Long-distance high-performance nonlinear pulse compression distributed acoustic sensing system

Abstract

This invention proposes a long-distance, high-performance nonlinear pulse compression distributed acoustic sensing system, belonging to the field of fiber optic sensing. The system includes: a single-frequency light output from a narrow-linewidth laser is split into signal light and local light by a first coupler; an optical modulation unit modulates the signal light into chirped frequency-modulated pulses; after amplification and filtering, the signal light is injected into the fiber under test via a circulator to generate a Rayleigh scattering time-domain signal; this signal is transmitted through a circulator to a second coupler to beat with the local light; a detector converts the beat-frequency light signal into an electrical signal; and a data acquisition module acquires the signal, which is then demodulated by a data processing module. Through a full-link collaborative architecture of "laser splitting → chirped modulation → scattering recovery → coherent beat frequency → digital processing," the system improves backlight recovery efficiency with a combination of circulators and dual couplers, and its modular design ensures a high signal-to-noise ratio over long distances. This overcomes the limitations of traditional DAS resolution and distance while maintaining hardware simplification.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, and in particular to a long-distance, high-performance nonlinear pulse-compressed distributed acoustic sensing (NPC-DAS) system. Background Technology

[0002] Distributed Acoustic Sensing (DAS) is an innovative fiber optic sensing technology that analyzes changes in backscattered light (such as Rayleigh scattering) generated by laser pulses in optical fibers to achieve real-time distributed monitoring of physical quantities such as vibration, sound waves, and strain in the environment along the transmission line. This technology can utilize ordinary communication optical fibers to create a continuously distributed sensor network, offering both long-distance coverage and high spatial resolution. Compared to traditional discrete sensor arrays, DAS systems exhibit significant advantages: a single optical fiber can replace a large number of independent sensors, significantly reducing deployment costs; they possess inherent immunity to electromagnetic interference and are adaptable to harsh environments such as high temperatures and corrosion. These characteristics have led to their widespread application in fields such as oil and gas pipeline monitoring, perimeter security, seismic exploration, and infrastructure health diagnostics. With the advancement of 5G networks and smart city construction, DAS technology is deeply integrating with the Internet of Things (IoT), providing key technical support for next-generation intelligent monitoring systems. Currently, the technology is continuously optimizing its sensitivity and positioning accuracy, and it has even broader application prospects in environmental monitoring and urban governance in the future.

[0003] Traditional DAS systems typically employ single-frequency pulse modulation technology, which involves injecting a fixed-frequency laser pulse into an optical fiber and detecting external disturbances by demodulating the phase change of the backscattered Rayleigh light. In this approach, spatial resolution directly depends on the pulse width—the narrower the pulse width, the higher the resolution. However, excessively narrow pulses can significantly degrade the signal-to-noise ratio (SNR) due to fiber nonlinear effects (such as stimulated Brillouin scattering and self-phase modulation), making it difficult for the system to achieve long-distance detection. Therefore, the spatial resolution and sensing distance of traditional DAS systems are mutually restrictive and cannot be improved simultaneously.

[0004] To overcome this limitation, pulse compression technology emerged. This approach eliminates the use of single-frequency pulses, instead modulating wide pulses using time-frequency modulation (such as linear frequency modulation or coded modulation) to carry varying frequency components. At the receiver, the scattered signal is processed through matched filtering or correlation demodulation algorithms, effectively compressing the wide pulse in the time domain, thus significantly improving spatial resolution. In this case, the system resolution no longer depends on the actual pulse width, but rather on the modulation frequency range (bandwidth). Therefore, even with wide pulses (ensuring high signal-to-noise ratio and long detection range), high-resolution detection can still be achieved through wide-bandwidth modulation, effectively resolving the contradiction between resolution and distance in traditional DAS systems. This technology significantly improves the performance of DAS systems, making them more advantageous in long-distance, high-precision monitoring scenarios (such as oil and gas pipeline safety, seismic exploration, and perimeter security).

[0005] However, DAS technology based on Rayleigh scattering, whether implemented in the traditional way or using pulse compression, inevitably faces the fundamental technical challenge of interference fading. Interference fading originates from the intensity fluctuations generated during interference by randomly distributed Rayleigh scattering points within the optical fiber. When the optical path difference at a specific location satisfies the destructive interference condition, the signal intensity at that detection point drops to an extremely low level, making it impossible to accurately extract phase information and ultimately leading to a decline in system performance.

[0006] Currently, frequency division multiplexing (FDM) technology is mainly used in engineering practice to address this challenge. This technology simultaneously transmits multiple sets of probe optical signals with different center frequencies. Utilizing the different fading distributions of each frequency component in the optical fiber, signal fusion processing is used to avoid signal failure at specific locations caused by a single frequency, thereby improving the overall reliability of the system. However, this approach has significant limitations in long-distance monitoring scenarios: as the transmission distance increases, signal attenuation leads to a continuous deterioration in the signal-to-noise ratio, and the density of interference fading points increases accordingly. To maintain effective fading suppression, the system needs to be configured with more operating frequencies, which not only significantly increases hardware implementation costs but also significantly increases system complexity. Therefore, developing new solutions suitable for long-distance monitoring scenarios that balance performance and complexity has become an important research direction in the current development of DAS technology. On the other hand, existing pulse compression schemes are based on linear frequency modulated (LFM) pulses. Due to their mathematical characteristic of exhibiting a sinc function after autocorrelation, time-domain pulse width compression can be achieved using demodulation algorithms such as matched filtering, thus improving spatial resolution. However, the pulse with the equivalent sinc function shape after LFM pulse compression has a low sidelobe suppression ratio, which leads to signal-to-noise ratio degradation and increases the false alarm probability in practical applications. Therefore, researching pulse compression technology with high sidelobe suppression ratio is a key technical challenge in developing high spatial resolution and high signal-to-noise ratio DAS technology. Summary of the Invention

[0007] This invention proposes a long-distance, high-performance nonlinear pulse compression distributed acoustic sensing system, which solves the problems in the prior art where the system needs to be configured with more operating frequencies to maintain effective fading suppression, which not only significantly increases the hardware implementation cost but also significantly increases the system complexity.

[0008] The technical solution of this invention is implemented as follows:

[0009] This invention provides a long-distance, high-performance nonlinear pulse compression distributed acoustic sensing system, comprising a narrow-linewidth laser, a first coupler, an optical modulation unit, an amplification and filtering unit, a circulator, an optical fiber under test, a second coupler, a detector, a data acquisition module, and a data processing module. The narrow-linewidth single-frequency continuous light output from the narrow-linewidth laser is split into signal light and local light by the first coupler at a specific ratio. The optical modulation unit modulates the signal light into a nonlinear chirped frequency-modulated pulse signal light. The amplification and filtering unit amplifies and filters the chirped frequency-modulated pulse signal light. The circulator is used to process the amplified and filtered signal light. A chirped frequency-modulated pulse signal is injected into the optical fiber under test. Based on the Rayleigh scattering effect, the optical fiber under test generates a Rayleigh scattered light time-domain signal corresponding to the chirped frequency-modulated pulse signal. The Rayleigh scattered light time-domain signal is transmitted to a second coupler through a circulator. The second coupler is used to perform optical beat frequency matching between the Rayleigh scattered light time-domain signal and the local light. The detector is used to detect the optical signal after optical beat frequency matching and convert it into an electrical signal. The data acquisition module is used to acquire the electrical signal converted by the detector. The data processing module is used to demodulate the acquired coherent time-domain signal to realize long-distance, high-performance distributed sensing.

[0010] Specifically, the optical modulation unit includes a voltage-controlled oscillator (VCO), an acousto-optic modulator (AOM), and a signal generator. The signal generator generates a voltage signal for the VCO, which outputs a chirped electrical signal based on the voltage signal to drive the AOM to chirp-modulate the signal light. The signal generator also generates a pulse signal for the VCO, which outputs periodic high and low levels based on the pulse signal to drive the AOM to pulse-modulate the chirped signal light, resulting in a nonlinear chirped frequency-modulated pulse signal light.

[0011] Preferably, the bandwidth of the detector is greater than the frequency modulation range of the acousto-optic modulator.

[0012] Preferably, the sampling rate of the data acquisition module is greater than twice the maximum operating frequency of the acousto-optic modulator.

[0013] Specifically, the detector is a polarization diversity balanced detector, and the data processing module uses a demodulation algorithm based on polarization multiplexing subband nonlinear matched filtering to demodulate the coherent time-domain signal acquired by the acquisition module.

[0014] Furthermore, the demodulation algorithm based on polarization multiplexing subband nonlinear filtering includes the following steps:

[0015] The time-domain signal of the original nonlinear chirped pulse generated by the acousto-optic modulator is detected and acquired using a detector, and used as a reference nonlinear chirped pulse signal.

[0016] The reference nonlinear chirped pulse signal is truncated using N sliding time-domain windows with a width of T and a sliding step size of Δt to obtain N nonlinear sub-pulses;

[0017] The N nonlinear sub-pulses are transformed into N nonlinear sub-pulse analytic signals using Hilbert transform, and then the N analytic signals are subjected to complex conjugation and time reversal operations to obtain N sub-band nonlinear matched filters.

[0018] The coherent Rayleigh scattering time-domain signals obtained from the polarization classification of the detector in two polarization directions are transformed by Hilbert to obtain the coherent Rayleigh scattering time-domain analytical signal. The coherent Rayleigh scattering time-domain analytical signal is then filtered sequentially by N sub-band nonlinear matched filters to obtain the sub-band nonlinear compressed time-domain analytical signal.

[0019] The nonlinear compressed time-domain analytic signals of different subbands are subjected to rotation vector operations and then superimposed to obtain nonlinear compressed signals in two polarization directions;

[0020] By superimposing the two nonlinear compressed signals in the polarization direction in the complex domain, a polarization-multiplexed nonlinear compressed signal is obtained.

[0021] Furthermore, the formula for filtering the coherent Rayleigh scattering time-domain analytic signal using a subband nonlinear matched filter is as follows:

[0022] ;

[0023] in, To utilize the first i The sub-band nonlinear compressed time-domain analytic signal is obtained by filtering the coherent Rayleigh scattering time-domain analytic signal with a sub-band nonlinear filter; conv is the convolution operation; This is a time-domain analytic signal of coherent Rayleigh scattering; For the first i Sub-band nonlinear filter.

[0024] Furthermore, the formula for superimposing the nonlinear compressed time-domain analytic signals of different subbands after performing rotation vector operations is as follows:

[0025] ;

[0026] in, This is the superimposed nonlinear compressed time-domain analytic signal; For the first i Sub-band nonlinear compressed time-domain analytic signal; It is the complex conjugate of the nonlinear compressed time-domain analytic signal of the first sub-band; This represents the magnitude of the nonlinear compressed time-domain analytic signal of the first subband.

[0027] Furthermore, the spatial resolution of the sensing system sr for:

[0028] ;

[0029] in, c At the speed of light, n To detect the effective refractive index of the optical fiber, B This represents the bandwidth of the subband nonlinear filter.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0031] (1) This invention constructs a full-link collaborative architecture of “laser beam splitting → chirped pulse modulation → backscatter recovery → coherent beat frequency detection → digital processing”, achieving a dual breakthrough in hardware integration and signal fidelity under the single-source single-frequency modulation framework. The combination design of circulator and dual coupler significantly improves the backscatter light recovery efficiency, the local optical beat frequency mechanism enhances the sensitivity of weak signal detection, and the modular structure (modulation unit, amplification and filtering unit, processing unit) provides a scalable physical basis for high-resolution demodulation while ensuring the signal-to-noise ratio of long-distance transmission. Thus, it overcomes the inherent contradiction between resolution and distance in traditional DAS systems under the premise of simplifying hardware.

[0032] (2) This invention generates chirped pulses with nonlinear frequency changes through the coordinated driving of a voltage-controlled oscillator and an acousto-optic modulator, replacing traditional linear frequency modulation. This results in a narrower main lobe width and lower side lobe energy in the equivalent time-domain signal after pulse compression, significantly improving spatial resolution while suppressing noise interference. This solves the signal-to-noise ratio degradation problem caused by high side lobes in linear frequency modulation schemes. The VCO module simultaneously performs chirped modulation and pulse switching control. Combined with the coordinated design of a polarization diversity balanced detector and a high-speed data acquisition module, the hardware structure is greatly simplified while ensuring signal integrity. The rigid constraints on detector bandwidth and sampling rate ensure distortion-free capture of nonlinear chirped components, laying the foundation for subsequent algorithm processing.

[0033] (3) This invention employs subband nonlinear matched filtering technology to divide a wide pulse into multiple sub-pulses for parallel processing. Based on the spatial resolution model of subband filter bandwidth (rather than pulse width), the system is freed from the limitation of pulse width on resolution. The wide pulse ensures the signal-to-noise ratio for long-distance detection, while the large-bandwidth subband filtering achieves high-resolution demodulation, fundamentally resolving the trade-off between resolution and distance in traditional DAS. By aligning and superimposing the subband phases through rotation vector operations, signal fading caused by random fiber interference is effectively suppressed. Furthermore, by combining polarization diversity detection with complex domain fusion, signal fluctuations caused by polarization disturbances are simultaneously eliminated. This method only requires single-frequency modulation to achieve dual fading suppression, significantly reducing the hardware cost of traditional multi-frequency multiplexing schemes. Attached Figure Description

[0034] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0035] Figure 1 A schematic diagram of the structure of the long-distance, high-performance NPC-DAS system provided by the present invention;

[0036] Figure 2 This is a schematic diagram illustrating the principle of the sub-band nonlinear matched filtering technology provided by the present invention.

[0037] Figure 3 A flowchart of the demodulation algorithm based on polarization multiplexing subband nonlinear filtering provided by the present invention;

[0038] Figure 4 A comparison diagram of the equivalent time-domain compressed pulse of the nonlinear chirped frequency modulation signal provided by the present invention and the equivalent time-domain compressed pulse of the traditional linear chirped frequency modulation signal.

[0039] Figure 5 A comparison diagram of a single sub-band nonlinear time-domain compressed signal and a superimposed nonlinear time-domain compressed signal provided by the present invention;

[0040] Figure 6 A comparison diagram of the nonlinear time-domain compressed signal of a single polarization state and the nonlinear time-domain compressed signal after polarization multiplexing provided by the present invention. Detailed Implementation

[0041] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0042] Reference Figure 1 This invention provides a long-distance, high-performance nonlinear pulse compression distributed acoustic sensing system, comprising a narrow linewidth distributed feedback laser (NLDFB), a first coupler (Tap coupler), an optical modulation unit, an amplification and filtering unit, a circulator, a fiber under test (FUT), a second coupler (2×2 coupler), a polarization diversity balanced photodetector (PDBPD), a data acquisition module (DAQ), and a data processing module (Data... (Processing), the narrow-linewidth single-frequency continuous light output from the narrow-linewidth laser is split into 90% signal light and 10% local light by a first coupler at a specific ratio (9:1 in this embodiment). The optical modulation unit is used to modulate the signal light into a nonlinear chirped frequency-modulated pulse signal light. The amplification and filtering unit is used to amplify and filter the chirped frequency-modulated pulse signal light. The circulator is used to inject the amplified and filtered chirped frequency-modulated pulse signal light into the fiber under test. The fiber under test generates a Rayleigh scattered light time-domain signal corresponding to the chirped frequency-modulated pulse signal based on the Rayleigh scattering effect. The Rayleigh scattered light time-domain signal is transmitted to the second coupler through the circulator. The second coupler is used to perform optical beat frequency conversion between the Rayleigh scattered light time-domain signal and the local light (the amplified and filtered chirped frequency-modulated pulse signal light is injected into the fiber under test through port 1 and port 2 of the circulator; the back Rayleigh scattered light returns to port 2 of the circulator through the fiber and is output to the receiver through port 3, forming the Rayleigh scattered light time-domain signal; this time-domain signal and the local light are optically beat frequency conversion through a 2×2 coupler). The detector is used to detect the optical signal after optical beat frequency conversion and convert it into an electrical signal. The data acquisition module is used to acquire the electrical signal converted by the detector. The data processing module is used to demodulate the acquired coherent time-domain signal to realize long-distance, high-performance distributed sensing.

[0043] This invention achieves a dual breakthrough in hardware integration and signal fidelity within a single-source, single-frequency modulation framework by constructing a full-link collaborative architecture of "laser beam splitting → chirped pulse modulation → backscatter recovery → coherent beat frequency detection → digital processing". The combination design of circulator and dual coupler significantly improves the backscatter light recovery efficiency, the local optical beat frequency mechanism enhances the sensitivity of weak signal detection, and the modular structure (modulation unit, amplification and filtering unit, and processing unit) ensures the signal-to-noise ratio for long-distance transmission while providing a scalable physical basis for high-resolution demodulation. Thus, it overcomes the inherent contradiction between resolution and distance in traditional DAS systems while simplifying the hardware.

[0044] Specifically, the optical modulation unit includes a voltage-controlled oscillator (VCO), an acousto-optic modulator (AOM), and a signal generator (SG). The signal generator generates a voltage signal for the VCO, which outputs a chirped electrical signal to drive the AOM to chirp-modulate the signal light. Simultaneously, the signal generator generates a pulse signal for the VCO, which outputs periodic high and low levels to drive the AOM to pulse-modulate the chirped signal light, resulting in a nonlinear chirped frequency-modulated pulse signal light.

[0045] This invention generates chirped pulses with nonlinear frequency variations through the coordinated driving of a voltage-controlled oscillator (VCO) and an acousto-optic modulator (ACEM), replacing traditional linear frequency modulation (LFM). This results in an equivalent time-domain signal with a narrower main lobe width and lower sidelobe energy, significantly improving spatial resolution while suppressing noise interference. It solves the signal-to-noise ratio degradation problem caused by high sidelobes in LFM schemes. The VCO module simultaneously performs chirped modulation and pulse switching control. Combined with the coordinated design of a polarization diversity balanced detector and a high-speed data acquisition module, the hardware structure is significantly simplified while ensuring signal integrity. Rigid constraints on detector bandwidth and sampling rate ensure distortion-free capture of the nonlinear chirped components, laying the foundation for subsequent algorithm processing.

[0046] Specifically, the amplification and filtering unit includes an erbium-doped optical fiber amplifier (EDFA) and a bandpass filter (BPF). The erbium-doped optical fiber amplifier is used to amplify the chirped frequency-modulated pulse signal light, and the bandpass filter is used to filter the amplified chirped frequency-modulated pulse signal light to remove spontaneous emission noise.

[0047] In this embodiment, the AOM can achieve continuous modulation of nonlinear optical frequencies in the range of several hundred megahertz.

[0048] Preferably, the bandwidth of the detector is greater than the frequency modulation range of the acousto-optic modulator to prevent the loss of high-frequency components of the optical beat frequency signal and ensure the complete detection of nonlinear chirp information.

[0049] Preferably, the sampling rate of the data acquisition module is greater than twice the maximum operating frequency of the acousto-optic modulator, satisfying the Nyquist sampling theorem and avoiding signal irreconstructability caused by spectral aliasing.

[0050] Specifically, the detector is a polarization diversity balanced detector, and the data processing module uses a demodulation algorithm based on polarization multiplexing subband nonlinear matched filtering to demodulate the coherent time-domain signal acquired by the acquisition module.

[0051] Furthermore, such as Figure 2 , 3 As shown, the demodulation algorithm based on polarization multiplexing subband nonlinear filtering includes the following steps:

[0052] The time-domain signal of the original nonlinear chirped pulse generated by the acousto-optic modulator is detected and acquired using a detector, and used as a reference nonlinear chirped pulse signal with a time-domain width of T0.

[0053] The reference nonlinear chirped pulse signal is truncated using N sliding time-domain windows with a width of T and a sliding step size of Δt, resulting in N nonlinear sub-pulses; the truncated ranges are as follows:

[0054] ;

[0055] The N nonlinear sub-pulses are each transformed into N nonlinear sub-pulse analytic signals using the Hilbert transform, and the calculation formula is as follows:

[0056] ;

[0057] in, It is a nonlinear sub-pulse signal. i The imaginary unit; Let Hilbert transform function, This is the nonlinear sub-pulse analytical signal after Hilbert transformation.

[0058] Then, perform complex conjugation and time reversal operations on the N analytical signals to obtain N sub-band nonlinear matched filters;

[0059] The coherent Rayleigh scattering time-domain signals obtained from the polarization classification of the detector in two polarization directions are used to obtain the coherent Rayleigh scattering time-domain analytic signal using Hilbert transform. The calculation formula is as follows:

[0060] ;

[0061] in, The signal is a coherent Rayleigh scattering time-domain signal. This is the time-domain analytic signal of coherent Rayleigh scattering after Hilbert transform.

[0062] The coherent Rayleigh scattering time-domain analytic signal is filtered sequentially using N sub-band nonlinear matched filters to obtain a sub-band nonlinear compressed time-domain analytic signal.

[0063] The nonlinear compressed time-domain analytic signals of different subbands are subjected to rotation vector operations and then superimposed to obtain nonlinear compressed signals in two polarization directions;

[0064] By superimposing the two nonlinear compressed signals in the polarization direction in the complex domain, a polarization-multiplexed nonlinear compressed signal is obtained.

[0065] This invention employs subband nonlinear matched filtering technology to divide a wide pulse into multiple sub-pulse segments for parallel processing. Based on a spatial resolution model of subband filter bandwidth (rather than pulse width), the system overcomes the limitation of pulse width on resolution. The wide pulse ensures the signal-to-noise ratio for long-distance detection, while the large-bandwidth subband filtering achieves high-resolution demodulation, fundamentally resolving the trade-off between resolution and distance in traditional DAS. By aligning and superimposing subband phases through rotation vector operations, signal fading caused by random fiber interference is effectively suppressed. Furthermore, by combining polarization diversity detection with complex domain fusion, signal fluctuations caused by polarization disturbances are simultaneously eliminated. This method requires only single-frequency modulation to achieve dual fading suppression, significantly reducing the hardware cost of traditional multi-frequency multiplexing schemes.

[0066] Furthermore, the formula for filtering the coherent Rayleigh scattering time-domain analytic signal using a subband nonlinear matched filter is as follows:

[0067] ;

[0068] in, To utilize the first i The sub-band nonlinear compressed time-domain analytic signal is obtained by filtering the coherent Rayleigh scattering time-domain analytic signal with a sub-band nonlinear filter; conv is the convolution operation; This is a time-domain analytic signal of coherent Rayleigh scattering; For the first i The sub-band nonlinear filter corresponds to a time-domain window range of the original time-domain pulse analytic signal. .

[0069] Furthermore, the spatial resolution of the sensing system sr for:

[0070] ;

[0071] in, c At the speed of light, n To detect the effective refractive index of the optical fiber, B This represents the bandwidth of the subband nonlinear filter. The final spatial resolution of the sensing system is determined by the bandwidth of the subband nonlinear filter, which is the frequency chirp range corresponding to the time-domain window.

[0072] like Figure 4 As shown, compared with the equivalent time-domain compressed pulse of the traditional linear chirped frequency modulation signal, the equivalent time-domain pulse of the nonlinear chirped frequency modulation signal proposed in this invention has the characteristics of high power and high sidelobe suppression ratio, which significantly enhances the signal-to-noise ratio of the system after pulse compression.

[0073] Furthermore, the formula for superimposing the nonlinear compressed time-domain analytic signals of different subbands after performing rotation vector operations is as follows:

[0074] ;

[0075] in, The superimposed nonlinear compressed time-domain analytic signal contains nonlinear compressed time-domain analytic signals in both X and Y polarization directions; For the first i Sub-band nonlinear compressed time-domain analytic signal; It is the complex conjugate of the nonlinear compressed time-domain analytic signal of the first sub-band; This represents the magnitude of the nonlinear compressed time-domain analytic signal of the first subband.

[0076] like Figure 5 As shown, compared to a single subband nonlinear time-domain compressed signal, the superimposed subband nonlinear time-domain compressed signal after rotation vector operation eliminates most of the interference fading and significantly improves the signal-to-noise ratio.

[0077] Specifically, the nonlinear compressed signals in two polarization directions are superimposed in the complex domain, as shown in the following expression:

[0078] ;

[0079] in, It is a nonlinear compressed signal with polarization multiplexing. , These are nonlinear compressed time-domain analytic signals with polarization directions X and Y, respectively.

[0080] like Figure 6As shown, by superimposing subband signals through rotation vector operations and eliminating interference fading, polarization multiplexing technology further eliminates polarization fading in the signal, achieving complete fading suppression and further improving the signal-to-noise ratio. Compared to the original signal, this brings a significant performance improvement to the sensing system.

[0081] The subband nonlinear matched filtering technique provided by this invention filters the time-domain signal obtained from the modulated nonlinear chirped pulse with different subband nonlinear matched filters. This effectively compresses the obtained time-domain signal, and the compressed equivalent time-domain pulse width, i.e., the spatial resolution, is determined by the bandwidth of the subband nonlinear filter. It also utilizes the fact that different subband nonlinear matched filters have different center frequencies, resulting in different bandwidths for the equivalently compressed time-domain signal. By superimposing these filters, interference fading can be suppressed and the signal-to-noise ratio improved. Furthermore, since the chirped pulse modulated by this invention is a nonlinear chirped pulse, its mathematical self-contradiction... The characteristic of the pulse compression mechanism determines that it has a higher sidelobe suppression ratio compared to linear chirped pulses. Therefore, the equivalent time-domain pulses corresponding to the equivalent compressed time-domain signals after filtering by different sub-band nonlinear matched filters also have high sidelobe suppression ratios. Thus, the pulse-compressed signal has a high signal-to-noise ratio and can effectively reduce false alarms during sensing. In addition, the polarization diversity detection used at the detection end, by processing the time-domain signals of different polarization states separately using sub-band nonlinear matched filtering technology and then superimposing them in the complex domain, can not only effectively eliminate polarization fading in the system, but also further improve the signal-to-noise ratio. Therefore, this invention provides a complete and high-performance DAS technology suitable for long-distance sensing.

[0082] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A long-distance, high-performance nonlinear pulse compression distributed acoustic sensing system, characterized in that, The system includes a narrow-linewidth laser, a first coupler, an optical modulation unit, an amplification and filtering unit, a circulator, an optical fiber under test, a second coupler, a detector, a data acquisition module, and a data processing module. The narrow-linewidth single-frequency continuous light output from the narrow-linewidth laser is split into signal light and local light at a specific ratio by the first coupler. The optical modulation unit modulates the signal light into a nonlinear chirped frequency-modulated (CFM) pulse signal light. The amplification and filtering unit amplifies and filters the CFM pulse signal light. The circulator injects the amplified and filtered CFM pulse signal light into the optical fiber under test. The optical fiber under test generates a Rayleigh scattered light time-domain signal corresponding to the chirped frequency-modulated pulse signal based on the Rayleigh scattering effect. The Rayleigh scattered light time-domain signal is transmitted to a second coupler through a circulator. The second coupler is used to perform optical beat frequency conversion between the Rayleigh scattered light time-domain signal and the local light. The detector is used to detect the optical signal after optical beat frequency conversion and convert it into an electrical signal. The data acquisition module is used to acquire the electrical signal converted by the detector. The data processing module is used to demodulate the acquired coherent time-domain signal to realize long-distance, high-performance distributed sensing. The optical modulation unit includes a voltage-controlled oscillator (VCO), an acousto-optic modulator (AOM), and a signal generator. The signal generator generates a voltage signal for the VCO, which outputs a chirped electrical signal to drive the AOM to chirp-modulate the signal light. The signal generator also generates a pulse signal for the VCO, which outputs periodic high and low levels to drive the AOM to pulse-modulate the chirped signal light, resulting in a nonlinear chirped frequency-modulated pulse signal light. The detector is a polarization diversity balanced detector, and the data processing module uses a demodulation algorithm based on polarization multiplexing subband nonlinear matched filtering to demodulate the coherent time-domain signal acquired by the acquisition module.

2. The long-distance, high-performance nonlinear pulse compression distributed acoustic sensing system as described in claim 1, characterized in that, The bandwidth of the detector is greater than the frequency modulation range of the acousto-optic modulator.

3. The long-distance, high-performance nonlinear pulse compression distributed acoustic sensing system as described in claim 1, characterized in that, The sampling rate of the data acquisition module is more than twice the maximum operating frequency of the acousto-optic modulator.

4. The long-distance, high-performance nonlinear pulse compression distributed acoustic sensing system as described in claim 1, characterized in that, The demodulation algorithm based on polarization multiplexing subband nonlinear matched filtering includes the following steps: The time-domain signal of the original nonlinear chirped pulse generated by the acousto-optic modulator is detected and acquired using a detector, and used as a reference nonlinear chirped pulse signal. The reference nonlinear chirped pulse signal is truncated using N sliding time-domain windows with a width of T and a sliding step size of Δt to obtain N nonlinear sub-pulses; The N nonlinear sub-pulses are transformed into N nonlinear sub-pulse analytic signals using Hilbert transform, and then the N analytic signals are subjected to complex conjugation and time reversal operations to obtain N sub-band nonlinear matched filters. The coherent Rayleigh scattering time-domain signals obtained from the polarization diversity of the detector in two polarization directions are transformed by Hilbert to obtain the coherent Rayleigh scattering time-domain analytical signal. The coherent Rayleigh scattering time-domain analytical signal is then filtered sequentially by N sub-band nonlinear matched filters to obtain the sub-band nonlinear compressed time-domain analytical signal. The nonlinear compressed time-domain analytic signals of different subbands are subjected to rotation vector operations and then superimposed to obtain nonlinear compressed signals in two polarization directions; By superimposing the two nonlinear compressed signals in the polarization direction in the complex domain, a polarization-multiplexed nonlinear compressed signal is obtained.

5. The long-distance, high-performance nonlinear pulse compression distributed acoustic sensing system as described in claim 4, characterized in that, The formula for filtering coherent Rayleigh scattering time-domain analytic signals using a subband nonlinear matched filter is as follows: ； in, To utilize the first i The sub-band nonlinear compressed time-domain analytic signal is obtained by filtering the coherent Rayleigh scattering time-domain analytic signal with a sub-band nonlinear matched filter; conv is the convolution operation; This is a time-domain analytic signal of coherent Rayleigh scattering; For the first i Sub-band nonlinear matched filter.

6. The long-distance, high-performance nonlinear pulse compression distributed acoustic sensing system as described in claim 4, characterized in that, The formula for superimposing the nonlinear compressed time-domain analytic signals of different subbands after performing rotation vector operations is as follows: ； in, This is the superimposed nonlinear compressed time-domain analytic signal; For the first i A sub-band nonlinear compressed time-domain analytic signal; It is the complex conjugate of the nonlinear compressed time-domain analytic signal of the first sub-band; This represents the magnitude of the nonlinear compressed time-domain analytic signal of the first subband.

7. The long-distance, high-performance nonlinear pulse compression distributed acoustic sensing system as described in claim 4, characterized in that, The spatial resolution of the sensing system sr for: ； in, c At the speed of light, n To detect the effective refractive index of the optical fiber, B This represents the bandwidth of the subband nonlinear matched filter.

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