saving hardware resources.

[0049] Channel 3 of the signal generator outputs a rising edge trigger signal synchronized with the pulse modulation signal. This trigger signal drives the data acquisition card of the signal processing unit to initiate the sampling process at the start of the pulse light emission, ensuring accurate capture and acquisition of the beat frequency signal data at that moment. The data acquisition card uses a 16-bit resolution high-speed ADC module with an input bandwidth greater than twice the intermediate frequency signal frequency to ensure distortion-free signal acquisition. Simultaneously, the acquisition card's storage depth is set to at least 1GB to meet the data stream storage requirements of long-term continuous monitoring scenarios.

[0050] Clock 1 and Clock 2 are the synchronization clock sources between the signal generator and the ADC acquisition card / FPGA, respectively. Their function is to eliminate phase errors introduced by clock drift in each unit, thereby ensuring the accuracy and stability of the system's phase measurement. The clock synchronization module uses a GPS discipline doscillator (GPSDO) as the reference clock source, with an output frequency stability better than 1×10⁻⁶. -12 The clock signal provides clock references for the signal generator and signal processing unit through the synchronous trigger interface, controlling the clock jitter error to within 1ps, thus fundamentally avoiding phase measurement deviations caused by clock drift.

[0051] After the above pulse time gating and frequency shift modulation processing, the modulation process of the detection signal light is completed.

[0052] Modulated light field It is injected into the sensing fiber via a circulator. For example... Figure 1 As shown, at a distance of [length missing] from the starting end of the optical fiber A piezoelectric ceramic (PZT) disturbance is installed at the location to simulate environmental vibration. A three-port fiber optic circulator is used, with an insertion loss of less than 0.5 dB and an isolation greater than 50 dB, effectively avoiding crosstalk between the back echo signal and the incident light signal, ensuring efficient unidirectional transmission of the optical signal. Single-mode fiber is used for sensing, with a loss coefficient of less than 0.2 dB / km and a dispersion coefficient controlled within ±3 ps / (nm・km), reducing attenuation and distortion of the optical signal during long-distance transmission.

[0053] Backscattered Rayleigh light is output through circulator port 3 and coupled to the local oscillator light via a coupler. Coherent mixing is performed, followed by input to the photodetector module for photoelectric conversion. The resulting received signal can be expressed as:

[0054] In the formula, Indicates the strength of the received signal after photoelectric conversion. This represents the frequency of the intermediate frequency signal. This represents the phase change caused by external stress disturbance, and its amplitude is proportional to the stress intensity. The core objective of this scheme is to demodulate... That's it. Demodulation can be achieved using the Hilbert transform:

[0055] In the formula, for The signal after Hilbert transform This refers to the Hilbert transform operator. The Hilbert transform, by performing a 90° phase shift on the signal in the frequency domain, effectively extracts the instantaneous phase and frequency information of the signal, providing a high-quality signal foundation for subsequent phase unwinding. In this embodiment, the Hilbert transform is implemented through the digital signal processing module inside the FPGA, with a processing delay of less than 1μs, ensuring real-time demodulation requirements.

[0056] Based on this, the phase is extracted and phase unwrapping is performed:

[0057] In the formula, This indicates the phase after unwinding. This is a phase unwinding operator used to eliminate phase jumps. It is the arctangent function. for The imaginary part, for The real part.

[0058] based on Interference signals between multiple scattering points are observed through differential relative measurements; therefore, time-domain difference is performed on the extracted phase.

[0059] In the formula, for The time-domain difference.

[0060] Because the acquisition card performs discrete sampling of the signal, the actual calculated phase is a discrete sequence. Each pulse, under the control of the synchronous trigger signal, can acquire a set of phase sequences. ,in Indicates the pulse sequence number. Located at a distance of... The perturbation at point 1 will act on the first point 2 in the sequence. Each sampling point, i.e. The phase corresponding to this position is extracted into a sequence according to pulse order:

[0061] Figure 3 A 200Hz vibration signal is shown. By performing a Fourier transform on this sequence, the vibration frequency information at that distance can be obtained, such as... Figure 4 As shown. Points at different distances are processed sequentially. By extracting and analyzing the relevant sequences, a two-dimensional distribution map of vibrations in the distance-frequency dimension can be constructed, ultimately yielding results such as... Figure 5 The two-dimensional distribution map shown indicates a vibration signal of 200 Hz at a location of 1 km.

[0062] Example 2: In terms of modulation scheme, the acousto-optic modulator (AOM) of the optical modulation module can also be used to implement the frequency shift time gating function. The specific system structure is as follows: Figure 6 As shown in the diagram, in this scheme, the modulation frequency is limited by the inherent frequency shift of the acousto-optic modulator, and the frequency parameters cannot be flexibly adjusted. Nevertheless, this embodiment still embodies the core technical concept protected, and therefore it is listed as Embodiment Two for explanation.

[0063] Since AOM itself has a fixed frequency shift characteristic, this solution only requires one drive signal (channel 1) for gate timing control. The drive signals and timing requirements for channel 1 and channel 2 are as follows: Figure 7As shown, channel 1 is responsible for controlling the pulse duration and repetition frequency, while channel 2 serves as the pulse trigger synchronization signal, functioning identically to the aforementioned embodiment. This scheme maintains the same signal demodulation section as Embodiment 1.

[0064] Example 3: In terms of demodulation schemes, both Examples 1 and 2 employ real sampling and perform phase extraction in the digital domain. If a 90° phase shift is introduced in the optical frequency domain, and a complex sampling structure is used, phase demodulation can be achieved more directly. The demodulation scheme and modulation scheme are architecturally separable; the demodulation scheme based on complex sampling can be combined with the modulation scheme in Example 1 to form Example 3, with the modulation channel signal being the same as in Example 1. The specific system implementation is as follows... Figure 8 As shown, in this embodiment, the local oscillator light and the scattered echo are optically mixed at 90° by the optical mixing module and then received by two balanced detectors of the photoelectric detection module, which output in-phase signals respectively. orthogonal to Signal demodulation only requires phase extraction:

[0065] In the formula, This indicates the phase after unwinding. For phase unwinding operator, It is the arctangent function. and These are the in-phase and quadrature signals output by two balanced detectors after 90° optical mixing, respectively. The subsequent algorithm is the same as in Example 1.

[0066] Example 4: Since demodulation and modulation can be separated, the gated frequency shifting based on Example 3 can also employ complex sampling, thus forming Example 4. The system architecture is as follows: Figure 9 As shown, the signal demodulation algorithm is the same as in Embodiment 3, and the signal modulation is the same as in Embodiment 2. The optical modulation module uses an acousto-optic modulator, the optical mixing module includes a 90° optical mixer, and the photoelectric detection module includes two balanced detectors.

[0067] In summary, this embodiment provides a heterodyne distributed acoustic wave sensing system based on a time-shift gate. By optimizing the combination of optical path modules, it simplifies the complex optical path structure of traditional systems, effectively reducing system size, hardware complexity, and deployment difficulty. By introducing a clock synchronization module, it ensures clock alignment between signal generation and data processing, eliminating error interference caused by clock jitter at its source and improving the long-term stability and reliability of the system. By applying a circulator to achieve efficient unidirectional transmission of optical signals, it reduces signal loss and interference caused by optical path reflection, further optimizing signal transmission quality and enabling the system to maintain stable sensing performance in complex and harsh field environments or long-term monitoring scenarios.

[0068] By optimizing the modulation and demodulation mechanisms, the accuracy of vibration signal recognition and the ability to detect weak signals are improved. The strict synchronization design of phase modulation and pulse modulation ensures the stability and consistency of frequency offset, making the intermediate frequency signal characteristics more distinct and effectively improving the identification of vibration signals. Compared with traditional solutions, this invention reduces the sampling frequency requirement, decreasing data acquisition and real-time processing pressure while ensuring signal integrity, thus lowering hardware costs and energy consumption, improving system response speed, and enabling rapid acquisition and analysis of vibration signals. The organic combination of Hilbert transform and phase unwinding technology effectively avoids interference from phase jumps and light intensity fluctuations, improving the accuracy of phase extraction. The combination of an optical mixer and a balanced detector further simplifies the phase demodulation process, enabling direct signal extraction and improving demodulation efficiency and accuracy.

[0069] Adopting a modular and replaceable design approach, it offers diverse technical combination solutions. The optical modulation module can be flexibly selected from combinations of phase modulators and semiconductor optical amplifiers, or acousto-optic modulators, depending on actual needs. The demodulation scheme can also be switched between real sampling and complex sampling architectures as needed, meeting the application requirements of high precision and high flexibility, as well as adapting to scenarios requiring low cost and simplified deployment.

[0070] Through the above description of the embodiments, those skilled in the art can clearly understand that each embodiment can be implemented by means of software plus necessary general-purpose hardware platforms, and of course, it can also be implemented by hardware. Based on this understanding, the above technical solutions, in essence or the part that contributes to the prior art, can be embodied in the form of a software product. This computer software product can be stored in a computer-readable storage medium, such as ROM / RAM, magnetic disk, optical disk, etc., and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute the methods described in the various embodiments or some parts of the embodiments.

[0071] 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 of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A heterodyne distributed acoustic wave sensing system based on a frequency-shift time gate, characterized in that, include: Narrow linewidth lasers are used to generate continuous laser beams. An optical coupling module, connected to the narrow linewidth laser, is used to split the continuous laser into two paths: a signal beam and a local oscillator beam. An optical modulation module, connected to the optical coupling module, is used to sequentially perform pulse modulation and frequency shift modulation on the signal light and output the modulated optical pulse; The sensing fiber is used to receive the optical pulses output by the optical modulation module and generate a back echo signal based on the back Rayleigh scattering effect. An optical mixing module, connected to the optical coupling module, is used to coherently mix the back echo signal with the local oscillator light; A photoelectric detection module, connected to the optical mixing module, is used to convert the coherently mixed optical signal into an electrical signal. The signal processing unit, connected to the photoelectric detection module, is used to acquire the electrical signal and extract vibration information at different positions of the optical fiber through a phase demodulation algorithm.

2. The heterodyne distributed acoustic wave sensing system based on frequency-shift time gate according to claim 1, characterized in that, It also includes a signal generator for providing a drive signal and a synchronization trigger signal to the optical modulation module. The synchronization trigger signal is used to control the signal processing unit to start acquisition at the start of optical pulse emission.

3. The heterodyne distributed acoustic wave sensing system based on a frequency-shift time gate according to claim 1, characterized in that, It also includes a circulator for realizing unidirectional transmission of optical signals. The modulated optical pulse is injected into the sensing optical fiber through the circulator, and the back echo signal is output to the optical mixing module through the circulator.

4. The heterodyne distributed acoustic wave sensing system based on frequency-shift time gate according to claim 1, characterized in that, It also includes a clock synchronization module to ensure that the clocks of the signal generator and the signal processing unit are synchronized, eliminating clock jitter errors.

5. The heterodyne distributed acoustic wave sensing system based on frequency-shift time gate according to claim 1, characterized in that, The optical modulation module includes a phase modulator and a semiconductor optical amplifier. The phase modulator is used to implement frequency shift modulation by applying a linear phase modulation signal synchronized with the pulse time gate to introduce a constant frequency offset. The semiconductor optical amplifier is used to implement pulse modulation, with its pulse width set in the range of 50ns to 500ns and synchronized with the phase modulation signal.

6. The heterodyne distributed acoustic wave sensing system based on a frequency-shift time gate according to claim 5, characterized in that, The modulation phase of the phase modulator satisfies Where k is a coefficient related to phase modulation, and t is a time variable; the modulated signal optical field satisfies The local oscillator light field satisfies ;in Let be the amplitude constant of the light field. This refers to the angular frequency of the output light from a narrow-linewidth laser. The coefficients are related to frequency shift. is the initial phase constant, and j is the imaginary unit.

7. The heterodyne distributed acoustic wave sensing system based on a frequency-shift time gate according to claim 5, characterized in that, The phase modulator has a pulse width Phase change introduced within the corresponding time period satisfy The frequency of the intermediate frequency signal and The sampling frequency of the signal processing unit satisfy .

8. The heterodyne distributed acoustic wave sensing system based on a frequency-shift time gate according to claim 2, characterized in that, The optical modulation module can be replaced by an acousto-optic modulator, which simultaneously performs pulse modulation and frequency shift modulation functions. The signal generator only needs to provide one gated time control drive signal and one synchronous trigger signal.

9. The heterodyne distributed acoustic wave sensing system based on a frequency-shift time gate according to claim 1, characterized in that, The optical mixing module includes a 90° optical mixer, and the photoelectric detection module includes two balanced detectors. After the local oscillator light and the back echo signal are mixed by the 90° optical mixer, they are received by the two balanced detectors and output as I(t) and Q(t) signals, respectively.

10. An adaptive heterodyne distributed acoustic wave sensing demodulation algorithm based on an intelligent frequency-shift time gate, applied to the adaptive heterodyne distributed acoustic wave sensing system based on an intelligent frequency-shift time gate as described in any one of claims 1 to 9, characterized in that, The demodulation algorithm includes: Perform a Hilbert transform on the electrical signal to obtain the transformed signal. ,in For Hilbert transform operators; The phase is calculated based on the transformed signal, and phase unwrapping is performed. The unwrapping formula is as follows: ,in This indicates the phase after unwinding. This is a phase unwinding operator used to eliminate phase jumps. It is the arctangent function. for The imaginary part, for The real part; Perform time-domain difference analysis on the unwound phase to obtain ; The phase difference sequence corresponding to different distance positions is extracted according to the pulse sequence number, and the Fourier transform is performed on each sequence to obtain the vibration frequency information at each position. Based on the vibration frequency information at each location, a two-dimensional distance-frequency distribution map is constructed.

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